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IB · CHEMISTRY SL

Chemistry SL

Structure 2: models of bonding and structure — Structure 2

Name: ____________________Date: October 10, 2026
  1. 1.

    Explain, in terms of electron transfer, how an ionic bond forms between a sodium atom and a chlorine atom.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the sodium atom transfers (loses) one electron from its outer shell to the chlorine atom. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that this forms a positively charged sodium ion, Na⁺, with a full outer shell. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the chlorine atom gains this electron, forming a negatively charged chloride ion, Cl⁻, also with a full outer shell. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that the oppositely charged ions are then held together by strong electrostatic attraction, forming the ionic bond. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Ionic bonding always involves complete transfer of electrons between atoms, resulting in oppositely charged ions with full outer electron shells.

    Marking points

    • States that the sodium atom transfers (loses) one electron from its outer shell to the chlorine atom.
    • States that this forms a positively charged sodium ion, Na⁺, with a full outer shell.
    • States that the chlorine atom gains this electron, forming a negatively charged chloride ion, Cl⁻, also with a full outer shell.
    • States that the oppositely charged ions are then held together by strong electrostatic attraction, forming the ionic bond.

    Examiner tip: Ionic bonding always involves complete transfer of electrons between atoms, resulting in oppositely charged ions with full outer electron shells.

  2. 2.

    Marking analysis: A learner attempts the following task: “Explain, in terms of electron transfer, how an ionic bond forms between a sodium atom and a chlorine atom.” Their response addresses only this point: “States that the sodium atom transfers (loses) one electron from its outer shell to the chlorine atom.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that the sodium atom transfers (loses) one electron from its outer shell to the chlorine atom. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that this forms a positively charged sodium ion, Na⁺, with a full outer shell. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the chlorine atom gains this electron, forming a negatively charged chloride ion, Cl⁻, also with a full outer shell. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that the oppositely charged ions are then held together by strong electrostatic attraction, forming the ionic bond. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that the sodium atom transfers (loses) one electron from its outer shell to the chlorine atom.
    • Identifies the missing requirement: States that this forms a positively charged sodium ion, Na⁺, with a full outer shell.
    • Identifies the missing requirement: States that the chlorine atom gains this electron, forming a negatively charged chloride ion, Cl⁻, also with a full outer shell.
    • Identifies the missing requirement: States that the oppositely charged ions are then held together by strong electrostatic attraction, forming the ionic bond.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  3. 3.

    Explain, in terms of shared electron pairs, how a covalent bond forms between two chlorine atoms in a Cl₂ molecule.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that each chlorine atom has 7 electrons in its outer shell and needs 1 more to achieve a stable, full outer shell. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that each atom contributes one electron to form a shared pair of electrons between the two nuclei. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this shared pair constitutes the covalent bond, attracted to both nuclei, holding the atoms together. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Covalent bonding involves sharing electrons, unlike ionic bonding where electrons are fully transferred from one atom to another.

    Marking points

    • States that each chlorine atom has 7 electrons in its outer shell and needs 1 more to achieve a stable, full outer shell.
    • States that each atom contributes one electron to form a shared pair of electrons between the two nuclei.
    • States that this shared pair constitutes the covalent bond, attracted to both nuclei, holding the atoms together.

    Examiner tip: Covalent bonding involves sharing electrons, unlike ionic bonding where electrons are fully transferred from one atom to another.

  4. 4.

    Marking analysis: A learner attempts the following task: “Explain, in terms of shared electron pairs, how a covalent bond forms between two chlorine atoms in a Cl₂ molecule.” Their response addresses only this point: “States that each chlorine atom has 7 electrons in its outer shell and needs 1 more to achieve a stable, full outer shell.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that each chlorine atom has 7 electrons in its outer shell and needs 1 more to achieve a stable, full outer shell. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that each atom contributes one electron to form a shared pair of electrons between the two nuclei. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that this shared pair constitutes the covalent bond, attracted to both nuclei, holding the atoms together. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that each chlorine atom has 7 electrons in its outer shell and needs 1 more to achieve a stable, full outer shell.
    • Identifies the missing requirement: States that each atom contributes one electron to form a shared pair of electrons between the two nuclei.
    • Identifies the missing requirement: States that this shared pair constitutes the covalent bond, attracted to both nuclei, holding the atoms together.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  5. 5.

    Use VSEPR theory to predict the shape and bond angle of a methane molecule, CH₄.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the central carbon atom has 4 bonding electron pairs and no lone pairs. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that these 4 electron pairs repel each other and arrange themselves as far apart as possible in three dimensions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States the shape as tetrahedral with bond angles of 109.5°. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: VSEPR theory predicts molecular shape from electron pair repulsion — lone pairs repel more strongly than bonding pairs and compress bond angles when present.

    Marking points

    • States that the central carbon atom has 4 bonding electron pairs and no lone pairs.
    • States that these 4 electron pairs repel each other and arrange themselves as far apart as possible in three dimensions.
    • States the shape as tetrahedral with bond angles of 109.5°.

    Examiner tip: VSEPR theory predicts molecular shape from electron pair repulsion — lone pairs repel more strongly than bonding pairs and compress bond angles when present.

  6. 6.

    Marking analysis: A learner attempts the following task: “Use VSEPR theory to predict the shape and bond angle of a methane molecule, CH₄.” Their response addresses only this point: “States that the central carbon atom has 4 bonding electron pairs and no lone pairs.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that the central carbon atom has 4 bonding electron pairs and no lone pairs. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that these 4 electron pairs repel each other and arrange themselves as far apart as possible in three dimensions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States the shape as tetrahedral with bond angles of 109.5°. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that the central carbon atom has 4 bonding electron pairs and no lone pairs.
    • Identifies the missing requirement: States that these 4 electron pairs repel each other and arrange themselves as far apart as possible in three dimensions.
    • Identifies the missing requirement: States the shape as tetrahedral with bond angles of 109.5°.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  7. 7.

    Explain why metals are good conductors of electricity, in terms of their bonding and structure.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that metallic bonding consists of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that these delocalised electrons are free to move throughout the entire structure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that when a voltage is applied, these mobile electrons carry electric charge through the metal. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: It is the delocalised (free) electrons, not the fixed metal ions, that are responsible for both electrical and thermal conductivity in metals.

    Marking points

    • States that metallic bonding consists of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons.
    • States that these delocalised electrons are free to move throughout the entire structure.
    • States that when a voltage is applied, these mobile electrons carry electric charge through the metal.

    Examiner tip: It is the delocalised (free) electrons, not the fixed metal ions, that are responsible for both electrical and thermal conductivity in metals.

  8. 8.

    Marking analysis: A learner attempts the following task: “Explain why metals are good conductors of electricity, in terms of their bonding and structure.” Their response addresses only this point: “States that metallic bonding consists of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that metallic bonding consists of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that these delocalised electrons are free to move throughout the entire structure. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that when a voltage is applied, these mobile electrons carry electric charge through the metal. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that metallic bonding consists of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons.
    • Identifies the missing requirement: States that these delocalised electrons are free to move throughout the entire structure.
    • Identifies the missing requirement: States that when a voltage is applied, these mobile electrons carry electric charge through the metal.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  9. 9.

    Explain why giant covalent structures, such as diamond, have very high melting points.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that in a giant covalent structure, every atom is joined to neighbouring atoms by strong covalent bonds throughout the entire structure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that melting requires breaking a very large number of these strong covalent bonds. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this requires a very large amount of energy, resulting in a very high melting point. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: It is the sheer number of strong covalent bonds that must break in a giant structure that explains the high melting point, not the strength of any single bond alone.

    Marking points

    • States that in a giant covalent structure, every atom is joined to neighbouring atoms by strong covalent bonds throughout the entire structure.
    • States that melting requires breaking a very large number of these strong covalent bonds.
    • States that this requires a very large amount of energy, resulting in a very high melting point.

    Examiner tip: It is the sheer number of strong covalent bonds that must break in a giant structure that explains the high melting point, not the strength of any single bond alone.

  10. 10.

    Marking analysis: A learner attempts the following task: “Explain why giant covalent structures, such as diamond, have very high melting points.” Their response addresses only this point: “States that in a giant covalent structure, every atom is joined to neighbouring atoms by strong covalent bonds throughout the entire structure.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that in a giant covalent structure, every atom is joined to neighbouring atoms by strong covalent bonds throughout the entire structure. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that melting requires breaking a very large number of these strong covalent bonds. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that this requires a very large amount of energy, resulting in a very high melting point. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that in a giant covalent structure, every atom is joined to neighbouring atoms by strong covalent bonds throughout the entire structure.
    • Identifies the missing requirement: States that melting requires breaking a very large number of these strong covalent bonds.
    • Identifies the missing requirement: States that this requires a very large amount of energy, resulting in a very high melting point.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  11. 11.

    Explain why simple molecular substances, such as iodine, have low melting and boiling points compared to giant ionic or covalent structures.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that within each molecule, the atoms are held together by strong covalent bonds. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that between different molecules, there are only weak intermolecular forces. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that melting or boiling only requires overcoming these weak intermolecular forces, requiring relatively little energy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A very common error is to say the covalent bonds break during melting — for simple molecular substances, only the weak forces between molecules break, not the bonds within them.

    Marking points

    • States that within each molecule, the atoms are held together by strong covalent bonds.
    • States that between different molecules, there are only weak intermolecular forces.
    • States that melting or boiling only requires overcoming these weak intermolecular forces, requiring relatively little energy.

    Examiner tip: A very common error is to say the covalent bonds break during melting — for simple molecular substances, only the weak forces between molecules break, not the bonds within them.

  12. 12.

    Marking analysis: A learner attempts the following task: “Explain why simple molecular substances, such as iodine, have low melting and boiling points compared to giant ionic or covalent structures.” Their response addresses only this point: “States that within each molecule, the atoms are held together by strong covalent bonds.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that within each molecule, the atoms are held together by strong covalent bonds. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that between different molecules, there are only weak intermolecular forces. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that melting or boiling only requires overcoming these weak intermolecular forces, requiring relatively little energy. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that within each molecule, the atoms are held together by strong covalent bonds.
    • Identifies the missing requirement: States that between different molecules, there are only weak intermolecular forces.
    • Identifies the missing requirement: States that melting or boiling only requires overcoming these weak intermolecular forces, requiring relatively little energy.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  13. 13.

    Explain why sodium chloride conducts electricity when molten but not when solid.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that sodium chloride contains charged ions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that in the solid, the ions are fixed in a lattice and cannot move to carry charge. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that when molten, the ions are free to move. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that the moving ions carry electric charge through the liquid, allowing conduction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Electrical conduction requires mobile charged particles — ionic solids only conduct once molten or dissolved, when their ions become free to move.

    Marking points

    • States that sodium chloride contains charged ions.
    • States that in the solid, the ions are fixed in a lattice and cannot move to carry charge.
    • States that when molten, the ions are free to move.
    • States that the moving ions carry electric charge through the liquid, allowing conduction.

    Examiner tip: Electrical conduction requires mobile charged particles — ionic solids only conduct once molten or dissolved, when their ions become free to move.

  14. 14.

    Marking analysis: A learner attempts the following task: “Explain why sodium chloride conducts electricity when molten but not when solid.” Their response addresses only this point: “States that sodium chloride contains charged ions.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that sodium chloride contains charged ions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that in the solid, the ions are fixed in a lattice and cannot move to carry charge. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that when molten, the ions are free to move. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that the moving ions carry electric charge through the liquid, allowing conduction. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that sodium chloride contains charged ions.
    • Identifies the missing requirement: States that in the solid, the ions are fixed in a lattice and cannot move to carry charge.
    • Identifies the missing requirement: States that when molten, the ions are free to move.
    • Identifies the missing requirement: States that the moving ions carry electric charge through the liquid, allowing conduction.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  15. 15.

    Use VSEPR theory to predict the shape of an ammonia molecule, NH₃, and explain why its bond angle (107°) is slightly smaller than the tetrahedral angle.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the central nitrogen atom has 3 bonding electron pairs and 1 lone pair. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States the shape as trigonal pyramidal. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the lone pair repels more strongly than bonding pairs, since it is held closer to the nucleus by only one atom. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that this stronger lone-pair repulsion compresses the bonding-pair angles slightly below the ideal tetrahedral 109.5°. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion — this ordering explains all bond-angle compressions from the ideal geometry.

    Marking points

    • States that the central nitrogen atom has 3 bonding electron pairs and 1 lone pair.
    • States the shape as trigonal pyramidal.
    • States that the lone pair repels more strongly than bonding pairs, since it is held closer to the nucleus by only one atom.
    • States that this stronger lone-pair repulsion compresses the bonding-pair angles slightly below the ideal tetrahedral 109.5°.

    Examiner tip: Lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion — this ordering explains all bond-angle compressions from the ideal geometry.

  16. 16.

    Marking analysis: A learner attempts the following task: “Use VSEPR theory to predict the shape of an ammonia molecule, NH₃, and explain why its bond angle (107°) is slightly smaller than the tetrahedral angle.” Their response addresses only this point: “States that the central nitrogen atom has 3 bonding electron pairs and 1 lone pair.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that the central nitrogen atom has 3 bonding electron pairs and 1 lone pair. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States the shape as trigonal pyramidal. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the lone pair repels more strongly than bonding pairs, since it is held closer to the nucleus by only one atom. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that this stronger lone-pair repulsion compresses the bonding-pair angles slightly below the ideal tetrahedral 109.5°. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that the central nitrogen atom has 3 bonding electron pairs and 1 lone pair.
    • Identifies the missing requirement: States the shape as trigonal pyramidal.
    • Identifies the missing requirement: States that the lone pair repels more strongly than bonding pairs, since it is held closer to the nucleus by only one atom.
    • Identifies the missing requirement: States that this stronger lone-pair repulsion compresses the bonding-pair angles slightly below the ideal tetrahedral 109.5°.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  17. 17.

    Distinguish between a polar covalent bond and a non-polar covalent bond, referring to electronegativity.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a non-polar covalent bond forms between atoms of equal or very similar electronegativity, so shared electrons are distributed evenly. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that a polar covalent bond forms between atoms of different electronegativity, so shared electrons are pulled closer to the more electronegative atom, creating partial charges (δ+ and δ−). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The greater the electronegativity difference between two bonded atoms, the more polar the bond — very large differences shift the bond towards fully ionic character instead.

    Marking points

    • States that a non-polar covalent bond forms between atoms of equal or very similar electronegativity, so shared electrons are distributed evenly.
    • States that a polar covalent bond forms between atoms of different electronegativity, so shared electrons are pulled closer to the more electronegative atom, creating partial charges (δ+ and δ−).

    Examiner tip: The greater the electronegativity difference between two bonded atoms, the more polar the bond — very large differences shift the bond towards fully ionic character instead.

  18. 18.

    Marking analysis: A learner attempts the following task: “Distinguish between a polar covalent bond and a non-polar covalent bond, referring to electronegativity.” Their response addresses only this point: “States that a non-polar covalent bond forms between atoms of equal or very similar electronegativity, so shared electrons are distributed evenly.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a non-polar covalent bond forms between atoms of equal or very similar electronegativity, so shared electrons are distributed evenly. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a polar covalent bond forms between atoms of different electronegativity, so shared electrons are pulled closer to the more electronegative atom, creating partial charges (δ+ and δ−). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a non-polar covalent bond forms between atoms of equal or very similar electronegativity, so shared electrons are distributed evenly.
    • Identifies the missing requirement: States that a polar covalent bond forms between atoms of different electronegativity, so shared electrons are pulled closer to the more electronegative atom, creating partial charges (δ+ and δ−).

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  19. 19.

    Explain why the boiling point of water is anomalously high compared to other hydrides of Group 16 elements (e.g. H₂S), in terms of hydrogen bonding.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that water molecules can form hydrogen bonds, a relatively strong intermolecular force, due to the highly electronegative oxygen bonded directly to hydrogen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that other Group 16 hydrides (e.g. H₂S) have a less electronegative central atom and rely only on weaker van der Waals forces between molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that overcoming the stronger hydrogen bonds in water requires more energy, resulting in a higher boiling point than expected from the general Group 16 hydride trend. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Hydrogen bonding requires H bonded directly to N, O or F — this is why water, ammonia and hydrogen fluoride all show anomalously high boiling points compared to their period's other hydrides.

    Marking points

    • States that water molecules can form hydrogen bonds, a relatively strong intermolecular force, due to the highly electronegative oxygen bonded directly to hydrogen.
    • States that other Group 16 hydrides (e.g. H₂S) have a less electronegative central atom and rely only on weaker van der Waals forces between molecules.
    • States that overcoming the stronger hydrogen bonds in water requires more energy, resulting in a higher boiling point than expected from the general Group 16 hydride trend.

    Examiner tip: Hydrogen bonding requires H bonded directly to N, O or F — this is why water, ammonia and hydrogen fluoride all show anomalously high boiling points compared to their period's other hydrides.

  20. 20.

    Marking analysis: A learner attempts the following task: “Explain why the boiling point of water is anomalously high compared to other hydrides of Group 16 elements (e.g. H₂S), in terms of hydrogen bonding.” Their response addresses only this point: “States that water molecules can form hydrogen bonds, a relatively strong intermolecular force, due to the highly electronegative oxygen bonded directly to hydrogen.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that water molecules can form hydrogen bonds, a relatively strong intermolecular force, due to the highly electronegative oxygen bonded directly to hydrogen. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that other Group 16 hydrides (e.g. H₂S) have a less electronegative central atom and rely only on weaker van der Waals forces between molecules. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that overcoming the stronger hydrogen bonds in water requires more energy, resulting in a higher boiling point than expected from the general Group 16 hydride trend. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that water molecules can form hydrogen bonds, a relatively strong intermolecular force, due to the highly electronegative oxygen bonded directly to hydrogen.
    • Identifies the missing requirement: States that other Group 16 hydrides (e.g. H₂S) have a less electronegative central atom and rely only on weaker van der Waals forces between molecules.
    • Identifies the missing requirement: States that overcoming the stronger hydrogen bonds in water requires more energy, resulting in a higher boiling point than expected from the general Group 16 hydride trend.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  21. 21.

    Use VSEPR theory to predict the shape and bond angle of a water molecule, H₂O, explaining why its bond angle (104.5°) is smaller than that of ammonia (107°).

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the central oxygen atom has 2 bonding electron pairs and 2 lone pairs. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States the shape as bent (V-shaped/angular). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that with two lone pairs, compared to ammonia's one, there is even greater lone pair-lone pair and lone pair-bonding pair repulsion. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that this compresses the bond angle further, to 104.5°, smaller than ammonia's 107°. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Water, ammonia and methane form a classic progression: as lone pairs increase from 0 to 2 (with bonding pairs decreasing correspondingly), the bond angle compresses from 109.5° to 107° to 104.5°.

    Marking points

    • States that the central oxygen atom has 2 bonding electron pairs and 2 lone pairs.
    • States the shape as bent (V-shaped/angular).
    • States that with two lone pairs, compared to ammonia's one, there is even greater lone pair-lone pair and lone pair-bonding pair repulsion.
    • States that this compresses the bond angle further, to 104.5°, smaller than ammonia's 107°.

    Examiner tip: Water, ammonia and methane form a classic progression: as lone pairs increase from 0 to 2 (with bonding pairs decreasing correspondingly), the bond angle compresses from 109.5° to 107° to 104.5°.

  22. 22.

    Marking analysis: A learner attempts the following task: “Use VSEPR theory to predict the shape and bond angle of a water molecule, H₂O, explaining why its bond angle (104.5°) is smaller than that of ammonia (107°).” Their response addresses only this point: “States that the central oxygen atom has 2 bonding electron pairs and 2 lone pairs.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that the central oxygen atom has 2 bonding electron pairs and 2 lone pairs. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States the shape as bent (V-shaped/angular). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that with two lone pairs, compared to ammonia's one, there is even greater lone pair-lone pair and lone pair-bonding pair repulsion. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that this compresses the bond angle further, to 104.5°, smaller than ammonia's 107°. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that the central oxygen atom has 2 bonding electron pairs and 2 lone pairs.
    • Identifies the missing requirement: States the shape as bent (V-shaped/angular).
    • Identifies the missing requirement: States that with two lone pairs, compared to ammonia's one, there is even greater lone pair-lone pair and lone pair-bonding pair repulsion.
    • Identifies the missing requirement: States that this compresses the bond angle further, to 104.5°, smaller than ammonia's 107°.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  23. 23.

    Explain, in terms of structure and bonding, why metals are malleable (can be hammered into shape) without shattering.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that metals consist of layers of positive metal ions arranged in a regular structure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the delocalised electrons are not fixed to any particular ion or bond, allowing layers of ions to slide over each other when a force is applied. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that as the layers slide, the delocalised electrons continue to hold the ions together (metallic bonding is non-directional and maintained), so the metal deforms rather than shattering. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This is the key contrast with ionic solids: metallic bonding is non-directional and survives layers sliding, whereas ionic bonding breaks down through like-charge repulsion when layers shift, causing brittleness instead.

    Marking points

    • States that metals consist of layers of positive metal ions arranged in a regular structure.
    • States that the delocalised electrons are not fixed to any particular ion or bond, allowing layers of ions to slide over each other when a force is applied.
    • States that as the layers slide, the delocalised electrons continue to hold the ions together (metallic bonding is non-directional and maintained), so the metal deforms rather than shattering.

    Examiner tip: This is the key contrast with ionic solids: metallic bonding is non-directional and survives layers sliding, whereas ionic bonding breaks down through like-charge repulsion when layers shift, causing brittleness instead.

  24. 24.

    Marking analysis: A learner attempts the following task: “Explain, in terms of structure and bonding, why metals are malleable (can be hammered into shape) without shattering.” Their response addresses only this point: “States that metals consist of layers of positive metal ions arranged in a regular structure.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that metals consist of layers of positive metal ions arranged in a regular structure. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that the delocalised electrons are not fixed to any particular ion or bond, allowing layers of ions to slide over each other when a force is applied. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that as the layers slide, the delocalised electrons continue to hold the ions together (metallic bonding is non-directional and maintained), so the metal deforms rather than shattering. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that metals consist of layers of positive metal ions arranged in a regular structure.
    • Identifies the missing requirement: States that the delocalised electrons are not fixed to any particular ion or bond, allowing layers of ions to slide over each other when a force is applied.
    • Identifies the missing requirement: States that as the layers slide, the delocalised electrons continue to hold the ions together (metallic bonding is non-directional and maintained), so the metal deforms rather than shattering.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  25. 25.

    Sodium chloride forms a giant ionic lattice. Describe this structure, and explain why ionic compounds generally have high melting points.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that positive and negative ions are arranged in a regular, repeating (alternating) pattern extending in all directions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that each ion is strongly attracted electrostatically to several oppositely charged ions surrounding it. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that melting requires overcoming this very large number of strong electrostatic forces throughout the lattice, requiring a large amount of energy and giving a high melting point. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Each ion in a giant ionic lattice is attracted to several neighbouring ions of opposite charge, not just one — this is why ionic bonding is often described as non-directional and very strong overall.

    Marking points

    • States that positive and negative ions are arranged in a regular, repeating (alternating) pattern extending in all directions.
    • States that each ion is strongly attracted electrostatically to several oppositely charged ions surrounding it.
    • States that melting requires overcoming this very large number of strong electrostatic forces throughout the lattice, requiring a large amount of energy and giving a high melting point.

    Examiner tip: Each ion in a giant ionic lattice is attracted to several neighbouring ions of opposite charge, not just one — this is why ionic bonding is often described as non-directional and very strong overall.

  26. 26.

    Marking analysis: A learner attempts the following task: “Sodium chloride forms a giant ionic lattice. Describe this structure, and explain why ionic compounds generally have high melting points.” Their response addresses only this point: “States that positive and negative ions are arranged in a regular, repeating (alternating) pattern extending in all directions.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that positive and negative ions are arranged in a regular, repeating (alternating) pattern extending in all directions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that each ion is strongly attracted electrostatically to several oppositely charged ions surrounding it. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that melting requires overcoming this very large number of strong electrostatic forces throughout the lattice, requiring a large amount of energy and giving a high melting point. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that positive and negative ions are arranged in a regular, repeating (alternating) pattern extending in all directions.
    • Identifies the missing requirement: States that each ion is strongly attracted electrostatically to several oppositely charged ions surrounding it.
    • Identifies the missing requirement: States that melting requires overcoming this very large number of strong electrostatic forces throughout the lattice, requiring a large amount of energy and giving a high melting point.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  27. 27.

    Explain why the boiling points of the halogens (F₂, Cl₂, Br₂, I₂) increase down the group, in terms of van der Waals (London dispersion) forces.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that halogen molecules are held together in the solid/liquid state only by van der Waals (London dispersion) forces between molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that as molecular size (number of electrons) increases down the group, the strength of these instantaneous induced dipole-induced dipole forces increases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that stronger van der Waals forces require more energy to overcome, resulting in increasing boiling points down the group. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Larger molecules with more electrons have larger, more easily distorted electron clouds, producing stronger instantaneous dipoles — this is why van der Waals forces consistently strengthen with molecular (and atomic) size.

    Marking points

    • States that halogen molecules are held together in the solid/liquid state only by van der Waals (London dispersion) forces between molecules.
    • States that as molecular size (number of electrons) increases down the group, the strength of these instantaneous induced dipole-induced dipole forces increases.
    • States that stronger van der Waals forces require more energy to overcome, resulting in increasing boiling points down the group.

    Examiner tip: Larger molecules with more electrons have larger, more easily distorted electron clouds, producing stronger instantaneous dipoles — this is why van der Waals forces consistently strengthen with molecular (and atomic) size.

  28. 28.

    Marking analysis: A learner attempts the following task: “Explain why the boiling points of the halogens (F₂, Cl₂, Br₂, I₂) increase down the group, in terms of van der Waals (London dispersion) forces.” Their response addresses only this point: “States that halogen molecules are held together in the solid/liquid state only by van der Waals (London dispersion) forces between molecules.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that halogen molecules are held together in the solid/liquid state only by van der Waals (London dispersion) forces between molecules. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that as molecular size (number of electrons) increases down the group, the strength of these instantaneous induced dipole-induced dipole forces increases. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that stronger van der Waals forces require more energy to overcome, resulting in increasing boiling points down the group. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that halogen molecules are held together in the solid/liquid state only by van der Waals (London dispersion) forces between molecules.
    • Identifies the missing requirement: States that as molecular size (number of electrons) increases down the group, the strength of these instantaneous induced dipole-induced dipole forces increases.
    • Identifies the missing requirement: States that stronger van der Waals forces require more energy to overcome, resulting in increasing boiling points down the group.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

  29. 29.

    Explain, using the formation of the ammonium ion NH₄⁺ from ammonia and a hydrogen ion, what is meant by a dative (coordinate) covalent bond.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a dative covalent bond forms when one atom provides both electrons of the shared pair, rather than one electron from each atom. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the nitrogen atom in ammonia uses its lone pair to form a bond with the H⁺ ion, which has no electrons of its own to contribute. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that once formed, a dative bond is identical to (indistinguishable from) any ordinary covalent bond. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The only thing special about a dative bond is how it forms (both electrons from one atom) — once formed, all four N-H bonds in NH₄⁺ are completely identical in length, strength and behaviour.

    Marking points

    • States that a dative covalent bond forms when one atom provides both electrons of the shared pair, rather than one electron from each atom.
    • States that the nitrogen atom in ammonia uses its lone pair to form a bond with the H⁺ ion, which has no electrons of its own to contribute.
    • States that once formed, a dative bond is identical to (indistinguishable from) any ordinary covalent bond.

    Examiner tip: The only thing special about a dative bond is how it forms (both electrons from one atom) — once formed, all four N-H bonds in NH₄⁺ are completely identical in length, strength and behaviour.

  30. 30.

    Marking analysis: A learner attempts the following task: “Explain, using the formation of the ammonium ion NH₄⁺ from ammonia and a hydrogen ion, what is meant by a dative (coordinate) covalent bond.” Their response addresses only this point: “States that a dative covalent bond forms when one atom provides both electrons of the shared pair, rather than one electron from each atom.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a dative covalent bond forms when one atom provides both electrons of the shared pair, rather than one electron from each atom. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that the nitrogen atom in ammonia uses its lone pair to form a bond with the H⁺ ion, which has no electrons of its own to contribute. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that once formed, a dative bond is identical to (indistinguishable from) any ordinary covalent bond. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a dative covalent bond forms when one atom provides both electrons of the shared pair, rather than one electron from each atom.
    • Identifies the missing requirement: States that the nitrogen atom in ammonia uses its lone pair to form a bond with the H⁺ ion, which has no electrons of its own to contribute.
    • Identifies the missing requirement: States that once formed, a dative bond is identical to (indistinguishable from) any ordinary covalent bond.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.