Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a linear molecule? (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{CH}_{4}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{BeCl}_{2}\)

Step-by-Step Solution

Answer

The linear molecule is (d) \( \text{BeCl}_2\ \).

Step 1: Identify molecule shapes

To determine if a molecule is linear, we need to evaluate its molecular geometry. A linear molecule generally has a central atom bonded to two other atoms with a bond angle of 180 degrees.

Step 2: Evaluate SO2

The molecule \(\text{SO}_2\\) is bent due to the presence of lone pairs of electrons on the sulfur atom. This results in a bond angle of about 120 degrees, which makes it a non-linear molecule.

Step 3: Evaluate CH4

The molecule \(\text{CH}_4\\) is tetrahedral. It has a carbon atom in the center bonded to four hydrogen atoms with bond angles of roughly 109.5 degrees, making it a non-linear molecule.

Step 4: Evaluate H2O

Water, \(\text{H}_2\text{O}\\), forms a bent shape because of the two lone pairs on the oxygen atom. The bond angle is around 104.5 degrees, thus it is not linear.

Step 5: Evaluate BeCl2

The molecule \(\text{BeCl}_2\\) is linear. Beryllium chloride has a beryllium atom bonded to two chlorine atoms with no lone pairs on the central atom, resulting in a bond angle of 180 degrees.

Key Concepts

Linear Molecules

A linear molecule is one where the atoms align in a straight line. This configuration occurs when the central atom of the molecule bonds directly to only two other atoms, resulting in a bond angle of 180 degrees. For example, beryllium chloride (\(\text{BeCl}_2\)) is considered linear.

• Beryllium acts as the central atom with no lone pairs of electrons.
• It is symmetrically bonded to two chlorine atoms.
• Both chlorine atoms are 180 degrees apart, maintaining the linear shape.

This simplicity in structure makes identifying linear molecules relatively straightforward when compared to other geometries.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory helps in predicting the geometry and shape of molecules. According to VSEPR theory, electron pairs around a central atom will arrange themselves to minimize repulsion.

• This means electron pairs, whether as bonded atoms or lone pairs, will orient themselves as far apart as possible.
• In the case of linear molecules like \(\text{BeCl}_2\), there are no lone pairs, and the bonded pairs are arranged 180 degrees apart.

The theory accounts for different molecular shapes beyond linear, like bent or tetrahedral, which are based on the number of bonding and lone pairs present. It is a fundamental theory in chemistry to understand molecular geometry.

Chemical Bonding

Chemical bonding is the force that holds atoms together in a molecule. It involves interactions among the valence electrons, the outermost electrons, of the atoms involved.

• In a linear molecule like \(\text{BeCl}_2\), the bond is a covalent bond, where electrons are shared between beryllium and chlorine atoms.
• Beryllium forms these bonds despite having an incomplete octet, which is a peculiarity as it often hosts only four electrons in its valence shell.
• This sharing of electrons allows both beryllium and chlorine atoms to achieve more stable electronic arrangements.

Understanding chemical bonding is crucial as it provides insights into how molecules form and behave in various chemical reactions.