Consider the formate ion, \(\mathrm{HCO}_{2}^{-},\) which is the anion formed when formic acid loses an \(\mathrm{H}^{+}\) ion. The \(\mathrm{H}\) and the two \(\mathrm{O}\) atoms are bonded to the central \(\mathrm{C}\) atom. (a) Write one or more appropriate Lewis structures for this ion. (b) Are resonance structures needed to describe the structure? (c) What would you predict for the \(\mathrm{C}-\mathrm{O}\) bond lengths in the formate ion relative to those in \(\mathrm{CO}_{2} ?\)

First, count the total number of valence electrons available in the ion. For H: 1 valence electron For C: 4 valence electrons For O: 2 * 6 valence electrons (since there are two oxygen atoms) For the negative charge: 1 extra electron Total valence electrons: 1 + 4 + 12 + 1 = 18 Now, draw the Lewis structure using these electrons by connecting atoms with single, double, or triple bonds and adding non-bonding electron pairs. 1. Place the central atom (Carbon) in the middle and attach the other atoms (Hydrogen and two oxygens) to it using single bonds. 2. Calculate the remaining electrons after forming single bonds: 18 - (1 * 3) = 15 3. Complete the octet of the other atoms by adding remaining electrons to them. In this case, each oxygen will receive three lone pairs and hydrogen will retain its original electron. Now, 3 electrons are left. 4. Add the remaining electrons to the central atom (Carbon), forming a double bond with one of the oxygen atoms. So, the Lewis structure for HCO2- is: O=C-O | H

Resonance structures are required when there is more than one possible way to distribute electrons in a molecule. In the formate ion, we can see that the double bond between carbon and oxygen can be formed with either of the two oxygen atoms. Therefore, resonance structures are necessary to describe the structure for the formate ion. The resonance structures are: Structure 1: O=C-O Structure 2: O-C=O | | H H

In the formate ion, the C-O bonds are a combination of single and double bonds due to resonance, meaning that both bonds are somewhat shorter than a single C-O bond but longer than a double C-O bond. In CO2, the carbon atom forms double bonds with both the oxygen atoms. Therefore, we can predict that the C-O bond length in the formate ion will be longer than in CO2 as the average bond length is between a single and double bond.