Ionization energy: definition in chemistry
Ionization energy: The minimum energy needed to remove the most loosely held electron from a neutral atom in the gas phase: X(g) → X⁺(g) + e⁻.
Ionization energy is usually given in kilojoules per mole and shows how tightly an atom holds its outer electrons. Removing a second or third electron always takes more energy, and there is a very large jump once the outer shell is empty, which reveals how many valence electrons the atom has. Elements with low ionization energies, such as the alkali metals, form positive ions easily and are highly reactive metals.
Examples
- Helium — 2,372 kJ/mol, the highest of any element
- Cesium — about 376 kJ/mol, among the lowest
- Sodium — 496 kJ/mol for the first electron, 4,562 kJ/mol for the second
- Magnesium's ionization energy jumps sharply at the third electron, revealing two valence electrons
On the periodic table
First ionization energy generally rises across a period and falls down a group, so it is highest for helium at the top right and lowest near cesium at the bottom left.