Does this mean that all the extra electrons skip the 4s subshell and go into the 3d subshell. Like say, sulfur in oxidation state +6 (which I think means forming 6 covalent bonds).
Not quite. Elements in period 3, like sulfur, have empty orbitals (3d, 4s). These orbitals can participate in bonding and allow these atoms to expand their octets and thus their valencies. The "normal" valence for a Group 16 (6A) element is 2 for example, meaning they will form two bonds in most compounds. However, sulfur can form up to 6 bonds since it can use the empty orbitals.
Now the oxidation state is due to the valence electrons as well. Sulfur has six valence electrons. The other electrons are already energetically stable. Sulfur having a maximum oxidation state of +6 is due to it having 6 valence electrons. The oxidation state can span from +6 to -2.
Does the electronic configuration become
1s2 2s2 2p6 3s2 3p6 4s2 3d2
1s2 2s2 2p6 3s2 3p6 3d10
or I am just getting the wrong idea?
It is difficult to get an idea of electron configuration with atoms that can expand their octets, especially in Lewis bonding theory. 4s is not looked at in this bonding theory since using it "does not give the right answers". If you think about molecules like SF6
, using the 3d orbitals to form bonds gives you the correct bond angles. Including another s orbital does not.
Hope this helps some.