Discontinuities occur because electron sublevel energy and electron-pair repulsion sometimes outweigh the increasing nuclear attraction across a period. The characteristic decreases occur from group 2 to group 13 and from group 15 to group 16.
The first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms: X(g) → X⁺(g) + e⁻.
Across a period, proton number increases while electrons are added to the same principal energy level. Nuclear charge therefore increases, but shielding changes relatively little. Atomic radius generally decreases, so the attraction between the nucleus and the outer electron becomes stronger and first ionization energy generally rises.
However, electron configurations explain two discontinuities:
| Discontinuity | Explanation |
|---|---|
| Group 2 to group 13, such as Be to B or Mg to Al | The electron removed from the group 13 atom occupies a higher-energy p sublevel, whereas the group 2 electron is removed from an s sublevel. The p electron is also more shielded and is therefore easier to remove, causing ionization energy to decrease. |
| Group 15 to group 16, such as N to O or P to S | Group 15 has a relatively stable half-filled p sublevel, with one electron in each p orbital. In group 16, one p orbital contains a pair of electrons. Electron-pair repulsion makes one paired electron easier to remove, so ionization energy decreases. |
A common misconception is that increasing nuclear charge means ionization energy must rise without exception. The overall trend rises, but electron configuration produces the observed discontinuities.
Exam technique: For an HL explain question linked to Structure 3.1.7, state the relevant electron configurations and compare sublevel energy, shielding, or electron-pair repulsion. Do not merely state that one configuration is “more stable”; identify the physical reason the electron requires less energy to remove.