[deutdeut]

I know that the ethanoate ion is stabilized by resonance, there are four electrons delocalized over the -COO group, so, what is the type of bond exists between C and O, I think it's not a pure single bond as revealed in the resonance structure, but what's it?

[Neecze]

Bond order between C and O in acetate can be considered as equal 1.5.

[deutdeut]

But say, in carbonate ion, the bond order is 4/3, that means there are 8/3 electrons between each carbon and oxygen, but how can the number of electrons be a fraction? Is it just an average number?

[Bakegaku]

But say, in carbonate ion, the bond order is 4/3, that means there are 8/3 electrons between each carbon and oxygen, but how can the number of electrons be a fraction? Is it just an average number?

It's related to the uncertain nature of an electron.  Although we draw the resonance structures separately, they really exist simultaneously, making the bonds have strengths between the different states.

I'm bad with wording; if you need something clarified... 

[deutdeut]

But how come the number of electrons between the atoms is a fraction?

[lemonoman]

It's actually the probability that any given electron will be found in the bond - so yes, it's an average value.

Like for the COO^- group, the electrons will be in one CO bond OR the other...not 1/2 an electron in each (although, averaged out, each electron spends half its time in one bond, half its time in the other)

[deutdeut]

So that means, in ethanote ion, there are 4 delocalized pi-electrons. And a pair of them will spend half a time in one C-O bond and a half time in the othe C-O bond and that forms a half pi-bond?