1.4.5—Ionisation energies are due to the attraction

Ionisation energy reflects the attraction between the nucleus and the electron being removed. A larger ionisation energy means that, under the same definition and gaseous-state conditions, more energy is needed to overcome that attraction.

Compare four linked factors: greater nuclear charge strengthens attraction; greater distance from the nucleus weakens it; inner-shell electrons shield the outer electron; and spin-pair repulsion can make a paired electron easier to remove. The observed value is the combined result, not a proton count alone.

Use the electron’s actual shell and sub-shell before predicting a value. A 3p electron is generally easier to remove than a 2p electron because it is farther from the nucleus and more shielded; within a paired orbital, electron–electron repulsion also lowers the energy needed for removal.

Do not say that more protons always give a higher ionisation energy without checking distance, shielding, sub-shell energy and pairing. Keep these attraction factors distinct from the separate definition of first ionisation energy and from the successive-ionisation jump method.