Earlier we talked about the octet rule, which says that elements like carbon and oxygen are most stable when they have eight valence electrons. For transition metals, as we’ve mentioned, the d-orbitals start to become important, so eight electrons just aren’t enough anymore. Since there are five d-orbitals, we need an additional ten electrons to fill the valence shell. And 8 + 10 = 18, hence the 18-electron rule for transition metals.