Okay, I'm going with pairing energy. Thank you for that direction. Another question about the same matter.
Nitrogen (2s2 2p3) has all p orbitals half-filled and so there is electron-electron repulsion making it less likely to accept another electron.
Carbon ((2s2 2p2) has an unfilled orbital so is apparently more likely to accept an electron than Nitrogen.
Why is Oxygen (2s2 2p4) more likely to accept an electron than Nitrogen? This is essentially what is being said in the question asked, that minus one C and O ions have been observed but not N (untrue but no choice on that matter.)
Much appreciation,
Sheryl