Quantum Numbers

Question

Electron affinity is generally highest for which of the following?

RankGuru · Accepted Answer

1. **Definition**: Electron affinity is the energy change when an electron is added to a neutral gaseous atom. High affinity means energy is released.
2. **Expected Trend**: EA usually increases towards the top-right of the periodic table.
3. **Exception (Cl vs F)**: Fluorine is the most electronegative element, but its atom is extremely small. Adding an electron to the compact $2p$ shell causes significant **electron-electron repulsion** with the existing $7$ electrons.
4. **Mechanism**: Chlorine has a larger $3p$ shell, which can accommodate an extra electron with much less repulsion.
5. **Result**: Energy released is higher for **Chlorine** ($-349$ kJ/mol) than for Fluorine ($-328$ kJ/mol).

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