Account for the large decrease in electron affinity between `Li` and `Be` despite the increase in nuclear charge.

Explanation

The electron configurations of `Li` and `Be` are `[He]2s^(1)` and `[He]2s^(2)`, respectively. The additional electron enters the `2s` orbital of `Li` but the `2p` orbit of `Be` and hence is much less tightly bound. In fact, the nuclear charge is so well shielded in `Be` that electron gain is endothermic.