Question

Which molecules can exceed the octet rule? And why do they do this?

Answer 1

Elements that have three or more occupied principal energy levels (shells)can accommodate more than 8 electrons in their valence shells when bonding

Explanation:

Principal energy levels have sublevels. Because the first shell has only one sublevel which contains only one orbital (1s), the first shell fills with only 2 electrons.
The second shell has two sublevels, s (1 orbital) and p (3 orbitals). since there are four total orbitals, 8 electrons can be accommodated.
The third shell has three sublevels: s (1orbital), p(3 orbitals) and d(5 orbitals), totaling 9 orbitals which can hold up to 18 electrons.
When electrons are involved in bonding, the orbitals change form atomic orbitals to molecular orbitals, but they still fill from lowest to highest energy levels.