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In the formate ion (HCOO-), the two carbon-oxygen bonds are observed to have identical lengths. What is the reason for this?

1. The carbon atom's orbitals are hybridised
2. The C=O bond is weaker than the C-O bond
3. The HCOO- ion has two equivalent resonating structures
4. The anion is formed by removing a proton from the acid molecule

Correct answer: The HCOO- ion has two equivalent resonating structures

Solution

In the formate ion, two equivalent resonance structures exist in which the C=O double bond and the C-O(-) single bond interchange between the two oxygen atoms. The actual structure is a resonance hybrid, so each C-O bond has a bond order of 1.5 and both bonds are identical in length. Hybridisation alone does not equalize the two bonds, and the other options do not explain the equality of bond lengths.

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