S, Cl, Se and Al are all able to expand their valence shell. H and F cannot.
expanded valence shells occur when an atom has empty d orbitals in their valence shell (period 3 or higher). Look it up in your chem book and it will help a lot. This is very important to know especially when drawing lewis structures.
P.S. it’s also good to know that Be, B, and Al commonly have fewer than an octet.Source(s): Just going to class and studying.