Question

Compound \(\mathrm{X}\) is an anhydride of sulphuric acid. The number of sigma bonds and the number of pi bonds present in \(\mathrm{X}\) are respectively: (a) 3,3 (b) 4,2 (c) 2,4 (d) 4,3

Short answer

The number of sigma and pi bonds in SO₃ are 3 and 3, respectively - Option (a) 3,3.

Step-by-step solution

Identify Anhydride of Sulphuric Acid

First, recognize what the anhydride of sulphuric acid is. Sulfuric acid ( H₂SO₄) has the chemical formula of SO₃ when it loses water (H₂O). Therefore, the compound X is sulfur trioxide, SO₃.

Draw the SO₃ Lewis Structure

Draw the Lewis structure for SO₃. Sulfur (S) is in the center connected to three oxygen atoms (O) with double bonds. Each S=O bond consists of one sigma (σ) bond and one pi (π) bond.

Count Sigma Bonds

In the structure of SO₃, each S=O double bond contains one sigma bond. Since there are three oxygen atoms bonded to sulfur with double bonds, the total number of sigma bonds is 3.

Count Pi Bonds

Each S=O double bond also contains one pi bond. As there are three double bonds in SO₃, the total number of pi bonds is 3.

Select the Correct Option

From the options given: (a) 3,3, (b) 4,2, (c) 2,4, (d) 4,3 - the compound SO₃ has 3 sigma bonds and 3 pi bonds. Thus, the correct answer is (a) 3,3.

Key concepts

Sigma Bonds

Sigma bonds are the basic building blocks of chemical bonding. They form the framework that holds molecules together. In a sigma bond, two atomic orbitals overlap linearly or "end-to-end." This overlap creates a strong, stable bond.

Key characteristics of sigma bonds include:

• Single Bonds: Sigma bonds are often found in single bonds between atoms. For example, in a molecule of hydrogen gas (H₂), the H-H bond is a sigma bond.
• Direct Overlap: The overlap can happen between any of the following combinations of atomic orbitals: s-s, s-p, or p-p orbitals, usually oriented along the internuclear axis.
• Rotational Symmetry: An interesting property of sigma bonds is their ability to allow rotation around the bond axis. This is not possible with pi bonds.

In the case of sulfur trioxide (SO₃), each sulfur-oxygen (S=O) double bond contains one sigma bond. This means that the strong sigma interactions are part of the double bond framework.

Pi Bonds

Pi bonds add a special twist to the structure of molecules. Unlike sigma bonds, pi bonds are formed by the side-to-side overlap of atomic orbitals, typically p orbitals. These bonds are usually found in conjunction with sigma bonds to form double or triple bonds.

Important aspects of pi bonds are:

• Side-to-Side Overlap: Pi bonds are formed by the lateral overlap of orbitals that are parallel to each other, creating a bonding region above and below the nuclei.
• Weaker Than Sigma Bonds: Due to the nature of the overlap, pi bonds are generally weaker than sigma bonds and contribute less to the overall bond strength.
• Restrict Rotation: A major consequence of pi bonding is its restriction of rotation. Molecules with pi bonds cannot freely rotate along the axis of the bond, impacting the molecule's chemical properties.

In sulfur trioxide (SO₃), each sulfur-oxygen double bond also includes a pi bond. Therefore, SO₃ contributes three pi bonds to the molecule.

Lewis Structures

Lewis structures are diagrams that help us visualize the valence electrons in molecules. They are an essential tool in predicting molecular geometry, understanding reactivity, and determining the nature of a molecule's bonds.

Components of a Lewis structure:

• Element Symbols: Represent the atoms in the molecule.
• Lines/Stick Notation: Each line represents a pair of shared electrons, or in other words, a covalent bond.
• Dots: These depict unshared, or lone pair electrons surrounding the atoms.

Drawing a Lewis structure involves placing the most electronegative atoms around the central atom and ensuring that the total electron count matches the sum of the valence electrons. In the Lewis structure of sulfur trioxide, sulfur is at the center connected to three oxygen atoms with double bonds.
The Lewis structure helps us understand that each S=O double bond contains both a sigma and a pi bond, guiding us to foresee both the number of bonds and molecular shape.