What is Ionisation Enthalpy?

In short

Ionisation enthalpy is the energy needed to remove the outermost electron from a gaseous atom, measured in kilojoules per mole ( $\text{kJ mol}^{-1}$ ).

In simple words

The first ionisation enthalpy is for $\text{X(g)} \rightarrow \text{X}^+\text{(g)} + e^-$ . It is always positive, because energy must be supplied. It rises across a period and falls down a group. There are two small dips across a period: beryllium to boron, and nitrogen to oxygen. The second ionisation enthalpy is always larger than the first.

Example

Sodium has a first ionisation enthalpy of $496\ \text{kJ mol}^{-1}$ and a second of $4562\ \text{kJ mol}^{-1}$ . The big jump shows that the second electron comes from an inner, full shell.

Also searched as: ionization energy, ionisation energy.