Successive Ionisation Energies
The series of energies required to remove electrons one after another from the same atom.
Formula / equation
IE₁ < IE₂ < IE₃ …
Unit
kJ/mol
In depth
Each removal leaves a more positive ion, so every successive value is larger. A sudden jump marks the point at which a stable filled shell must be broken into, which reveals how many valence electrons an atom has and hence its group. The pattern was historic evidence for shell structure.
Examples in the real world
Magnesium's third ionisation energy (7733 kJ/mol) dwarfs its second (1451 kJ/mol), confirming just two valence electrons.