# Why is the first electron affinity exothermic?

Dec 7, 2015

Sometimes it is; especially for elements to the right of the Periodic Table.

#### Explanation:

Electron affinity is the enthalpy associated with the reaction:

$M \left(g\right) + {e}^{-} \rightarrow {M}^{-} \left(g\right)$

We would expect this enthalpy to increase across a Period from left to right (because of the increased nuclear charge), and to decrease down a Group, as the nuclear charge is insulated by intervening electron shells.

Metals tend to have low electron affinity, as these are reducing species.