Ionization Enthalpy & Electron Gain Enthalpy

CompleteFundamentals of Chemistry

Ionization enthalpy is the energy to remove an electron; electron gain enthalpy is the energy change on adding one.

Full notes

Definition: Ionization enthalpy (ΔᵢH) — minimum energy needed to remove one mole of electrons from one mole of gaseous atoms. Electron gain enthalpy (ΔₑgH) — energy change when one mole of gaseous electrons combine with atoms.

1. Trends in ionization enthalpy

Across a period Z* rises → ΔᵢH rises, with small breaks: Be > B (Be has fully filled 2s) and N > O (N has half-filled 2p). Down a group, shell size dominates → ΔᵢH falls.

2. Successive ionization enthalpies

IE₂ > IE₁ always (removing from a positive ion is harder). A giant jump between IEᵢ and IEᵢ₊₁ indicates crossing into a new shell — this jump is used to confirm valence electron count (e.g., Na: small IE₁, huge IE₂).

3. Electron gain enthalpy

Generally exothermic (negative) for atoms that want electrons: Cl ΔₑgH −349 kJ/mol, F −328 kJ/mol (Cl more negative than F because F's small size causes electron-electron repulsion). Down a group, ΔₑgH becomes less negative (except the same anomaly). Electron affinity = −ΔₑgH.

Element	ΔₑgH (kJ/mol)	Reason
F	−328	Small 2p repels added electron
Cl	−349	Larger, accepts better
N	≈ +7	Half-filled 2p — extra stability
O	−141	Takes electron into half-filled

Note: Exam point: the F vs Cl electron-gain anomaly and the IE jump method for valence-electron count are favourite questions.

Exam notes

ΔᵢH ↑ across period (breaks at Be/B, N/O), ↓ down group. ΔₑgH normally negative; Cl > F anomaly; large IE₂ jump ⇒ 1 valence electron.

Important points

These two properties together give the metallic/non-metallic character series.

Common mistakes

Writing electron affinity with a positive sign — electron affinity = −ΔₑgH.

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