Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Mark out the incorrect match of shape for a given molecule/ion. (a) \(\mathrm{ICl}_{4}^{-}-\)square planar (b) \(\mathrm{NH}_{2}^{-}-\)Pyramidal (c) \(\mathrm{SF}_{4}-\) See \(-\) saw shape (d) \(\mathrm{XeOF}_{2}-\) Trigonal planar

Step-by-Step Solution

Answer

The incorrect shape match is (d) \(\mathrm{XeOF}_{2}\) - Trigonal planar.

Step 1: Analyze the Shape of Each Molecule/Ion

Start by analyzing the molecular geometry of each molecule/ion based on its electron pair geometry. For (a) \(\mathrm{ICl}_{4}^{-}\), it has 4 bonded pairs and 2 lone pairs, leading to a square planar shape. For (b) \(\mathrm{NH}_{2}^{-}\), it has 2 bonded pairs and 2 lone pairs, resulting in a bent or angular shape, not pyramidal. For (c) \(\mathrm{SF}_{4}\), it has 4 bonded pairs and 1 lone pair, resulting in a see-saw shape. Lastly, (d) \(\mathrm{XeOF}_{2}\), it has 3 bonded pairs and 2 lone pairs, resulting in a T-shaped molecule, not trigonal planar.

Step 2: Identify Incorrect Shape Match

Compare the actual shapes identified in Step 1 with the provided shapes in the question. The statement for \(\mathrm{NH}_{2}^{-}\) (option (b)) suggests it is pyramidal, which does not match its actual angular shape. Similarly, option (d) suggests \(\mathrm{XeOF}_{2}\) is trigonal planar, which does not match its actual T-shape.

Step 3: Mark the Incorrect Match

Having determined the mismatches, \(\mathrm{NH}_{2}^{-}\) does not fit the description of pyramidal, and \(\mathrm{XeOF}_{2}\) does not fit as trigonal planar. Choose either one as incorrect. Since \(\mathrm{XeOF}_{2}\) having a T-shape is furthest from the given description of trigonal planar, option (d) is clearly incorrect compared to small misunderstanding for \(\mathrm{NH}_{2}^{-}\).

Key Concepts

Electron Pair Geometry

Understanding electron pair geometry is crucial in predicting the shape of a molecule. It refers to the arrangement of electron pairs around a central atom. These pairs include both bonding pairs (shared between atoms) and lone pairs (unshared electrons localized on the central atom).

This concept is rooted in the Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that electron pairs will arrange themselves to minimize repulsion, leading to specific geometric arrangements.

• For example, if there are two electron pairs around a central atom, they will be opposite each other, creating a linear geometry.
• If there are four electron pairs, they will form a tetrahedral arrangement.

Regardless of whether the pairs are bonding or lone, the electron pair geometry provides a framework that underlies the more specific 3D arrangement of atoms."

Molecular Shapes

While electron pair geometry sets the stage, molecular shape focuses specifically on the positioning of atoms. This means ignoring lone pairs and only considering the arrangement of the bonding pairs and the physical shape that results.

For example, in the case of water, its tetrahedral electron pair geometry results in a bent molecular shape due to the presence of two lone pairs.

• The shape of a molecule like \(\text{SF}_4\) is described as see-saw because despite its trigonal bipyramidal electron pair geometry, the presence of lone pairs causes a unique arrangement.
• In \(\text{XeOF}_2\), it has a T-shaped molecular geometry due to the presence of three bonding pairs and two lone pairs.

The three-dimensional arrangement of atoms determines the molecule's function and its interaction with other molecules."

Bonding Pairs and Lone Pairs

Identifying the number and type of electron pairs (bonding and lone pairs) is essential to predict both electron pair geometry and molecular shape.

Bonding pairs are electrons shared between two atoms, forming covalent bonds. The number of bonds alone doesn't always dictate geometry when lone pairs are present, as these unshared electrons exert repulsive force on bonding pairs, impacting the overall shape.

• For \(\text{ICl}_4^-\), it has four bonding pairs and two lone pairs resulting in a square planar shape, where the lone pairs occupy opposite sides.
• With \(\text{NH}_2^-\), despite having a similar electron count as tetrahedral structures, the two lone pairs on nitrogen alter the shape to be bent.

Understanding the interaction between bonding pairs and lone pairs is key to accurately predicting molecular behavior and properties."