Why doesn't arsenic have any valence electrons in the third orbital?
This is because in arsenic, the outer shell is the fourth shell, and it contains the valence electrons.
I will assume your question refers to why the
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
This effect is enough to raise the energy of the
I hope that was what you wanted!