Electron affinity is the energy change that happens when a neutral atom in the gaseous state accepts an extra electron and becomes negatively charged.
- If energy is released, the atom has a high electron affinity (it strongly wants the extra electron).
- If little or no energy is released, or energy is required, the atom has a low or even positive electron affinity (it does not want the extra electron).
Periodic Trends:
- Across a period (left to right): Electron affinity increases, because atoms become smaller and hold electrons more tightly.
- Down a group (top to bottom): Electron affinity decreases, because atoms get larger and the attraction to an extra electron weakens.
- Halogens have the highest electron affinity since they need only one electron to reach stability.
- Noble gases have very low or even positive electron affinity since they are already stable.