Why can one double bond exist in some resonance structures but two can't?

Question

I am doing some biology and have not done chemistry in ages. I have two concepts I am struggling with.

1. When drawing Lewis structures, am I right in assuming that the central atom will always require covalent bonds to fulfil its octet before the outer atom's octets are filled? For example when drawing lewis structures, the central carbon atom should be 'given' enough covalent bonds or lone electrons to have a full octet before lone electrons are allocated to the outer atoms in the structure.

2. I do not get why, for example, $\ce{HCO2-}$ can have resonant structures with one double bond (but not two double bonds) between the carbon and oxygens. (please see picture attached)

Answer 1

In your drawn example, the red-boxed structure requires the carbon atom to have five bonds, which, to channel my secondary school teacher, says "yuck, I don't like that".

This is a case of breaching the octet rule which has to with how the orbitals hybridise. Five full bonds on a carbon is not "allowed". However three full bonds and two partial bonds is common in both resonance and transition structures, and is in fact a much more accurate representation of the molecule you're drawing.

Don't forget too, that the location of the negative charge matters, and I can't see it in any of your drawings.

Answer 2

1) the rule of an octet must always be satisfied. That being said,a carbon can have a maximum of 4 covalent bonds. 2) Think of a resonance bond as the bonds being kind of 'smeared' between all the atoms, much as shown in the figure as the dotted line between two oxygens and carbon. In that way, the rule of an octet is never broken.