Electron Affinity Calculator
Look up electron affinity values for common elements and calculate the energy released (or absorbed) when a neutral atom gains an electron. Results in both kJ/mol and electron volts.
Electron affinity (EA) is the energy change when a neutral atom in the gas phase gains one electron to form a negative ion. For most nonmetals, this process releases energy (positive EA in the convention used here).
The Reaction
X(g) + e- -> X-(g) + energy
Sign Convention
This calculator uses the thermodynamic convention where a positive value means energy is released (exothermic). Some textbooks flip the sign -- always check which convention is in use.
Periodic Trends
Electron affinity generally increases across a period (left to right) and decreases down a group, with notable exceptions:
- Chlorine has the highest EA (349 kJ/mol), not fluorine, because fluorine's tiny size causes strong electron-electron repulsion
- Nitrogen and phosphorus have near-zero or negative EA because their half-filled p orbitals are already quite stable
- Noble gases have very negative (unfavorable) EA because adding an electron means starting a new shell
Common Values
| Element | EA (kJ/mol) |
|---|---|
| Cl | 349.0 |
| F | 328.0 |
| Br | 324.6 |
| I | 295.2 |
| S | 200.4 |
| O | 141.0 |
| C | 121.8 |
| H | 72.8 |