Strip the electrons from one atom one at a time and the energy needed climbs steadily, then leaps every time you break into a new shell. Those leaps let you read an element's group straight from a table of numbers. Electron affinity is the same idea run in reverse.
Successive ionisation energies always increase: each electron leaves an ion that is more positive, so the same nuclear charge holds fewer electrons more tightly and the ion shrinks. A large jump appears when the next electron comes from an inner shell, closer to the nucleus and less shielded. The number of electrons removed before the first large jump equals the number of outer-shell electrons, which is the group number. Electron affinity, X(g) + e− → X−(g), is exothermic for the first electron but endothermic for a second, because a negative ion repels the incoming electron.
Fig. 1: log of the successive ionisation energies of aluminium
Section 01
Why the energy keeps rising.
Choose an element in the simulator and remove its electrons one at a time. Watch the energy climb, then jump, and count how many electrons came off before the first jump.
Each electron leaves a more positive ion. The nuclear charge is unchanged but there are fewer electrons for it to hold, so each remaining electron is attracted more strongly.
Less repulsion, smaller ion. With fewer electrons the cloud contracts, bringing the remaining electrons closer to the nucleus.
Equations follow the pattern: second ionisation energy is X+(g) → X2+(g) + e−, third is X2+(g) → X3+(g) + e−.
Section 02
Reading the jumps.
Aluminium's first three ionisation energies are 578, 1817 and 2745 kJ per mole. The fourth is 11 577. That five-fold jump is the signature of a new shell.
Small steps within a shell: electrons 1 to 3 all come from n = 3, so each is only somewhat harder to remove than the last.
Large jump between shells: the fourth electron comes from n = 2, much closer to the nucleus and shielded only by the two 1s electrons.
Group number: three electrons before the first big jump, so aluminium has three outer electrons and is in Group 13. Magnesium (738, 1451, then 7733) has two: Group 2.
Log scale: plotting log10(IE) against electron number squashes the huge range so that the steps within a shell and the jumps between shells are both visible, as in Fig. 1.
Stage 1 · Learn
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Successive ionisation energies of an element always increase because:
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An element has successive ionisation energies of 590, 1145, 4912 and 6491 kJ mol−1. It is in:
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The large increase between the 3rd and 4th ionisation energies of aluminium occurs because the 4th electron:
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Which equation represents the second ionisation energy of an element X?
Section 03
Electron affinity.
Ionisation energy removes an electron; electron affinity adds one. The first electron affinity is the enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1− ions.
First electron affinity is usually exothermic:Cl(g) + e− → Cl−(g), ΔH = −349 kJ mol−1. The incoming electron is attracted by the nucleus.
Second electron affinity is endothermic:O−(g) + e− → O2−(g), ΔH = +798 kJ mol−1. The electron is being forced onto an ion that is already negative, and the repulsion must be overcome.
Same factors as ionisation energy: greater nuclear charge, smaller radius and less shielding all make the first electron affinity more exothermic, which is why the halogens have the most negative values.
Examiner trap
Count the electrons removed before the jump, not the position of the jump itself. A large increase between the 2nd and 3rd ionisation energies means two outer electrons, Group 2. And successive ionisation energies are always positive and always increase; a smaller number than the one before is a sign you have misread the table.
Stage 2 · Exam
Exam-style questions
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The first six ionisation energies of an element are 1012, 1903, 2912, 4957, 6274 and 21 269 kJ mol−1. The element is in:
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The first and second ionisation energies of sodium are 496 and 4562 kJ mol−1. Why is the second so much larger?
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Which equation represents the first electron affinity of chlorine?
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Why is the second electron affinity of oxygen endothermic although the first is exothermic?
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Successive ionisation energies, mastered.
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