AP Chemistry · Free-response skill

Explain an ionization trend with Coulomb's law

The move. Explain why one atom holds an electron more or less tightly than another by comparing the nuclear charge the electron feels and its distance from the nucleus.

How to do it

  1. Start by saying which electron is removed: its shell and subshell. Write the configuration if it helps.
  2. Compare the two atoms on both parts of Coulomb's law: the charge the electron feels (protons, minus shielding by core electrons) and how far it is from the nucleus (which shell).
  3. If one variable is the same for both atoms, say so. That is often what makes the other variable decide the answer.
  4. Finish by tying the stronger or weaker attraction to the property asked about: more energy to remove the electron, a smaller atom, and so on.

Practice prompt Try it

Measured first and second ionization energies (kJ/mol)

ElementFirst ionization energySecond ionization energy
Sodium4964562
Magnesium7381451
Aluminum5781817

A student compares three elements in period 3 using the data in the table. Answer each part.

  1. Write the ground-state electron configuration of a magnesium atom, listing every subshell (not the noble-gas shorthand).
  2. In both sodium and magnesium, the first electron removed comes from the 3s subshell. Explain why magnesium's first ionization energy is greater than sodium's, using the number of protons in each nucleus.
  3. Aluminum has more protons than magnesium, yet its first ionization energy is lower. Name the subshell that aluminum's first removed electron comes from. Then use the energy of that subshell, compared with the 3s subshell, to explain the lower value.
  4. Sodium's second ionization energy is about nine times its first. Explain why, by saying where the second electron comes from.

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Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

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Model answer, annotated

(a) A magnesium atom has 12 electrons: 1s² 2s² 2p⁶ 3s².

  • Point 1 Gives the full configuration with every subshell filled in order.

(b) Both removed electrons are 3s electrons, in the same shell, and both atoms have 10 core electrons. Magnesium has 12 protons and sodium has 11, so magnesium's 3s electron feels a greater effective nuclear charge. It is attracted more strongly and takes more energy to remove.

  • Point 2 Names the variable that differs, the number of protons, and ties it to a stronger attraction and more energy.

(c) Aluminum is 1s² 2s² 2p⁶ 3s² 3p¹, so its first removed electron comes from 3p. The 3p subshell is higher in energy than the 3s subshell, so a 3p electron is held less tightly than a 3s electron, and less energy is needed to remove it, even though aluminum has one more proton.

  • Point 3 Names the subshell, 3p, from the configuration.
  • Point 4 Compares the energies of the two subshells and links the higher-energy 3p electron to a lower ionization energy.

(d) After sodium loses its one 3s electron, Na⁺ is 1s² 2s² 2p⁶. The second electron must come from 2p, in the second shell. It is much closer to the nucleus than the 3s electron was and is less shielded, so it is attracted far more strongly and takes far more energy to remove.

  • Point 5 Names the second shell, 2p, as the source of the second electron.
  • Point 6 Compares it with the 3s electron: closer and less shielded, so held far more tightly.