Why do octets expand?

[Michaela Capps 1l]

I have memorized that there can be octet expansions for elements in the 3rd period and beyond but I don't think I really understand why that is? I usually remember things better when I understand everything versus just memorizing so can someone please explain? Thanks!

[Mitch Mologne 1A]

They are able to expand their valence shells by adding electrons to the d orbital!

[Adriana Rangel 1A]

Nonmetal atoms in period 3 or later can expanded their valence shells because they have empty d-orbitals where they can store electrons.
For example, Sulfur e-configuration is: [Ne] 3s2 3p4. However, it has a 3d orbital available for bonding

[Jessica Jones 2B]

All atoms in the 3p and past that like 4p,5p,6p, etc all have access to the d orbitals and can form more bonds than just their expected amount.

[Harrison Wang 1H]

According to the principal quantum number (n) of period 3, the angular momentum number (l) can be 2, 1, or 0. Therefore, elements in the third period have empty d shells even though they come before elements with electrons in them already on the periodic table