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AS · Practice questions · Successive ionisation energies and electron affinity

Count before the jump.

Six original Cambridge-style questions on successive ionisation energies and electron affinity: the trend, the jumps, group deduction, the log plot, and first versus second electron affinity.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Steps within a shell, jumps between shells.

01
Explain
[2 marks]

Explain why the successive ionisation energies of an element always increase.

  • Each electron is removed from an ion that is more positive than the last, so the remaining electrons are attracted more strongly by the same nuclear charge. ✓
  • With fewer electrons there is less repulsion between them, the ion becomes smaller, and the electrons are closer to the nucleus. ✓
02
Analysis
[3 marks]

The first four ionisation energies of magnesium are 738, 1451, 7733 and 10 541 kJ mol−1. Explain the pattern in terms of the electronic structure of magnesium.

  • Magnesium is 1s22s22p63s2. The first two electrons are removed from the 3s sub-shell, so the increase from 738 to 1451 is modest. ✓
  • The third electron is removed from the n = 2 shell, which is much closer to the nucleus and shielded only by the 1s electrons, so there is a large jump to 7733. ✓
  • The fourth electron is also from n = 2, so the rise from 7733 to 10 541 is again modest. ✓
03
Analysis
[2 marks]

An element has successive ionisation energies of 1086, 2353, 4621, 6223, 37 831 and 47 277 kJ mol−1. Deduce its group and identify the element, given that it is in Period 2.

  • The large jump is between the 4th and 5th values, so the element has four outer electrons and is in Group 14. ✓
  • A Period 2 element with four outer electrons (2s22p2) is carbon. ✓
04
Explain
[3 marks]

Describe the shape of a graph of log10(ionisation energy) against the number of electrons removed for silicon (Z = 14), and explain why a logarithmic scale is used.

  • Four points rising gently (the 3s and 3p electrons), then a large jump to eight points rising gently (the n = 2 shell), then a second large jump to two points (the 1s electrons). ✓
  • The jumps occur where the next electron comes from a shell closer to the nucleus with less shielding. ✓
  • The values range from hundreds to hundreds of thousands of kJ per mole; a log scale compresses this range so that both the small steps within a shell and the large jumps between shells can be seen on one graph. ✓
05
Recall
[2 marks]

Define first electron affinity and write an equation, with state symbols, for the first electron affinity of fluorine.

  • The enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1− ions. ✓
  • F(g) + e− → F−(g). ✓
06
Explain
[3 marks]

The first electron affinity of chlorine is −349 kJ mol−1. The second electron affinity of oxygen is +798 kJ mol−1. Explain the difference in sign.

  • In the first electron affinity the electron is added to a neutral atom and is attracted by the nuclear charge, so energy is released and the change is exothermic. ✓
  • In the second electron affinity the electron is added to an ion that is already negative, O−. ✓
  • The incoming electron is repelled by the negative charge, so energy must be supplied to overcome the repulsion and the change is endothermic. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.