Electron Affinity
Definition and meaning of Electron Affinity in chemistry.
Electron affinity is the energy change that occurs when a neutral gaseous atom gains an electron to form a negative ion. For most elements this process releases energy, so electron affinity is commonly reported as a negative value (exothermic) under the IUPAC convention, though some sources report the magnitude released as a positive number.
In more detail
Electron affinity generally becomes more negative (more energy released) moving left to right across a period, as increasing nuclear charge pulls the added electron in more strongly, and less negative moving down a group as atomic radius increases and electron-electron repulsion grows. Exceptions occur at groups 2, 15, and 18, where filled or half-filled subshells make electron addition less favorable or even endothermic. Electron affinity helps explain trends in reactivity, ionic bond formation, and why nonmetals like halogens readily form anions.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical units | kJ/mol or eV |
| Example value (Cl) | -349 kJ/mol |
| Periodic trend | More negative left to right; less negative down a group |
Chlorine has an electron affinity of about -349 kJ/mol: Cl(g) + e⁻ → Cl⁻(g) releases 349 kJ/mol of energy, reflecting chlorine's strong pull on an incoming electron to complete its octet.
Frequently asked questions
Is electron affinity always negative?
No. Most elements release energy when gaining an electron (negative electron affinity), but some, like noble gases and nitrogen, have positive (unfavorable/endothermic) electron affinities because the added electron would go into a less stable configuration.
How is electron affinity different from electronegativity?
Electron affinity is a measurable energy change for a single gaseous atom gaining one electron, while electronegativity is a relative, dimensionless index describing an atom's tendency to attract shared electrons within a chemical bond.