Problem 101 - Chemistry The Central Science

Question

In ozone, \(\mathrm{O}_{3}\) , the two oxygen atoms on the ends of the molecule are equivalent to one another. (a) What is the best choice of hybridization scheme for the atoms of ozone? (b) For one of the resonance forms of ozone, which of the orbitals are used to make bonds and which are used to hold nonbonding pairs of electrons? (c) Which of the orbitals can be used to delocalize the \(\pi\) electrons? (d) How many electrons are delocalized in the \(\pi\) system of ozone?

Step-by-Step Solution

Answer

In ozone (O3), the hybridization scheme is sp², which gives a trigonal planar molecular geometry. In one of the resonance forms, σ bonds are formed by the overlap of sp² hybrid orbitals, and π bond is formed by the overlap of unhybridized p orbitals. The unhybridized p orbitals on all three oxygen atoms are involved in the delocalization of π electrons, which is responsible for the resonance in ozone. There are 4 electrons delocalized in the π system of ozone.

Step 1: (a) Hybridization Scheme for Ozone

In ozone (O3), the central oxygen atom forms bonds with two equivalent terminal oxygen atoms. To determine the best hybridization scheme for ozone, consider the electron configuration of the oxygen atom. Oxygen has 6 valence electrons and its electron configuration is 1s²2s²2p⁴. When three orbitals are mixed (hybridize), they form three sp² hybrid orbitals. This would give a trigonal planar molecular geometry, and each of the sp² hybrid orbitals would be 120° apart from each other. This hybridization scheme allows the central atom to form two sigma (σ) bonds, making it the best choice for ozone.

Step 2: (b) Orbitals for Bonding and Nonbonding Pairs

In one of the resonance forms of ozone, the central oxygen forms a double bond with one of the terminal oxygen atoms and a single bond with the other. The two σ bonds between the central and terminal oxygen atoms are formed by the overlap of their sp² hybrid orbitals. The nonbonding pairs of electrons (lone pairs) are also located in the sp² hybrid orbitals of the terminal oxygen atoms. The π bond in the double bond is formed by the overlap of an unhybridized p orbital from the central oxygen atom and an unhybridized p orbital from the terminal oxygen atom.

Step 3: (c) Orbitals for Delocalizing π Electrons

The unhybridized p orbitals on all three oxygen atoms can be involved in the delocalization of π electrons. These p orbitals are perpendicular to the plane of the molecule, and they allow the π electrons to be spread over all three oxygen atoms. This delocalization of π electrons is responsible for the resonance in ozone.

Step 4: (d) Number of Electrons Delocalized in the π System

The π system in ozone is formed by the overlap of the unhybridized p orbitals from all three oxygen atoms. In one of the resonance forms, there is one π bond between the central and terminal oxygen atoms. Each π bond has 2 electrons. Besides, there are two nonbonding electrons (lone pairs) in the unhybridized p orbital of the other terminal oxygen atom. Therefore, there are a total of 2 + 2 = 4 electrons delocalized in the π system of ozone.

Key Concepts

Hybridization

Hybridization is a key concept in understanding the structure of ozone (\(\text{O}_3\)). Each oxygen atom has 6 valence electrons, leading to the formation of hybrid orbitals to facilitate bonding. In ozone, the central oxygen atom participates in bonding with two terminal oxygen atoms. To achieve this, it uses \(\text{sp}^2\) hybridization.
This type of hybridization involves mixing one \(s\) orbital and two \(p\) orbitals, creating three \(\text{sp}^2\) hybrid orbitals. These orbitals arrange themselves in a trigonal planar shape, about 120° apart.

• This configuration allows the central oxygen to form two sigma (\(\sigma\)) bonds, one with each terminal oxygen atom.
• Such a structure results in a stable molecule with lower energy.

Resonance

Resonance describes how some molecules can be represented by multiple structures. In the case of ozone, resonance is crucial in capturing the true nature of the electron structure. Ozone can be depicted by different resonance forms, showing the shifting of electrons between atoms.
In one resonance form, the central oxygen forms a double bond with one terminal oxygen and a single bond with the other.

• The \(\sigma\) bonds are formed using the \(\text{sp}^2\) hybrid orbitals.
• The lone pairs of electrons found on the terminal atoms reside in the hybrid orbitals as well.
• The \(\pi\) bond results from the overlap of unhybridized \(p\) orbitals, adding to the molecule’s stability.

Resonance allows the \(\pi\) electrons to move, distributing the charge more evenly across the molecule.

Delocalization

Delocalization is a process in which electrons are shared among more than two atoms, leading to extra stability. In ozone, the \(\pi\) electrons are delocalized across the molecule.
All three oxygen atoms have unhybridized \(p\) orbitals that contribute to this electron sharing.

• The delocalization arises because these \(p\) orbitals are parallel and overlap side-by-side.
• This configuration allows the \(\pi\) electrons to spread over the entire molecule, not just between two atoms.
• Such spreading contributes to the residence stability, allowing it to switch between structures.

The presence of multiple resonance structures is a clear indication of electron delocalization in ozone.

π System

The \(\pi\) system in ozone is an integral aspect of its bonding. It involves the overlap of \(p\) orbitals that are not hybridized with the others, creating a \(\pi\) bond.
This system includes the electrons that are not involved in \(\sigma\) bonding.

• In ozone, the \(\pi\) system includes a total of four delocalized electrons.
• Two of these electrons come from the \(\pi\) bond formed by the central and one terminal oxygen atom.
• The remaining two are a lone pair on the third unhybridized \(p\) orbital of the other terminal oxygen atom.

This system allows the electron density to be more distributed, contributing to the molecule's resonance capability. Understanding this system helps explain why ozone is stable and reactive at the same time.