Ionisation energiesEdexcel A-Level Chemistry: Revision notes

The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. It is measured in kJ mol⁻¹.

Equation, with state symbols: X(g) → X⁺(g) + e⁻

The second ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions: X⁺(g) → X²⁺(g) + e⁻. Successive ionisation energies are defined in the same way for each further electron.

The energy needed to remove an electron depends on how strongly the nucleus attracts it. Three factors matter:

• Nuclear charge: more protons means a stronger attraction.
• Distance and shielding: electrons in inner shells shield outer electrons from the nuclear attraction, and an electron in a higher shell is further from the nucleus, so it is held less strongly.
• Sub-shell: within a shell, p sub-shell electrons are higher in energy than s sub-shell electrons, so they are easier to remove.

Successive ionisation energies always increase, because each electron is removed from an ion with a greater positive charge: the same number of protons attracts fewer electrons, so they are held more strongly.

There is a large jump when electrons begin to be removed from an inner shell, because that shell is closer to the nucleus and less shielded. The number of electrons before the first big jump gives the group number. For magnesium, the jump from the 2nd (1451) to the 3rd (7733 kJ mol⁻¹) ionisation energy shows two outer electrons, so Group 2.

The first ionisation energy shows a general increase across a period (for example, sodium 496 to argon 1521 kJ mol⁻¹). The nuclear charge increases, while extra electrons are added to the same shell, so shielding is similar and the atomic radius decreases. The outer electron is held more strongly.

There are small dips caused by sub-shells. Aluminium (578) has a lower value than magnesium (738) because its outer electron is in a 3p sub-shell, higher in energy and shielded partly by the 3s electrons.

The first ionisation energy decreases down a group (Group 1: Li 520, Na 496, K 419, Rb 403 kJ mol⁻¹). Each element has an extra shell, so the outer electron is further from the nucleus, and more inner shells shield it. These effects outweigh the greater nuclear charge, so the outer electron is attracted less strongly and less energy is needed.