Atoms with a full subshells (or in some instances half-full subshells) tend to have a lower electron affinity. Essentially, electron affinity correlates with electron configuration. A notable exception to the trend with regards to it increasing up a group is Flourine (period 2) which has a lower electron affinity than chlorine (period 3) due to a higher magnitude of electron repulsions in the smaller 2p orbital opposed to chlorine's 3p orbital. Within a period, Carbon has a lower electron affinity than nitrogen, because nitrogen already has half-full p-subshell (which is more stable). As the name suggests, electron affinity is the ability of an atom to accept an electron. If electron affinity changes then it will also change the efficiency because the change in affinity leads to the change in the conduction band and valence band offset of the p-n junction. There are exceptions, however, such as group 2A (has a lower electron affinity than 1A) because the addition causes the previously unoccupied p-subshell to gain an electron (which increases instability). There is a tendency for electron affinity to increase as you move left to right across a period. Nobles gases have an electron affinity of approximately zero. Electron affinity tends to increase (as in more energy is released when an electron is added to an atom) as you move up a group on the periodic table. It is equal approximately to the negative energy of the virtual orbital on which the electron lands (if attaching an electron means energy lowering). A more negative electron affinity corresponds with greater attraction for an electron. The electron affinity is the difference of the energies of the system without an electron and that of the anion.
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