Octet exceptions

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Hannah Padilla 1H
Posts: 29
Joined: Fri Sep 28, 2018 12:19 am

Octet exceptions

Postby Hannah Padilla 1H » Wed Oct 31, 2018 1:17 pm

I know that certain elements can have more than an octet, but in which form are they more stable.

Ashley Zhu 1A
Posts: 69
Joined: Fri Sep 28, 2018 12:16 am

Re: Octet exceptions

Postby Ashley Zhu 1A » Wed Oct 31, 2018 2:47 pm

When the elements that can have an expanded octet form compounds, they are most stable at their lowest energy state. You can determine this by calculating formal charges on each of the elements in the compound. Usually, the Lewis structures with the most 0's/formal charges closest to 0 are the most stable. For example, HPO4^2- can have P in the middle forming 4 bonds to satisfy the octet rule. However, when we use a double bond for one of the O's (3 single bonds and 1 double bond instead of 4 single bonds with the P atom), we find that the formal charges on P and one of the O's becomes 0 instead of +/-1. Thus, the second structure where P has an expanded octet is more stable and has lower energy.

Clarissa Cabil 1I
Posts: 66
Joined: Fri Sep 28, 2018 12:19 am

Re: Octet exceptions

Postby Clarissa Cabil 1I » Wed Oct 31, 2018 5:35 pm

Molecules are the most stable when they have a formal charge of 0. After drawing the lewis structure, to check if the element is at it's most stable state, calculate its formal charge. To calculate formal charges, use the equation: FC = V - ( L + S/2 ).

FC : formal charge
V : valence electrons
L : # of lone pair electrons
S : # of shared electrons

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