what is variable covalency?

Variable covalency refers the tendency of certain atoms to show different covalencies as a result of excitation of the atom. In the atoms that exhibit variable covalency the number of unpaired electrons increases as a result of excitation of the atom that causes it to have variable covalency. 

For example chlorine by absorbing a certain amount of energy moves in to the excited state with 3, 5 and 7 unpaired electrons and thus show a variable covalency of 3, 5 and 7.