Successive ionisation energies

Successive ionisation energies always increase: 2nd > 1st, 3rd > 2nd, and so on.

• After each electron is removed, the same number of protons is pulling on fewer electrons.
• There's less electron–electron repulsion and the ion is smaller.
• So each remaining electron is held more tightly than the one before.

Sometimes the increase is much bigger than usual. That happens when the next electron has to come from a shell closer to the nucleus:

• it is much nearer the nucleus, and
• it is much less shielded (fewer electrons inside it).

So the big jumps mark the boundaries between shells.

Sodium's values run from 496 to 159 076 kJ mol⁻¹. On an ordinary scale the first few would be squashed flat against the axis. A log scale gives equal space to each factor of 10, so every jump shows up.