Q.Fluorine has less negative electron gain affinity than chlorine. Why ?
F's 2p orbitals are so compact that electron-electron repulsion partly cancels the attraction felt by an incoming electron; Cl's larger 3p orbitals accommodate the new electron with far less repulsion, so Cl's electron gain is MORE exothermic.
Step 1. The size of the fluorine atom is smaller than that of the chlorine atom -- fluorine adds its incoming electron to the compact n = 2 shell, chlorine to the roomier n = 3 shell.
Step 2. In fluorine's relatively small 2p orbitals the electrons are already crowded, so there are strong inter-electronic repulsions; the incoming electron therefore does not experience much net attraction.
Step 3. In chlorine the added electron enters the larger 3p subshell where repulsion is much weaker, so more energy is released on electron gain.
Thus fluorine has less negative electron gain affinity than chlorine.
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