Ionisation energy

Key Definition The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous +1 ions.

First IE: X(g) → X⁺(g) + e⁻ Second IE: X⁺(g) → X²⁺(g) + e⁻

Ionisation requires energy because work must be done to overcome the electrostatic attraction between the positive nucleus and the negative outer electron being removed.

Four factors control the magnitude of an ionisation energy:

• Nuclear charge — a greater number of protons increases the attraction on the outer electron, raising the ionisation energy.
• Atomic radius — the further the outer electron is from the nucleus, the weaker the attraction, so the ionisation energy is lower.
• Shielding by inner electrons — filled inner shells screen the outer electron from the nucleus, reducing the effective nuclear attraction and lowering the ionisation energy.
• Spin-pair repulsion — two electrons paired in the same orbital repel one another, so a paired electron is slightly easier to remove.

Across a period, first ionisation energy generally increases because the nuclear charge rises while shielding stays roughly constant and the atomic radius decreases, so the outer electron is held more strongly.

Two characteristic dips occur within a period: one between Groups 2 and 13, caused by the change in sub-shell from s to the slightly higher-energy p, and one between Groups 15 and 16, caused by spin-pair repulsion when an electron first pairs up in a p orbital.

Down a group, ionisation energy decreases because the outer electron is in a shell further from the nucleus and is more heavily shielded by additional inner shells, so the attraction it feels is weaker.

Successive ionisation energies remove electrons one at a time from the same species, and each value is always larger than the last because each electron is pulled from an increasingly positive ion.

Large jumps in the sequence reveal shell boundaries, because after a shell is emptied the next electron must come from a shell closer to the nucleus, which is held much more strongly.

Counting the number of electrons removed before the first large jump gives the group number of the element directly.

The full pattern of jumps reveals the complete electronic configuration, allowing the element to be identified once its period is known.

Exam Tip Examiners reward precise definitions, correct state symbols in ionisation equations (all species gaseous), and explanations that name the four factors specifically rather than referring vaguely to "attraction".