AsO43- (Arsenate Ion)

[ashleyhchung]

For Arsenate Ion's Lewis structure, why is there only one double bond to the oxygen and not for all 4? I thought expanded octet meant you could have many more valence electrons.

[Brandon Noimany 2C]

Hi,

Arsenic does have an expanded octet because it is a bigger molecule, although its outermost shell only has FIVE valence e-, therefore it can only hold a maximum of five bonds (instead of the standard four). Thus, in the arsenate ion, it has only one double bond with an oxygen to make 5 total shared e- pairs (5 bonds, effectively; 3 single bonds and one double bond).

Hope this helps!

[Anton Baranov 2F]

Using one double bond minimizes the formal charge and keeps the structure stable.