Explanation:
In most cases, the formation of an anion by the addition of an electron to a neutral atom releases energy. This can be shown for the chloride ion formation below:
Cl + e− → Cl− + energy
The energy change that occurs when a neutral atom gains an electron is called its electron affinity . When energy is released in a chemical reaction or process, that energy is expressed as a negative number. The figure below shows electron affinities in kJ/mole for the representative elements. Electron affinities are measured on atoms in the gaseous state and are very difficult to measure accurately.
