Ionisation energy

AS Level Chemistry · Atomic structure. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, to form one mole of gaseous 1+ ions. It measures how strongly the nucleus holds the outer electron.

Four things decide its size: the nuclear charge, the distance of the electron from the nucleus, the shielding by inner electrons, and spin-pair repulsion between two electrons in the same orbital.

Successive ionisation energies always increase, because each electron leaves a more positive ion. A very large jump shows that the next electron comes from a shell closer to the nucleus, and this tells you the group.

Rules to remember

Common mistake

Students write the second ionisation energy as X(g) → X2+(g) + 2e−. Each ionisation energy removes one electron only, and every species must be gaseous.

Worked example

The first four ionisation energies of an element are 738, 1451, 7733 and 10 540 kJ mol−1. In which group is the element?

  1. Compare each value with the one before: 1451 ÷ 738 ≈ 2.0, 7733 ÷ 1451 ≈ 5.3, 10 540 ÷ 7733 ≈ 1.4.
  2. The big jump is between the second and third values.
  3. So two electrons are removed easily, and the third comes from an inner shell.

Answer: Group 2 (two outer-shell electrons).

Practice questions

Try each one, then open the answer.

1. Which equation represents the first ionisation energy of chlorine?

  1. A
    Cl(g) + e− → Cl−(g)
  2. B
    ½Cl2(g) → Cl+(g) + e−
  3. C
    Cl(g) → Cl+(g) + e−
  4. D
    Cl2(g) → 2Cl+(g) + 2e−
Show answer

Answer: C. First ionisation energy refers to one mole of gaseous atoms each losing one electron. Equations starting from Cl2 include bond breaking, and gaining an electron is electron affinity.

2. Why is the second ionisation energy of an element always greater than its first?

  1. A
    the second electron is always removed from an inner shell
  2. B
    the second electron is removed from a positive ion, in which the same nuclear charge attracts fewer electrons
  3. C
    the nuclear charge increases after the first electron is removed
  4. D
    the second electron is always in a higher-energy sub-shell
Show answer

Answer: B. After one electron has gone, the same number of protons holds fewer electrons, repulsion is less and the ion is smaller, so more energy is needed. The second electron is not always from an inner shell (for magnesium both come from 3s).

3. Why does the first ionisation energy decrease down Group 1 from lithium to caesium?

  1. A
    the nuclear charge decreases down the group
  2. B
    the outer electron is in a p sub-shell for the heavier elements
  3. C
    spin-pair repulsion increases down the group
  4. D
    the outer electron is further from the nucleus and more shielded, which outweighs the increase in nuclear charge
Show answer

Answer: D. Nuclear charge increases down the group, but each element has an extra shell, so the outer s electron is further away and better shielded. The attraction is weaker, so less energy is needed.

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