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Consider the following image which shows the resonance structures of $\ce{CO_3^{2-}}$ ion. What is the hybridization of $\ce{O}$? How do we proceed in such cases? In one resonance structure, the top oxygen has $\ce{sp^3}$ hybridization and in others, it has $\ce{sp^2}$ hybridization. In the hybrid structure, shown in the second image, I don't even know how to proceed.

resonance of carbonate

resonance hybrid of carbonate

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    $\begingroup$ All the oxygens should be considered to be sp² hybridised, with its resonance-contibuting p orbitals being unhybridised and acting solely as pi-bonds. $\endgroup$ Mar 2 at 10:09
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    $\begingroup$ @KanghunKim Not really, sp is an accurate description for, like, all terminal atoms. $\endgroup$
    – Mithoron
    Mar 2 at 14:57
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    $\begingroup$ Does this answer your question? What is the hybridization of terminal fluorine atoms in molecules like boron trifluoride? $\endgroup$
    – Mithoron
    Mar 2 at 14:58
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    $\begingroup$ Boron trifluoride may seem different, but is isoelectronic with carbonate anion. $\endgroup$
    – Mithoron
    Mar 2 at 15:00

1 Answer 1

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The only important thing to consider for the description in terms of hybridisation is the molecular structure. It is therefore irrelevant if you are using a resonance structure, all resonance structures, or the resonance hybrid, or even some kind of completely different diagram. The structure for all of these is the same.

Therefore, to determine a reasonable hybridisation scheme, you should first look at local coordination of the atom. Please keep in mind that the $x$ in $\mathrm{sp}^x$ doesn't need to be an integer value.
For this, please look up Bent's rule and Coulson's theorem. You may start with my answer here: How is Bent's rule consistent with LCAO MO theory?
Please also keep then in mind that only orbitals hybridise and any combination of hybrid orbitals my be a valid description.*

If you keep this in mind, the following definition makes quite a bit more sense, hybridization in the Gold Book (DOI: 10.1351/goldbook.H02874)

Linear combination of atomic orbitals on an atom. Hybrid orbitals are often used in organic chemistry to describe the bonding molecules containing tetrahedral (sp³), trigonal (sp²) and digonal (sp) atoms.

Considering this, here are some zero order approximations, which my come in handy:

Coordination Examples Approximate Hybrid Orbital
linear carbon in acetylene or carbon dioxide, or any terminal atom** sp
trigonal carbon in $\ce{CO3^2-}$, boron in $\ce{BH3}$, carbon in acetone sp²
tetrahedral carbon in methane, sulfur in $\ce{SO4^2-}$ sp³
bent oxygen in water sp² - sp³
trigonal pyramidal nitrogen in ammonia close to sp³
trigonal bipyramidal phosphorus in $\ce{PCl5}$ model breaks down
octahedral sulfur in $\ce{SF6}$ model breaks down

After writing all this, I've realised that I have commented on this before: What is the hybridization of the carbonyl oxygen in a carboxylic acid?


Footnotes:

* The terminology we use for hybridisation actually is just an abbreviation: $$\mathrm{sp}^x = \mathrm{s}^\frac{1}{x+1}\mathrm{p}^\frac{x}{x+1}$$ In theory $x$ can have any value; since it is just a unitary transformation, the representation does not change, hence \begin{align} 1\times\mathrm{s}, 3\times\mathrm{p} &\leadsto 4\times\mathrm{sp}^3 \\ &\leadsto 3\times\mathrm{sp}^2, 1\times\mathrm{p} \\ &\leadsto 2\times\mathrm{sp}, 2\times\mathrm{p} \\ &\leadsto 2\times\mathrm{sp}^3, 1\times\mathrm{sp}, 1\times\mathrm{p} \\ &\leadsto \text{etc. pp.}\\ &\leadsto 2\times\mathrm{sp}^4, 1\times\mathrm{p}, 1\times\mathrm{sp}^{(2/3)} \end{align}

There are virtually infinite possibilities of combination.

Or see more: Are the lone pairs in water equivalent?

** I have written about this a few times already. These should provide also data:

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  • $\begingroup$ Thank you for the answer, Martin. (❁´◡`❁) $\endgroup$
    – Osmium
    Mar 4 at 14:33

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