*Fluorine's electron affinity is smaller than chlorine's because of the higher electron - electron repulsions in the smaller 2p orbital compared to the larger 3p orbital of chlorine.

(More negative energy = greater affinity) Element Most notably, noble gases have a positive electron affinity, meaning it actually requires input of energy to add another electron. For some elements, however, this is not the case.

Cl (g) + e - Cl -(g) For most atoms, this is a negative quantity - or energy is released. Obviously, the halogens, which are one electron away from a noble gas electron configuration, have high affinities for electrons: While ionization energy is the energy change incurred from losing an electron, electron affinity is the energy change incurred from gaining an electron. Since a half-filled 'p' subshell is more stable, carbon has a greater affinity for an electron than nitrogen. This occurs because of the same subshell rule that governs ionization energies. The change is small and there are many exceptions.Įlectron affinity decreases or increases across a period depending on electronic configuration. (An unbound electron has an energy of zero.)Īs with ionization energy, there are two rules that govern the periodic trends of electron affinities:Įlectron affinity becomes less negative down a group.Īs the principal quantum number increases, the size of the orbital increases and the affinity for the electron is less. A more negative electron affinity corresponds to a greater attraction for an electron. This is because energy is usually released in this process, as opposed to ionization energy, which requires energy.

Notice the sign on the energy is negative.

Electron affinity