Hello Dennis Chamberlin! 

Na- and K- do not exist, this is due to metals being very reactive elements, so it bonds with other non-metals to complete their octet state.

If the value of the electron affinity is negative, meaning energy is required to attach an electron. Hence the negative values are seen on surfaces and are seen for the nitrogen atoms; furthermore, they capture secondary electrons. 

Electron affinities have negative numbers because energy is released - elements of the halogen group 17 gain electrons easily, due to their large negative electron affinities; more energy is released in the formation of a halide ion than for the anions in other group elements. The more negative the electron affinity is the higher an atom's affinity in electrons.

Upper numbers in the halogen group will have the most negative electron affinity, as the electron affinity group decreases, the non-metals gain electrons, the energy change is always negative because they give the energy to form an anion, resulting in less energy released to form anions. 

- Hope this was helpful!

~Galaxy.

Hey, thanks for responding. Your explanation makes sense, and fits with what I had believed. However, my periodic table (ptable.com) lists positive numbers for both Cl and Na electron affinity. Perhaps they are expressing the energy given off when an electron attaches to make an ion. What puzzles me is that both atoms list positive numbers, and therefore both appear to yield energy when a - ion is formed, even though the Cl energy is much larger (349 kJ/mol) than Na (59 kJ mol). The Cl figure I understand, but the claim that the Na- formation yields any energy at all seems to imply that an Na- ion  could potentially exist.