Electron affinity: definition in chemistry
Electron affinity: The energy change when a neutral atom in the gas phase gains an electron to form a negative ion. Halogens have especially high electron affinities.
A large electron affinity means a lot of energy is released when the atom accepts an electron, which is typical of nonmetals. Chlorine, not fluorine, has the highest electron affinity of any element, because the extra electron is crowded in fluorine's very small atom. Sign conventions differ: some books list the energy released as a positive number, while others write it as a negative energy change.
Examples
- Chlorine releases about 349 kJ/mol when it gains an electron
- Fluorine releases about 328 kJ/mol, slightly less than chlorine
- Noble gases such as neon do not form stable negative ions
- Adding a second electron to O⁻ to form O²⁻ requires energy
On the periodic table
Electron affinity generally grows from left to right across a period, peaks at the halogens in group 17 and drops sharply at the noble gases in group 18.