Electron affinities for the simplest
ten elements are shown across the top of this page.
Electron affinities are harder to measure than ionization energies,
and these values are only approximate.
Neon has no electron affinity,
because it already has its full complement of eight outer electrons.
Fluorine has the largest electron affinity, 83.5 kilocalories per
mole; thus the F ion
is formed easily. Oxygen has a smaller electron affinity because
the net charge pulling on the outer electrons is +6 rather than
+7. The affinities of other neutral atoms of the second-shell elements
for extra electrons are weaker still.