Ionisation Energy
The minimum energy needed to remove the most loosely held electron from one mole of gaseous atoms in their ground state.
Formula / equation
X(g) → X⁺(g) + e⁻
Unit
kJ/mol
In depth
Ionisation energy increases across a period as effective nuclear charge rises and decreases down a group as the valence shell gets further from the nucleus. Small dips occur where a new subshell begins or a paired p electron is first introduced, as at boron and oxygen.
Examples in the real world
Sodium requires 496 kJ/mol, chlorine 1251 kJ/mol, and helium 2372 kJ/mol — the highest of any element.