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 \(\mathrm{C}\) atom? (c) Are there multiple equivalent resonance structures for the ion? (d) Which of the atoms in the ion have \(p_{\pi}\) orbitals? (e) How many electrons are in the \(\pi\) system of the ion?

Short answer

The Lewis structure for the formate ion, HCO2-, has a central carbon atom bonded to one hydrogen atom and two oxygen atoms, with one of the oxygen atoms forming a double bond: O || H - C - O - The central carbon atom exhibits sp2 hybridization, and there are multiple resonance structures for the ion. The carbon atom and the two oxygen atoms have pπ orbitals, and there are 2 electrons in the π system of the ion.

Step-by-step solution

Draw the Lewis structure of the formate ion

To draw the Lewis structure, start by counting the total number of valence electrons present in the ion. For the formate ion, HCO2-, we have: - 1 Hydrogen atom: 1 valence electron - 1 Carbon atom: 4 valence electrons - 2 Oxygen atoms: 6 valence electrons each (total of 12) - 1 extra electron from the negative charge This gives us a total of 18 valence electrons to distribute. Place the Carbon atom at the center and connect it to one Hydrogen atom and two Oxygen atoms. Distribute the remaining electrons among the atoms, starting with the more electronegative Oxygen atoms, to complete their octets. The Lewis structure for the formate ion will be: O || H - C - O -

Determine the hybridization of the central carbon atom

The hybridization of an atom is determined by the number of electron domains (regions of electron density) surrounding it. In the case of the central Carbon atom in the formate ion, we have three electron domains: - one single bond to Hydrogen - one single bond to Oxygen - one double bond to Oxygen (we can consider this as one electron domain) Since there are three electron domains, the hybridization for the Carbon atom is sp2.

Check for resonance structures

Resonance structures can occur when there is the possibility to distribute the π electrons differently without changing the placement of the atoms. In the formate ion, we have a double bond between the Carbon and one of the Oxygen atoms. We can draw an equivalent structure by moving the double bond to the other Oxygen atom: O || H - C - O - and O || H - C - O - Since we can draw more than one equivalent Lewis structure, there are multiple resonance structures for the formate ion.

Identify atoms with pπ orbitals

Atoms involved in π systems, like double bonds or resonance, have pπ orbitals. In the formate ion, the central Carbon atom and the two Oxygen atoms are involved in the double bond and resonance. Therefore, these three atoms have pπ orbitals.

Count the number of electrons in the π system

The π system in the formate ion consists of a double bond between the Carbon atom and one of the Oxygen atoms (it can be on either side due to resonance). In a double bond, there are 2 π electrons. So, there are 2 electrons in the π system of the formate ion.

Key concepts

Lewis Structure

Understanding the arrangement of atoms in a molecule begins with the Lewis structure, which represents atoms and their valence electrons through symbols and dots. To draw the Lewis structure of the formate ion, \( \mathrm{HCO_2}^- \), manage the total count of 18 valence electrons — one from hydrogen, four from carbon, six each from the two oxygens, and an extra one due to the negative charge. Place carbon at the center, connecting to a hydrogen and two oxygens. Electrons are first paired around oxygen atoms to complete their octets, which are preferred due to their high electronegativity. The resulting structure shows a single bond between carbon and hydrogen, and a double bond between carbon and one oxygen, with the remaining oxygen single-bonded to carbon, carrying a negative charge.

Correct representation of the Lewis structure ensures clear visualization and is crucial for exploring further molecular properties, including hybridization, resonance, and pi systems.

Hybridization

Hybridization explains the mixing of atomic orbitals to form new hybrid orbitals, which influences molecular geometry and bonding properties. In the formate ion, the carbon atom's hybridization is determined by the number of 'electron domains' or regions of electron density it possesses. With one single bond to hydrogen, another to oxygen, and a double bond to the second oxygen, carbon demonstrates three domains and thus an sp2 hybridization. This implies the formation of three sp2 hybrid orbitals that lie in a plane with 120-degree angles between them, accommodating the sigma bonds, and one unhybridized p orbital that participates in the pi bond formation.

Resonance Structures

Resonance structures depict alternate ways of arranging electrons in molecules with conjugated pi systems, offering a more complete picture of the electronic distribution. The formate ion demonstrates resonance as the pi electrons of the double bond can be located between carbon and either oxygen atom without altering the atoms' positions. Essential to understand, neither individual resonance form is actually accurate on its own; it's the concept of resonance stabilization—the blending of these forms—that accurately reflects the ion's true structure.

p\(\pi\) Orbitals

P\(\pi\) orbitals are involved in pi bond formation and are present in atoms that have regions of overlapping p orbitals. In the case of the formate ion, both oxygen atoms and the central carbon atom contain p\(\pi\) orbitals due to their involvement in the pi bond system. These orbitals extend above and below the plane of the atoms creating the pi bond through lateral overlap, which is essential for the resonance stability of the ion.

\(\pi\) Electron System

The \(\pi\) electron system includes the delocalized electrons in pi bonds or lone pairs that can contribute to resonance. In the formate ion, there are 2 \(\pi\) electrons found within the double bond shared between carbon and oxygen, forming part of the ion's resonating structure. Despite resinous depiction suggesting the double bond could be with either oxygen, there are always only 2 \(\pi\) electrons contributing to the system. Understanding the electron count in the \(\pi\) system is fundamental in predicting the molecule's reactivity and the distribution of electron density.