How are valence electrons used by an element?
Helps us determine a compound and its reactivity.
Valence electrons are the bonding electrons when looking at a compound's ability to form (I'm not talking about charge). An element's valence electron tells us about its ability to react and not react. More rules to this, but that's the gist of it.
E.g. Na = 1 valence electron and Cl has 7 valence electrons. They can form an ionic bond because they make up an octet atom.
Hope this helps :)