Question

Some of the important pollutants in the atmosphere are ozone \(\left(\mathrm{O}_{3}\right)\), sulfur dioxide, and sulfur trioxide. Write Lewis structures for these three molecules. Show all resonance structures where applicable.

Short answer

The Lewis structures for the three atmospheric pollutants are as follows: 1. Ozone (O3): The central oxygen atom forms a single bond with one outer oxygen atom and a double bond with the other outer oxygen atom, resulting in resonance structures. 2. Sulfur dioxide (SO2): The central sulfur atom forms a double bond with one oxygen atom and a single bond with the other oxygen atom, resulting in resonance structures. 3. Sulfur trioxide (SO3): The central sulfur atom forms double bonds with all three oxygen atoms, resulting in resonance structures as the double bonds can rotate among the oxygen atoms.

Step-by-step solution

Determine the total number of valence electrons in the molecule

Before drawing the Lewis structure for each molecule, we need to determine the total number of valence electrons. For each of these molecules, we only have two types of atoms: oxygen (O) and sulfur (S). Oxygen has 6 valence electrons, and sulfur has 6 valence electrons as well.

Draw the Lewis structure for ozone (O3)

a) Calculate the total number of valence electrons in ozone. Ozone has 3 oxygen atoms, so the total number of valence electrons is 3 × 6 = 18 electrons. b) Place the central atom and connect the surrounding atoms with single bonds. Place one oxygen atom (O) at the center and connect the other two oxygen atoms to the central oxygen atom with single bonds. c) Distribute the remaining electrons as lone pairs. We have used 4 electrons for 2 single bonds, so there are 14 electrons left. Fill the outer O atoms with lone pairs and distribute any remaining electrons on the central O atom. d) Check the octet rule and adjust as necessary. All three O atoms have a full octet. The central oxygen atom has a positive formal charge of +1, and one outer oxygen atom has a negative formal charge of -1. e) Show resonance structures where applicable. Resonance structures for ozone can be displayed by moving the double bonds between the central oxygen atom and the outer oxygen atoms.

Draw the Lewis structure for sulfur dioxide (SO2)

a) Calculate the total number of valence electrons in sulfur dioxide. Sulfur dioxide has 1 sulfur atom and 2 oxygen atoms, so the total number of valence electrons is 1 × 6 + 2 × 6 = 18 electrons. b) Place the central atom and connect the surrounding atoms with single bonds. Place the sulfur (S) atom at the center and connect the two oxygen (O) atoms to the central sulfur atom with single bonds. c) Distribute the remaining electrons as lone pairs. We have used 4 electrons for 2 single bonds, so there are 14 electrons left. Fill the outer O atoms with lone pairs and distribute any remaining electrons on the central S atom. d) Check the octet rule and adjust as necessary. Since sulfur can expand its octet, form a double bond with one of the oxygen atoms to complete the octet on each oxygen atom. e) Show resonance structures where applicable. Resonance structures for SO2 can be displayed by moving the double bond between the sulfur atom and the oxygen atom.

Draw the Lewis structure for sulfur trioxide (SO3)

a) Calculate the total number of valence electrons in sulfur trioxide. Sulfur trioxide has 1 sulfur atom and 3 oxygen atoms, so the total number of valence electrons is 1 × 6 + 3 × 6 = 24 electrons. b) Place the central atom and connect the surrounding atoms with single bonds. Place the sulfur (S) atom at the center and connect the three oxygen (O) atoms to the central sulfur atom with single bonds. c) Distribute the remaining electrons as lone pairs. We have used 6 electrons for 3 single bonds, so there are 18 electrons left. Fill the outer O atoms with lone pairs and distribute any remaining electrons on the central S atom. d) Check the octet rule and adjust as necessary. Since sulfur can expand its octet, form double bonds with two of the oxygen atoms to complete the octet on each oxygen atom. e) Show resonance structures where applicable. Resonance structures for SO3 can be displayed by moving the double bonds between the sulfur atom and the oxygen atoms.

Key concepts

Valence Electrons

Valence electrons are crucial in understanding how atoms interact in a molecule. They are the outermost electrons of an atom and are responsible for forming chemical bonds. Each element has a specific number of valence electrons, which can typically be found in its column on the periodic table. For example, both oxygen and sulfur have six valence electrons. This information is foundational in drawing Lewis structures where electrons interact with one another when forming bonds between atoms.

When creating a Lewis structure, we calculate the total number of valence electrons available. For instance:

• Ozone (\( ext{O}_3 \)): 6 valence electrons per oxygen atom times three atoms equals 18.
• Sulfur dioxide (\( ext{SO}_2 \)): 6 valence electrons from sulfur plus 12 from two oxygen atoms equals 18.
• Sulfur trioxide (\( ext{SO}_3 \)): 6 valence electrons from sulfur plus 18 from three oxygen atoms equals 24.

Understanding the distribution of these electrons helps us build accurate molecular models.

Resonance Structures

Resonance structures are multiple representations of a molecule that illustrate its potential electron configurations. They occur when more than one valid Lewis structure is possible due to electron mobility. In reality, the actual structure of the molecule is a hybrid of these possibilities, a concept that helps explain properties like bond lengths and stability.

Let's explore resonance structures further:

• For ozone (\( ext{O}_3 \)), resonance occurs as electrons rearrange to form different patterns of double and single bonds.
• Sulfur dioxide (\( ext{SO}_2 \)) also exhibits resonance. The double bond between sulfur and oxygen can shift, demonstrating different possible structures.
• Sulfur trioxide (\( ext{SO}_3 \)) can be represented with differing patterns of double bonds between sulfur and oxygen atoms.

By showing all resonance structures, we gain insight into how flexible the electron arrangement can be and understand why the molecular shape does not fully match a single Lewis structure.

Octet Rule

The octet rule is a guiding principle in chemistry that asserts atoms are generally most stable when they have eight electrons in their valence shell. This rule helps predict how electrons are shared or transferred when atoms bond.
While helpful, the octet rule does have its exceptions, notably with elements like sulfur, which can accommodate more than eight electrons due to available d-orbitals.

• In \( ext{O}_3 \), each oxygen atom aims to follow the octet rule by forming bonds and sharing electrons with the surrounding oxygen atoms.
• Sulfur in \( ext{SO}_2 \) and \( ext{SO}_3 \), however, have expanded valences. This is because sulfur can utilize d-orbitals, allowing it to hold more than the typical eight electrons, forming resonance structures with double bonds.

The application of the octet rule helps chemists understand the necessary bonding arrangements to achieve stable molecules. Yet it is also crucial to recognize where its limits lie, especially when dealing with elements capable of expanded valence shells.