Question

Why doesn't arsenic have any valence electrons in the third orbital?

Answer 1

This is because in arsenic, the outer shell is the fourth shell, and it contains the valence electrons.

Explanation:

I will assume your question refers to why the `3d` electrons of arsenic are not included as valence electrons, since they populate the shells of the atom after the `4s` electrons, which are valence electrons.

This is because the order in which subshells are filled is based on energy of the subshell rather than distance from the nucleus. If it were distance, then each shell would completely fill before any electrons populated subshells of a higher shell, because the shells contain those electrons having greatest probability at similar distance from the nucleus.

But, the situation is much more complex than that! Without going into the (lengthy) explanation of why this occurs, I will simply state that it is the presence of the non-valence electrons in a multi-electron atom that causes the energies of the orbitals with more complex shapes (such as `p` and especially `d` and `f`) to have higher energies that the `s` orbitals in the same shell.

This effect is enough to raise the energy of the `3d` subshell above that of `4s` (although not above `4p`).

So, the `3d` subshell fills after `4s`, even though it is not part of the fourth shell, and in particular is not at the same distance from the nucleus as either `4s` or `4p`, and is not part of the valence shell.

I hope that was what you wanted!