Electron Gain or Electron Affinity - QS Study
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The electron gain (or electron affinity) (ΔH0eg) 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) ΔH0eg = -364 kJ mol-1

If the non-metal NM concerned forms NM2- 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 NM2- ion.

O (g) + e → O2 -(g) ΔH0eg = +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)