Question

Giving suitable reasons, arrange halogens in the decreasing order of electron affinity.

Answer 1

1. Electron affinity generally increases with electronegativity, so fluorine is expected to have higher electron affinity than chlorine.
2. However, fluorine has a smaller atomic size, resulting in very high electron density around the atom.
3. This high electron density causes strong repulsion for the incoming electron, making it difficult for fluorine to accept an extra electron smoothly.
4. Due to this repulsion, less energy is released when fluorine gains an electron, resulting in a lower (less negative) electron affinity compared to chlorine.
5. Chlorine, being larger, has lower electron density and less repulsion, so it releases more energy on gaining an electron.

Hence, the electron affinity trend among halogens is F < Cl > Br > I (fluorine is unexpectedly lower than chlorine).