Chemistry: why 3rd row nonmetals can have more than 8 valence electrons

Tutoring chemistry, ideas about valence electrons are important. The tutor mentions how some atoms have more than 8.

Valence electrons are those in the outer shell: electrons involved in bonding or reactions are valence electrons. An early rule chemistry students learn is that the outer shell has capacity for 2 electrons (first row of the Periodic Table) or else 8 (second row). Said model carries over into the third row for some explanations, but doesn’t work for some compounds: for instance, PCl₅.

Since we imagine that each bond comprises 2 electrons, in PCl₅, the P (phosphorus) must contain 10 valence electrons at least. How can it, when the outer shell is meant to hold at most 8?

The answer is that 3rd-row (aka 3rd-period) elements, of which phosphorus is one, have access to more electron capacity because of an extra subshell the third shell has but the second and first ones lack. The d subshell, introduced in shell 3, has capacity for 10 extra electrons, meaning that the third shell of electrons actually has capacity of 18 electrons. This capacity is not necessarily used. However, it enables compounds like PCl₅ and SF₆.

Source:

lavelle.chem.ucla.edu