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This has to do with the orbitals. All three of N’s p orbitals already have one e- each in them. C has one open p orbital, so it has a higher electron affinity because it has space to put another e-. N does not have the space unless it pairs electrons, which would cause it to have higher energy due to electron-electron repulsion than if it didn’t have that extra e-. Therefore, it has a lower affinity.
Electron affinity is described as the propensity for an atom or ion to gain an electron. The way that you can tell that C has a higher electron affinity than N is by looking at the 2p sublevels of a C and N atom. C has one electron in two of its three orbitals. N has one electron in each of its orbitals. If N were to gain another electron, the electron would have to be added to an already occupied orbital. If O were to gain another electron, the electron would be added to the empty third orbital. The energy required to add an electron to N is higher than the energy required to add an electron to O because there are more interelectronic repulsions for N. Therefore, it's harder to add an electron to N than C. Therefore, C has a higher electron affinity than N.
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