Question

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 \(\mathrm{CO}_{2} ?\)

Short answer

The Lewis structure for the formate ion, HCO2-, is: O=C-O | H Resonance structures are needed, as there are two possible ways to distribute electrons: Structure 1: O=C-O Structure 2: O-C=O | | H H The C-O bond length in the formate ion is longer than in CO2 since the C-O bond is a combination of single and double bonds due to resonance, while in CO2, the carbon forms double bonds with the oxygen atoms.

Step-by-step solution

1. Draw the Lewis structure of the formate ion, HCO2-.

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

2. Determine if resonance structures are needed.

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

3. Predict the C-O bond lengths relative to CO2.

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.

Key concepts

Resonance Structures

Resonance structures are a crucial concept in understanding the behavior of electrons in molecules. They allow us to describe the distribution of electrons across different possible structures of a molecule.
In the case of the formate ion \( \text{HCO}_2^- \), there are two resonance structures. These arise because the double bond can be between carbon and either oxygen atom. Each resonance structure shows a different placement of the pi electrons, illustrated as either \( \text{O}=\text{C}-\text{O} \) or \( \text{O}-\text{C}=\text{O} \).
Even though these structures seem distinct, the true structure of the formate ion is a hybrid of these resonance forms.
* Resonance provides a better understanding of the electron distribution.* Actual bond lengths in such structures are between those of a typical single and double bond.
Resonance structures help in stabilizing molecules and ions by distributing electron density more evenly.

Formate Ion

The formate ion \( \text{HCO}_2^- \) is a classic example of an anion that results from the deprotonation of formic acid. When formic acid loses an \( \text{H}^+ \) ion, the resulting formate ion is formed. In this ion, carbon is the central atom, with hydrogen and two oxygens bonded to it.
Key points about formate ion:

• Carbon forms one double bond with oxygen and shares single bonds with the other oxygen and hydrogen atom.
• It is an example of a carboxylate, where deprotonation of the acidic hydrogen occurs.

Understanding the structure of the formate ion can aid in comprehending how such ions stabilize charge distribution and contribute to the acid-base balance.

Valence Electrons

Valence electrons are the electrons that exist in the outermost shell of an atom and are crucial for determining an atom's chemical properties and bonding behavior.
For the formate ion \( \text{HCO}_2^- \), the number of valence electrons is key to drawing its Lewis structure accurately.
Here's how we determine the valence electrons:

• Hydrogen contributes 1 valence electron.
• Carbon contributes 4 valence electrons because it is in group 14 of the periodic table.
• Each oxygen atom contributes 6 valence electrons, giving us \( 2 \times 6 = 12 \) electrons from oxygen.
• An additional electron comes from the negative charge, providing a total of 18 valence electrons.

These valence electrons are distributed across the atoms to ensure "octet" satisfaction for each atom, except for hydrogen, which only accommodates 2 electrons. Understanding valence electrons is essential for predicting molecular structure and reactivity.