Electron affinity, in general, becomes more negative as we move left to right
across the periodic table (makes sense since
Z_{eff}
is increasing!). Several exceptions
occur in this trend as well due to especially stable electron configurations
having filled subshells or shells. The group 2 and group 8 elements have
positive electron affinities since adding electrons to a new subshell (group
2) or shell (group 8) will be unfavorable. There is no clear trend down a
group.