Final answer:
A carbon vacancy has 3 unpaired valence electrons because one of the 2s electrons is promoted to the empty 2p orbital.
Step-by-step explanation:
Looking at the 2s²2p² valence electron configuration of carbon, we can understand why a carbon vacancy has 3 unpaired valence electrons. Carbon typically forms compounds with four covalent bonds, even though we might expect it to use its two unpaired 2p electrons to form compounds with only two covalent bonds. To account for this, one of the 2s electrons is promoted to the empty 2p orbital, resulting in 3 unpaired valence electrons.