C) The number of valence electrons.
Valence electrons are in the highest energy
Electron Configurations of Alkali Metals Using Noble Gas Notation
By using the noble gas notation, it is very easy to see that all of the alkali metals have the same number of valence electrons, which is
The diagram below shows the valence electrons for the main group elements (representative elements) in periods 2 and 3. You can see that the members of each group have the same number of valence electrons.