Question

What orbitals do valence electrons occupy?

Answer 1

See here for further explanation.

For NON-transition-metal elements (i.e. non-d-block, non-f-block), we have that the valence electrons occupy the orbitals where the quantum level `n` matches the current period on the periodic table.

(NOTE: I am not simply saying "valence orbitals for all elements" because sometimes valence orbitals are not occupied at all, like in americium, with electron configuration `[Rn] 5f^7 7s^2`, i.e. empty `6d` orbitals! By the way, americium has `7` valence electrons, as its highest oxidation state so far is `+7`.)

For example, the valence electrons occupy the `5s` and `5p` orbitals of antimony, while the valence electrons occupy the `3s` orbitals of magnesium.

EXCEPTIONS BELOW!

d-block metals

For many `d`-block metals (such as scandium or tungsten), the `(n-1)d` orbitals also hold some of the valence electrons, not just the `ns` and `np` orbitals.

f-block metals

For many `f`-block metals, the `(n-2)f` orbitals could also contain some of the valence electrons (such as for protactinium), not just the `ns` and `np` orbitals.

Sometimes the `(n-1)d` orbitals ALSO hold some of the valence electrons, but sometimes they don't (such as for berkelium), depending on the particular `f`-block metal.