Question

Why does the octet rule have exceptions?

Answer 1

Exceptions to the octet rule come from the presence of the orbital d

Explanation:

Elements of the third raw in the periodic table and beyond can carry more than 8 electrons on their outermost shell because of the additional room (space) on their orbital d.

Only elements of the first and second raws of the periodic table can carry a maximum of 8 electrons on their outermost shell because they only have s and p orbitals available to host electrons (with the exception of hydrogen and helium they obey the duet rule).

For example, the electronic configuration of oxygen (second raw element) O: 1`s^2`2`s^2`2`p^4`
Oxygen can host 2 more electrons on its outer shell 2 and therefore 2 + 4 + 2 = 8 electrons.

However, sulfur (third raw element) S: 1`s^2`2`s^2`2`p^6`3`s^2`3`p^4`3`d^0`
Sulfur can host up to 12 more electrons on its outer shell 3, two electrons on p and 10 electrons on d and therefore 2 + 4 + 2 + 10 = 18 electrons.

This is why oxygen has an oxidation number of -2 in general with the exception of `H_2O_2` where the oxidation number is -1; however, sulfur has many oxidation numbers such as -2 in `S^(2-)`, +6 in `SO_4^(2-)` and +4 in `S_2O_3^(2-)`.

If we look at the Lewis structure of `SO_4^(2-)` we can count 12 electrons around sulfur: