1.4.4—The variation in successive ionisation

Successive ionisation energies generally increase because each electron is removed from a more positively charged gaseous ion. Shielding decreases and the proton-to-electron ratio increases, so the remaining electrons are held more strongly by the nucleus.

A large jump means the next electron is being removed from a new inner shell. It is closer to the nucleus and experiences stronger attraction, so much more energy is required; smaller changes can occur within a shell or sub-shell.

To infer the outer-shell count, find the first large change in the successive-ionisation data and count the electrons removed before it. For example, if the third value is the first very large value, two outer electrons were removed before the shell change, supporting a Group 2 inference for the supported case.

Do not treat every increase as a shell jump, or read a jump without checking the gaseous-ion sequence and the stated period or element information. The method infers shell structure from a change of scale; it does not replace the separate first-ionisation trend explanation or justify unsupported configurations.