Question

How many unpaired electrons are in a boron atom?

Answer 1

In the ground state, only one. But for bonding, all three valence electrons are used, and the atom acquires a slightly excited state.

Explanation:

The configuration of boron is `1s^2 2s^2` `2p^1` which means only the `p` electron is unpaired.

However, when bonding in a compound, one of the `2s` electrons is promoted into the `2p` subshell, with the result that `sp^2` hybrid orbitals are created, and three bonding orbitals exist.