Question

In the formate ion, \(\mathrm{HCO}_{2}^{-}\) , the carbon atom is the central atom with the other three atoms attached to it. (a) Draw a Lewis structure for the formate ion. (b) What hybridization is exhibited by the C atom? (c) Are there multiple equivalent resonance structures for the ion? (d) How many electrons are in the \(\pi\) system of the ion?

Short answer

The formate ion (\(\mathrm{HCO}_{2}^{-}\)) has the following properties: (a) Its Lewis structure has the carbon atom in the center with a single bond to hydrogen, a single bond to one oxygen, and a double bond to another oxygen, and the oxygen atoms have two lone pairs. (b) The carbon atom exhibits sp2 hybridization. (c) There are two equivalent resonance structures for the ion. (d) There are 2 electrons in the π system of the ion.

Step-by-step solution

Draw the Lewis structure for the formate ion

First, we need to find the total number of valence electrons in the formate ion. For hydrogen (H), there is 1 valence electron; carbon (C) has 4 valence electrons; oxygen (O) has 6 valence electrons. Additionally, the ion has a negative charge, which means it has an extra valence electron. So, the total number of valence electrons is: \(1 + 4 + 6 + 6 + 1 = 18\). Now, we will draw the Lewis structure: 1. Place the least electronegative atom (C) in the center. 2. Place the remaining atoms (H and two O) around the central atom. 3. Connect the atoms with single bonds (each using 2 electrons). 4. Distribute the remaining electrons (12 left) as lone pairs to complete the octets of the outer atoms. The Lewis structure for formate ion is: ``` O || C - O - H | O- ``` with two lone pairs on each oxygen atom.

Identify the hybridization of the carbon atom

To determine the hybridization of the carbon atom, we need to count the number of electron groups (bonding pairs and lone pairs) around it. In the formate ion, carbon forms three single bonds - one with hydrogen and two with oxygen atoms. There are no lone pairs on the carbon atom. So, there are three electron groups in total, which corresponds to an sp2 hybridization.

Determine the multiple equivalent resonance structures for the ion

Different resonance structures can be formed by moving the π electrons between the carbon and oxygen atoms. In the formate ion, there exists 2 equivalent resonance structures: ``` O || C - O - H | O- ``` and ``` O- | C - O - H || O ``` These two structures have the same bond arrangements but differ in the location of the π bond.

Calculate the number of electrons in the π system of the ion

In the formate ion, there is a double bond between the carbon and one of the oxygen atoms. A double bond consists of one σ bond and one π bond. The π bond is formed by the overlap of the \(p_z\) orbitals of carbon and oxygen, which contains 2 π electrons. Therefore, the formate ion has a total of 2 electrons in its π system.

Key concepts

Lewis Structure

The Lewis structure is a graphical representation of the valence electrons in molecules or ions, which illustrates how these electrons are arranged around the atoms and how the atoms are bonded to each other. In the case of the formate ion electron count is a necessary first step. With a total of 18 valence electrons to work with, carbon is placed in the center being the least electronegative atom, with the hydrogen and oxygen atoms arranged around it. Single bonds are drawn between the central carbon atom and the other atoms, using 2 electrons each, with the remaining electrons distributed as lone pairs on the oxygen atoms to complete their octets. It is essential to understand that in such structures, the atoms aim to fulfill the octet rule, gaining a stable electron configuration.
Visualizing this structure is crucial for understanding the molecular geometry and reactivity of the formate ion.

sp2 Hybridization

Hybridization in chemistry refers to the concept of mixing atomic orbitals to form new hybrid orbitals that can better explain the molecular geometry of atoms within a molecule. In the formate ion, we determine the hybridization of the carbon atom by counting the number of electron groups around it. For carbon, three single bonds mean there are three electron groups, which corresponds to an sp2 hybridization. This means that one s orbital and two p orbitals mix to form three sp2 hybrid orbitals, each occupied by one electron group.

In the context of formate ion chemistry, recognizing that the carbon atom is sp2 hybridized helps us predict that the atoms bonded to it will lie on a plane, with bond angles approximately 120 degrees apart, characterizing the molecule's overall structure.

Resonance Structures

Resonance structures are a set of two or more Lewis structures that collectively describe the electron distribution in a molecule where the atomic arrangement remains constant, but the electron positions may differ. The formate ion has two equivalent resonance structures, which can be depicted by shifting the position of the pi (π) bond between the carbon and the oxygen atoms while keeping the sigma (σ) bonds intact.

The existence of resonance structures has profound implications for the stability and reactivity of the formate ion. It indicates that the actual structure is a hybrid of the resonance forms, suggesting delocalization of electrons across the molecule. This delocalization contributes to the stability of the ion, making it an essential concept to grasp when studying molecular chemistry.

Pi (π) Electron System

The pi (π) electron system of a molecule includes the electrons found in π bonds, which are formed by the sideways overlap of p orbitals on adjacent atoms. In the formate ion, the double bond between the carbon and one of the oxygen atoms contains one π bond. A π bond is a region of electron density located above and below the plane of the nuclei of the bonding atoms, contributing 2 π electrons to the ion's electron system.

Understanding the π electron system is vital for learning about the reactivity and properties of molecules. π electrons are often the electrons involved in chemical reactions, particularly those that occur via mechanisms that involve electron shifts, such as in the case of resonance structures. The π system in the formate ion allows for the delocalization of electrons, which is a key factor in determining the ion's stability and its ability to engage in chemical reactions.