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title: "Experimental measurements of the carbonate ion, \\(\\text{CO}_3^{2-}\\), show that all three carbon-oxygen bonds are chemically equivalent with an identical bond length of \\(128\\text{ pm}\\), which is intermediate between a typical \\(\\text{C}-\\text{O}\\) single bond (\\(143\\text{ pm}\\)) and a \\(\\text{C}=\\text{O}\\) double bond (\\(120\\text{ pm}\\)). The three traditional Lewis resonance structures for \\(\\text{CO}_3^{2-}\\) are shown below.  Which of the following statements best reconciles the equivalent bond lengths with the molecular geometry and orbital hybridization of the central carbon atom?"
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url: "https://nerd-notes.com/ubq/123749/"
date_modified: "2026-09-28T12:04:54+00:00"
---

# Experimental measurements of the carbonate ion, \(\text{CO}_3^{2-}\), show that all three carbon-oxygen bonds are chemically equivalent with an identical bond length of \(128\text{ pm}\), which is intermediate between a typical \(\text{C}-\text{O}\) single bond (\(143\text{ pm}\)) and a \(\text{C}=\text{O}\) double bond (\(120\text{ pm}\)). The three traditional Lewis resonance structures for \(\text{CO}_3^{2-}\) are shown below.

Which of the following statements best reconciles the equivalent bond lengths with the molecular geometry and orbital hybridization of the central carbon atom?

Experimental measurements of the carbonate ion, \(\text{CO}_3^{2-}\), show that all three carbon-oxygen bonds are chemically equivalent with an identical bond length of \(128\text{ pm}\), which is intermediate between a typical \(\text{C}-\text{O}\) single bond (\(143\text{ pm}\)) and a \(\text{C}=\text{O}\) double bond (\(120\text{ pm}\)). The three traditional Lewis resonance structures for \(\text{CO}_3^{2-}\) are shown below.

Which of the following statements best reconciles the equivalent bond lengths with the molecular geometry and orbital hybridization of the central carbon atom?

![A horizontal arrangement of three Lewis resonance structures for \(\text{CO}_3^{2-}\) separated by two double-headed horizontal arrows. Each structure contains 24 valence electrons enclosed in square brackets with a \(2-\text{ superscript}\) at the top right. In structure 1, central \(\text{C}\) has no lone pairs; it forms a vertical double bond upward to an \(\text{O}\) atom with 2 lone pairs (one top, one right; no lone pairs on left or bottom of this \(\text{O}\)), a single bond down-left to an \(\text{O}\) atom with 3 lone pairs (top, left, bottom), and a single bond down-right to an \(\text{O}\) atom with 3 lone pairs (top, right, bottom). In structure 2, the double bond is directed down-left to the \(\text{O}\) with 2 lone pairs, while the top and down-right \(\text{O}\) atoms each have single bonds and 3 lone pairs. In structure 3, the double bond is directed down-right to the \(\text{O}\) with 2 lone pairs, while the top and down-left \(\text{O}\) atoms each have single bonds and 3 lone pairs. All structures show \(120^\circ\) bond angles. No other particles, labels, text, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790597094-YpwLFV.jpg)

- **A.** The \(\text{C}-\text{O}\) bonds have an average bond order of \(1\) because the ion rapidly oscillates between three discrete resonance structures containing localized single and double bonds.
- **B.** The \(\text{C}-\text{O}\) bonds have an identical bond order of \(\dfrac{4}{3}\) because the central \(\text{C}\) atom forms four equivalent \(sp^3\) hybrid orbitals that overlap with oxygen orbitals to distribute four \(\sigma\) bonds equally.
- **C.** The \(\text{C}-\text{O}\) bonds have an identical bond order of \(\dfrac{4}{3}\) because the central \(\text{C}\) atom is \(sp^2\) hybridized and forms three localized \(\sigma\) bonds, while its unhybridized \(2p\) orbital forms a delocalized \(\pi\) system across all three oxygen atoms.
- **D.** The \(\text{C}-\text{O}\) bonds have an identical bond order of \(\dfrac{4}{3}\) because the central \(\text{C}\) atom is \(sp\) hybridized and uses two unhybridized \(2p\) orbitals to form two localized \(\pi\) bonds that alternate between the three terminal oxygen atoms.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123749/*