are the electrons located in the outermost shell of an atom. For example, oxygen has six and bromine has seven. Sodium has one valence electron and magnesium has two.
You can get the number of valence electrons from the . For example, carbon has this configuration:
##1s^(2)## ##2s^(2)## ##2p^(2)##
and it has four valence electrons. The outermost shell is shell 2 and it holds 4 electrons, two in the 2s and two in the 2p.
