Chemistry

1. 1.

   An atom has atomic number 17 and mass number 37. State its proton, neutron and electron counts when it forms a singly negative ion.

   [3 marks] · no calculator

   Answer explanation

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

   1. Atomic number counts protons; subtract it from mass number for neutrons. A negative charge means one extra electron, not an extra proton.

   Marking points

   • 17 protons.
   • 20 neutrons.
   • 18 electrons.

   Examiner tip: Ion formation changes electrons, not the nucleus.

2. 2.

   Write the full electron configuration of a ground-state sodium atom and explain which electron is removed first.

   [3 marks] · no calculator

   Answer explanation

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

   1. Fill orbitals in increasing energy order. The single outer 3s electron experiences greater distance and shielding, so removing it leaves a filled second shell.

   Marking points

   • 1s^2 2s^2 2p^6 3s^1.
   • The 3s electron is removed.
   • It is in the outer shell and less strongly attracted than inner electrons.

   Examiner tip: Do not write a shell count when a full orbital configuration is requested.

3. 3.

   An element has isotopes of mass 24 (75%) and 26 (25%). Calculate its relative atomic mass and explain why the value is not a mass number.

   [3 marks]

   Answer explanation

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

   1. Multiply each isotopic mass by its fractional abundance: 24(0.75) + 26(0.25) = 24.5. Individual mass numbers are integers; the average need not be.

   Marking points

   • Use the abundance-weighted mean.
   • Relative atomic mass = 24.5.
   • It averages a mixture rather than counting nucleons in one atom.

   Examiner tip: Relative atomic mass is dimensionless; do not attach grams.

4. 4.

   Explain why the first ionisation energy of aluminium is lower than that of magnesium despite aluminium having more protons.

   [3 marks] · no calculator

   Answer explanation

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

   1. Compare the orbital being emptied, not merely the proton count. Aluminium starts a new subshell, whose electron is easier to remove than magnesium's 3s electron.

   Marking points

   • Magnesium loses a 3s electron; aluminium loses a 3p electron.
   • The 3p subshell is higher in energy.
   • The 3p electron is more shielded/less penetrating, outweighing the increased nuclear charge.

   Examiner tip: A new subshell is not a new principal shell.

5. 5.

   Successive ionisation energies are 590, 1150, 4940 and 6480 kJ mol^-1. Infer the main-group valence-electron count and explain why the jump is evidence, not an exact elemental identification.

   [4 marks] · no calculator

   Answer explanation

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

   1. After the outer pair has gone, removing an inner electron requires much more energy. Several group-2 elements share this qualitative pattern, so an identity needs additional evidence.

   Marking points

   • The large jump is between removal of the second and third electrons.
   • There are two valence electrons.
   • The third electron comes from an inner shell with stronger attraction.
   • The pattern suggests group 2 but does not by itself uniquely identify the period/element.

   Examiner tip: Locate the jump before assigning the group; do not count the high-energy electron as a valence electron.

6. 6.

   Predict which has the smallest radius among O^2-, F^- and Na^+, and explain the ordering using electron structure rather than charge alone.

   [4 marks] · no calculator

   Answer explanation

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

   1. This is an isoelectronic comparison: the occupied shells are the same. Increasing nuclear charge pulls that common electron cloud inward, so sodium's ion is smallest.

   Marking points

   • All three ions have the neon electron configuration.
   • Nuclear charge increases from O to F to Na.
   • Comparable shielding leaves stronger attraction with more protons.
   • Radius order: O^2- > F^- > Na^+; Na^+ is smallest.

   Examiner tip: Establish equal electron configurations before applying the nuclear-charge argument.

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