Chemistry

1. 1.

   Chlorine contains 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.

   [4 marks]

   Answer explanation

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

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Uses a weighted-average calculation. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Calculates 35×75 + 37×25. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Develop this part of the answer: Divides the result by 100. 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: Obtains 35.5. 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: Isotopic abundance must be used as a weighting, not simply averaged.

   Marking points

   • Uses a weighted-average calculation.
   • Calculates 35×75 + 37×25.
   • Divides the result by 100.
   • Obtains 35.5.

   Examiner tip: Isotopic abundance must be used as a weighting, not simply averaged.

2. 2.

   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: 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: In the solid, the ions are fixed in a lattice. 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: 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: The moving ions carry electric charge through the liquid. 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.

   Marking points

   • Sodium chloride contains charged ions.
   • In the solid, the ions are fixed in a lattice.
   • When molten, the ions are free to move.
   • The moving ions carry electric charge through the liquid.

   Examiner tip: Electrical conduction requires mobile charged particles.

3. 3.

   An atom of aluminium has atomic number 13 and mass number 27. State the number of protons, neutrons and electrons it contains.

   [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 13 protons (equal to the atomic number). 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 14 neutrons (mass number minus atomic number: 27 − 13). 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 13 electrons (equal to the number of protons in a neutral atom). 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: In a neutral atom, the number of electrons always equals the number of protons; only ions have unequal numbers.

   Marking points

   • States 13 protons (equal to the atomic number).
   • States 14 neutrons (mass number minus atomic number: 27 − 13).
   • States 13 electrons (equal to the number of protons in a neutral atom).

   Examiner tip: In a neutral atom, the number of electrons always equals the number of protons; only ions have unequal numbers.

4. 4.

   Draw (in words) and describe the electronic structure of a chlorine atom (atomic number 17), stating the number of electrons in each shell.

   [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 first shell contains 2 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 the second shell contains 8 electrons. 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 third (outer) shell contains 7 electrons, giving the electronic structure 2,8,7. 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: Fill inner shells to their maximum capacity (2, then 8, then 8) before placing remaining electrons in the outer shell.

   Marking points

   • States that the first shell contains 2 electrons.
   • States that the second shell contains 8 electrons.
   • States that the third (outer) shell contains 7 electrons, giving the electronic structure 2,8,7.

   Examiner tip: Fill inner shells to their maximum capacity (2, then 8, then 8) before placing remaining electrons in the outer shell.

5. 5.

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

   [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 (Na⁺ and Cl⁻) 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 (Na⁺ and Cl⁻) 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.

6. 6.

   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 of electrons constitutes the covalent bond, and is 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 (both atoms keep some claim to them), 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 of electrons constitutes the covalent bond, and is attracted to both nuclei, holding the atoms together.

   Examiner tip: Covalent bonding involves sharing electrons (both atoms keep some claim to them), unlike ionic bonding where electrons are fully transferred from one atom to another.

7. 7.

   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: Do not confuse the strength of individual covalent bonds (strong in both simple molecules and giant structures) with the weak intermolecular forces between separate simple molecules — it is the sheer number of bonds that must break in a giant structure that matters.

   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: Do not confuse the strength of individual covalent bonds (strong in both simple molecules and giant structures) with the weak intermolecular forces between separate simple molecules — it is the sheer number of bonds that must break in a giant structure that matters.

8. 8.

   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 (not breaking the strong covalent bonds within molecules), 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, it is only the weak forces between molecules that 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 (not breaking the strong covalent bonds within molecules), requiring relatively little energy.

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

9. 9.

   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 (free-moving) 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, allowing conduction. 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 specifically the delocalised (free) electrons, not the fixed metal ions, that are responsible for both electrical conductivity 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 (free-moving) 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, allowing conduction.

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

10. 10.

    An atom loses two electrons to form a 2+ ion. State how the number of protons, neutrons and electrons changes, if at all, when this happens.

    [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 number of protons stays the same (forming an ion does not change the identity of the element). 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 number of neutrons stays the same. 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 number of electrons decreases by 2, since the atom loses two electrons to form the 2+ ion. 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: Forming an ion only ever changes the number of electrons — the number of protons and neutrons in the nucleus is never affected by ionisation.

    Marking points

    • States that the number of protons stays the same (forming an ion does not change the identity of the element).
    • States that the number of neutrons stays the same.
    • States that the number of electrons decreases by 2, since the atom loses two electrons to form the 2+ ion.

    Examiner tip: Forming an ion only ever changes the number of electrons — the number of protons and neutrons in the nucleus is never affected by ionisation.

11. 11.

    An atom of oxygen has atomic number 8. State its electronic structure, and state which group of the periodic table it belongs to, explaining your reasoning.

    [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 the electronic structure as 2,6 (2 electrons in the first shell, 6 in the 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 oxygen belongs to Group 6 (Group 16). 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: Explains that the group number corresponds to the number of electrons in the outer shell, which is 6 for oxygen. 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: For main-group elements, the number of outer-shell electrons directly gives the (old-style) group number — this is one of the most useful shortcuts for predicting an element's properties from its position in the periodic table.

    Marking points

    • States the electronic structure as 2,6 (2 electrons in the first shell, 6 in the outer shell).
    • States that oxygen belongs to Group 6 (Group 16).
    • Explains that the group number corresponds to the number of electrons in the outer shell, which is 6 for oxygen.

    Examiner tip: For main-group elements, the number of outer-shell electrons directly gives the (old-style) group number — this is one of the most useful shortcuts for predicting an element's properties from its position in the periodic table.

12. 12.

    Explain, in terms of electron transfer, how an ionic bond forms between a magnesium atom and an oxygen atom to form magnesium oxide.

    [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 magnesium atom transfers (loses) its 2 outer-shell electrons to the oxygen 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 magnesium ion, Mg²⁺, 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 oxygen atom gains these 2 electrons, forming a negatively charged oxide ion, O²⁻, 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 (Mg²⁺ and O²⁻) 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: Unlike sodium chloride, where one electron transfers, magnesium oxide involves a transfer of two electrons, since magnesium has two outer electrons to lose and oxygen has two spaces to fill.

    Marking points

    • States that the magnesium atom transfers (loses) its 2 outer-shell electrons to the oxygen atom.
    • States that this forms a positively charged magnesium ion, Mg²⁺, with a full outer shell.
    • States that the oxygen atom gains these 2 electrons, forming a negatively charged oxide ion, O²⁻, also with a full outer shell.
    • States that the oppositely charged ions (Mg²⁺ and O²⁻) are then held together by strong electrostatic attraction, forming the ionic bond.

    Examiner tip: Unlike sodium chloride, where one electron transfers, magnesium oxide involves a transfer of two electrons, since magnesium has two outer electrons to lose and oxygen has two spaces to fill.

13. 13.

    Explain, in terms of electron transfer, how calcium chloride, CaCl₂, forms from calcium and chlorine atoms, accounting for the 1:2 ratio of calcium to chlorine in the formula.

    [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 a calcium atom transfers (loses) its 2 outer-shell 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 two electrons are each transferred to a separate chlorine atom, since each chlorine atom can only accept 1 electron to complete its 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 this forms one Ca²⁺ ion and two Cl⁻ ions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Work through this mathematical step: States that strong electrostatic attraction between the one Ca²⁺ ion and the two Cl⁻ ions gives the 1:2 ratio in the formula CaCl₂. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The formula ratio of an ionic compound is always determined by balancing the total positive and negative charge to zero overall — here, one 2+ ion requires two 1− ions to balance.

    Marking points

    • States that a calcium atom transfers (loses) its 2 outer-shell electrons.
    • States that these two electrons are each transferred to a separate chlorine atom, since each chlorine atom can only accept 1 electron to complete its outer shell.
    • States that this forms one Ca²⁺ ion and two Cl⁻ ions.
    • States that strong electrostatic attraction between the one Ca²⁺ ion and the two Cl⁻ ions gives the 1:2 ratio in the formula CaCl₂.

    Examiner tip: The formula ratio of an ionic compound is always determined by balancing the total positive and negative charge to zero overall — here, one 2+ ion requires two 1− ions to balance.

14. 14.

    Describe, using shared electron pairs, the covalent bonding in a molecule of water, H₂O, stating the number of shared pairs and the number of lone (non-bonding) pairs on the oxygen atom.

    [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 oxygen atom forms one shared pair of electrons (a single covalent bond) with each of the two hydrogen atoms, giving two shared pairs in total. 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 two covalent bonds, together with oxygen's remaining outer electrons, give the oxygen atom a full outer shell of 8 electrons. 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 oxygen atom has 2 lone (non-bonding) pairs of electrons remaining in its outer shell. 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: Oxygen has 6 outer-shell electrons: 2 are used in the two covalent bonds to hydrogen, leaving exactly 4 (two lone pairs) as non-bonding electrons.

    Marking points

    • States that the oxygen atom forms one shared pair of electrons (a single covalent bond) with each of the two hydrogen atoms, giving two shared pairs in total.
    • States that these two covalent bonds, together with oxygen's remaining outer electrons, give the oxygen atom a full outer shell of 8 electrons.
    • States that the oxygen atom has 2 lone (non-bonding) pairs of electrons remaining in its outer shell.

    Examiner tip: Oxygen has 6 outer-shell electrons: 2 are used in the two covalent bonds to hydrogen, leaving exactly 4 (two lone pairs) as non-bonding electrons.

15. 15.

    Sodium chloride forms a giant ionic lattice structure. 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 in the giant ionic lattice, 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 in the giant ionic lattice, 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.

16. 16.

    Explain why ionic compounds are typically hard but brittle, in terms of their lattice 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 ionic compounds are hard because of the strong electrostatic forces of attraction throughout the giant lattice, acting in every direction. 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 when a force is applied, layers of ions can be shifted (displaced) slightly out of alignment. 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 shift brings ions of the same charge next to each other, causing strong repulsion, which forces the layers apart and shatters (cracks) the crystal. 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: Brittleness in ionic solids comes specifically from like-charge repulsion after a layer shift — this is the key difference from metals, where delocalised electrons allow layers to slide without repulsion.

    Marking points

    • States that ionic compounds are hard because of the strong electrostatic forces of attraction throughout the giant lattice, acting in every direction.
    • States that when a force is applied, layers of ions can be shifted (displaced) slightly out of alignment.
    • States that this shift brings ions of the same charge next to each other, causing strong repulsion, which forces the layers apart and shatters (cracks) the crystal.

    Examiner tip: Brittleness in ionic solids comes specifically from like-charge repulsion after a layer shift — this is the key difference from metals, where delocalised electrons allow layers to slide without repulsion.

17. 17.

    Describe the structure of graphite, and explain why it is used as a lubricant and as an electrode, despite being an allotrope of carbon like diamond.

    [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 in graphite, each carbon atom is covalently bonded to 3 other carbon atoms, forming layers of hexagonal rings. 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 there are only weak forces between the layers, allowing the layers to slide over each other easily, making graphite soft and slippery, suitable as a lubricant. 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 each carbon atom has one delocalised (free) electron, not used in bonding. 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 these delocalised electrons can move along the layers, allowing graphite to conduct electricity, making it useful as an electrode. 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: Graphite and diamond are both pure carbon (allotropes) with entirely covalent bonding, yet have opposite properties (graphite soft and conducting, diamond hard and insulating) purely because of their different structural arrangements.

    Marking points

    • States that in graphite, each carbon atom is covalently bonded to 3 other carbon atoms, forming layers of hexagonal rings.
    • States that there are only weak forces between the layers, allowing the layers to slide over each other easily, making graphite soft and slippery, suitable as a lubricant.
    • States that each carbon atom has one delocalised (free) electron, not used in bonding.
    • States that these delocalised electrons can move along the layers, allowing graphite to conduct electricity, making it useful as an electrode.

    Examiner tip: Graphite and diamond are both pure carbon (allotropes) with entirely covalent bonding, yet have opposite properties (graphite soft and conducting, diamond hard and insulating) purely because of their different structural arrangements.

18. 18.

    Describe the structure of diamond, and explain why it is extremely hard and does not conduct electricity.

    [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 carbon atom in diamond forms 4 strong covalent bonds to 4 other carbon atoms in a rigid, giant tetrahedral (three-dimensional) 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 diamond is extremely hard because a very large number of strong covalent bonds must be broken to separate the atoms. 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 diamond does not conduct electricity because all electrons are held in fixed, localised covalent bonds, with no delocalised (free-moving) electrons or ions available to carry charge. 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: Every electron in diamond is committed to a specific covalent bond between two specific atoms — this total absence of free charge carriers is why diamond is an electrical insulator.

    Marking points

    • States that each carbon atom in diamond forms 4 strong covalent bonds to 4 other carbon atoms in a rigid, giant tetrahedral (three-dimensional) structure.
    • States that diamond is extremely hard because a very large number of strong covalent bonds must be broken to separate the atoms.
    • States that diamond does not conduct electricity because all electrons are held in fixed, localised covalent bonds, with no delocalised (free-moving) electrons or ions available to carry charge.

    Examiner tip: Every electron in diamond is committed to a specific covalent bond between two specific atoms — this total absence of free charge carriers is why diamond is an electrical insulator.

19. 19.

    Explain how the position of an element in the periodic table relates to its electronic structure, using magnesium (Period 3, Group 2) as an example.

    [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 period number equals the number of occupied electron shells in an atom of that element. 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 group number equals the number of electrons in the outermost (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 for magnesium, being in Period 3 and Group 2, this gives the electronic structure 2,8,2. 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 relationship works directly for main-group elements: an element's row tells you its number of shells, and its (old-style) column tells you its number of outer electrons.

    Marking points

    • States that the period number equals the number of occupied electron shells in an atom of that element.
    • States that the group number equals the number of electrons in the outermost (outer) shell.
    • States that for magnesium, being in Period 3 and Group 2, this gives the electronic structure 2,8,2.

    Examiner tip: This relationship works directly for main-group elements: an element's row tells you its number of shells, and its (old-style) column tells you its number of outer electrons.

20. 20.

    State the difference between mass number and relative atomic mass, and explain why the relative atomic mass of chlorine (35.5) is not a whole number.

    [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 mass number is the total number of protons and neutrons in a single atom, and is always a whole number. 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 relative atomic mass is the weighted average mass of all the naturally occurring isotopes of an element, compared with the mass of an atom of carbon-12. 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: Explains that chlorine's relative atomic mass is not a whole number because it exists as a mixture of isotopes (chlorine-35 and chlorine-37) in different proportions, and the weighted average of two different whole numbers need not itself be a whole number. 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: Mass number always describes one specific atom (always a whole number); relative atomic mass describes the average across a natural sample of the element (often not a whole number).

    Marking points

    • States that mass number is the total number of protons and neutrons in a single atom, and is always a whole number.
    • States that relative atomic mass is the weighted average mass of all the naturally occurring isotopes of an element, compared with the mass of an atom of carbon-12.
    • Explains that chlorine's relative atomic mass is not a whole number because it exists as a mixture of isotopes (chlorine-35 and chlorine-37) in different proportions, and the weighted average of two different whole numbers need not itself be a whole number.

    Examiner tip: Mass number always describes one specific atom (always a whole number); relative atomic mass describes the average across a natural sample of the element (often not a whole number).

21. 21.

    Copper consists of 69% copper-63 and 31% copper-65. Calculate the relative atomic mass of copper.

    [3 marks]

    Answer explanation

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

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses a weighted-average calculation. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Calculates (63 × 69 + 65 × 31) ÷ 100. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: Obtains 63.6 (to 3 significant figures). 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: Since the more abundant isotope (copper-63) contributes more heavily to the average, the relative atomic mass ends up closer to 63 than to 65.

    Marking points

    • Uses a weighted-average calculation.
    • Calculates (63 × 69 + 65 × 31) ÷ 100.
    • Obtains 63.6 (to 3 significant figures).

    Examiner tip: Since the more abundant isotope (copper-63) contributes more heavily to the average, the relative atomic mass ends up closer to 63 than to 65.

22. 22.

    State three general physical properties of ionic compounds.

    [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 ionic compounds have high melting and boiling points. 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 ionic compounds conduct electricity when molten or dissolved in water (aqueous), but not when solid. 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 many ionic compounds are soluble in water. 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: These three properties (high melting point, conducts only when molten/dissolved, often soluble in water) together are the standard checklist for identifying an ionic compound experimentally.

    Marking points

    • States that ionic compounds have high melting and boiling points.
    • States that ionic compounds conduct electricity when molten or dissolved in water (aqueous), but not when solid.
    • States that many ionic compounds are soluble in water.

    Examiner tip: These three properties (high melting point, conducts only when molten/dissolved, often soluble in water) together are the standard checklist for identifying an ionic compound experimentally.

23. 23.

    State three general physical properties of simple molecular (covalent) compounds.

    [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 simple molecular compounds have low melting and boiling points. 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 simple molecular compounds do not conduct electricity, since there are no free ions or delocalised electrons present. 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 many simple molecular compounds are insoluble in water, though often soluble in organic solvents. 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: These properties are the direct opposite of ionic compounds' properties, reflecting the fundamental structural difference: strong bonds only within small molecules, versus strong bonds (or attractions) throughout an entire giant lattice.

    Marking points

    • States that simple molecular compounds have low melting and boiling points.
    • States that simple molecular compounds do not conduct electricity, since there are no free ions or delocalised electrons present.
    • States that many simple molecular compounds are insoluble in water, though often soluble in organic solvents.

    Examiner tip: These properties are the direct opposite of ionic compounds' properties, reflecting the fundamental structural difference: strong bonds only within small molecules, versus strong bonds (or attractions) throughout an entire giant lattice.

24. 24.

    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 (the metallic bonding is maintained), so the metal bends or 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 (the metallic bonding is maintained), so the metal bends or 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.

25. 25.

    Explain why alloys, such as brass (a mixture of copper and zinc), are generally harder than the pure metals from which they are made.

    [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 an alloy contains atoms of more than one element, of different sizes. 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 different-sized atoms distort the regular layers of the metal 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 this disrupts the ability of layers to slide easily over each other, making the alloy harder than the pure 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: A pure metal's layers of identical-sized atoms slide past each other easily; introducing even a small proportion of differently sized atoms is enough to jam this sliding and noticeably increase hardness.

    Marking points

    • States that an alloy contains atoms of more than one element, of different sizes.
    • States that these different-sized atoms distort the regular layers of the metal structure.
    • States that this disrupts the ability of layers to slide easily over each other, making the alloy harder than the pure metal.

    Examiner tip: A pure metal's layers of identical-sized atoms slide past each other easily; introducing even a small proportion of differently sized atoms is enough to jam this sliding and noticeably increase hardness.

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