Electron Gain or Electron Affinity

Definition:

The electron gain (or electron affinity) (ΔH⁰_eg) is the energy change when one electron is added to each atom to form a negative ion. Note that the standard refers to one mole of ions being formed in the gas phase.

Cl(g) + e^– → Cl^– (g) ΔH⁰_eg = -364 kJ mol^-1

If the non-metal NM concerned forms NM^2- ions, then the ionisation needs to be two stages. The second electron is the energy change needed when a second electron is added to the NM^– ion to form a NM^2- ion.

O^– (g) + e^– → O₂ -(g) ΔH⁰_eg = +844 kJ mol^-1

Although the first electron gain is always exothermic, the second is usually endothermic.

Lattice formation energy (ΔH⁰₁)

Definition: The lattice formation energy (ΔH⁰₁) is the enthalpy change when one mole of a solid ionic crystal is formed from the ions in a gaseous state under standard conditions as shown below for solid NaCl.

Na⁺ (g) + Cl^– (g) → NaCl(s)