Problem 87 - 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

The best hybridization scheme for the atoms of ozone is sp2. In one of the resonance forms of ozone, sp2 orbitals are used to make bonds, while 2p orbitals hold nonbonding pairs of electrons. The 2p orbitals can be used to delocalize the π electrons, and there are 4 electrons delocalized in the π system of ozone.

Step 1: Determine the best hybridization scheme for the atoms of ozone

The best hybridization scheme can be found by determining the number of electron groups surrounding each atom. In ozone, there are 3 electron groups: a single bond, a double bond, and a lone pair of electrons. Since there are 3 electron groups, the best hybridization scheme for the atoms of ozone is sp2.

Step 2: Identify the orbitals used for bonding and nonbonding electrons

For one of the resonance forms of ozone, the central oxygen atom forms a single bond with one oxygen atom and a double bond with the other. In the sp2 hybridization scheme, each atom will use three sp2 hybrid orbitals and one 2p orbital. The single bond and double bond are formed by the overlap of sp2 orbitals from each oxygen atom. The remaining 2p orbitals are perpendicular to the plane formed by the hybrid orbitals and can hold nonbonding pairs of electrons.

Step 3: Determine which orbitals can be used to delocalize the π electrons

The 2p orbitals can be used to delocalize the π electrons. Since all oxygen atoms have one unhybridized 2p orbital, these orbitals can overlap and create π bonding interactions, allowing for delocalization of the π electrons.

Step 4: Calculate the number of electrons delocalized in the π system of ozone

In the π system of ozone, there are two π electrons associated with the double bond and one lone pair of electrons from the central oxygen atom that can be delocalized using the molecule's 2p orbitals. Therefore, there are \(2 + 2 = 4\) electrons delocalized in the π system of ozone.

Key Concepts

Resonance forms

In chemistry, resonance forms are different structures that represent the same molecule. They show how it is possible to arrange electrons in multiple ways. For ozone (\( \mathrm{O}_3 \)), this concept helps explain the distribution of electrons. Usually, one resonance form might have a double bond between the central oxygen and one outer oxygen atom. At the same time, the other form might show the double bond with the opposite outer oxygen atom. This doesn't mean the molecule flips between these forms. Instead, the actual molecule is a blend of the forms. This blend is often called the resonance hybrid. This hybrid shows a more accurate picture of where electrons are in the molecule. This is crucial for understanding its stability and reactivity. In ozone, the resonance forms explain why the oxygen-oxygen bonds are neither single nor double, but somewhere in between.

sp2 hybridization

Hybridization helps explain how atoms form bonds in molecules. In ozone, the central oxygen atom uses sp2 hybridization. Here’s a simple way to understand it: Atoms have orbitals where they hold electrons. In sp2 hybridization, the atom mixes one s orbital with two p orbitals. This mixture creates three new equivalent orbitals, called sp2 hybrid orbitals.
These three sp2 hybrid orbitals are arranged in a flat, triangular shape, at angles of 120° from each other. In the ozone molecule, the central oxygen forms one single and one double bond using these hybrid orbitals. This arrangement aligns with the three electron groups around it: one lone pair, one single bond, and one double bond. This not only explains the molecule's shape but also its reactive nature.

π electrons delocalization

Delocalization of π (pi) electrons is a key factor in the stability of certain molecules. Delocalization means these electrons are not fixed between any two atoms but are spread out across several atoms. In ozone, the π electrons contribute to the molecule's functional stability.
In ozone, pi electron delocalization occurs through the overlapping of 2p orbitals. Each oxygen atom in ozone has an unhybridized 2p orbital. These unhybridized orbitals overlap, forming pi bonds that allow electrons to flow freely along the molecule. This explains why ozone can exhibit resonance, as the electrons’ movement across all three oxygen atoms gives rise to its dynamic behavior. This delocalization does not only stabilize the molecule, but it results in a uniform bond length across the oxygen atoms, making ozone a resonance hybrid.

Chemical bonding

Chemical bonding refers to the force holding atoms together in a molecule. In ozone, different types of bonds are formed involving the oxygen atoms.
For the ozone molecule, bonding includes both sigma (σ) and pi (π) bonds. The sigma bonds are formed between the central and outer oxygen atoms. Each sigma bond in ozone is created by the overlap of sp2 hybrid orbitals. As mentioned earlier, the pi bonds result from the overlap of unhybridized 2p orbitals. This gives rise to electron delocalization. These different types of bonding add special characteristics to the molecule colorations, like its reactivity and absorption of UV light in the atmosphere, which are crucial for understanding its role in environmental chemistry.