A sodium street lamp holds sodium vapour.

The first ionisation enthalpy is the energy to remove the outermost electron from an isolated gaseous atom: $\text{X(g)} \rightarrow \text{X}^+\text{(g)} + e^-$. It rises across a period and falls down a group, with dips at groups 13 and 16. A big jump among the successive ionisation enthalpies shows where an inner shell begins. Electron gain enthalpy is the enthalpy change when a gaseous atom gains an electron, negative when energy is released.