Skip to content
CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Ionization energy

The energy needed to remove the outermost electron from a gaseous atom, X(g)→XX+(g)+eX−\ce{X(g) -> X+(g) + e-}. It increases across a period and decreases down a group, with dips from group 2 to 13 and from group 15 to 16.

Bar chart of values in kJ/molLi: 520 kJ/mol; Be: 899 kJ/mol; B: 801 kJ/mol; C: 1086 kJ/mol; N: 1402 kJ/mol; O: 1314 kJ/mol; F: 1681 kJ/mol; Ne: 2081 kJ/mol0100020003000kJ/mol520Li899Be801B1086C1402N1314O1681F2081Ne

First ionization energies across period 2: an overall rise, with dips at B and O.

Example

Na needs 496 kJ/mol but K only 419 kJ/mol: potassium's 4s4s electron is farther from the nucleus, with about the same Zeff≈1Z_{\text{eff}} \approx 1.

See alsoSuccessive ionization energiesEffective nuclear chargeElectron affinityCation