Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The group of molecules having identical shape is : (a) \(\mathrm{PCl}_{5}, \mathrm{IF}_{5}, \mathrm{XeO}_{2} \mathrm{~F}_{2}\) (b) \(\mathrm{BF}_{3}, \mathrm{PCl}_{3}, \mathrm{XeO}_{3}\) (c) \(\mathrm{SF}_{4}, \mathrm{XeF}_{4}, \mathrm{CCl}_{4}\) (d) \(\mathrm{ClF}_{3}, \mathrm{XeOF}_{2}, \mathrm{XeF}_{3}^{+}\)

Step-by-Step Solution

Answer

Group (d): \( \mathrm{ClF}_{3}, \mathrm{XeOF}_{2}, \mathrm{XeF}_{3}^{+} \) have identical shapes.

Step 1: Analyze molecular shape of \( \mathrm{PCl}_{5} \), \( \mathrm{IF}_{5} \), \( \mathrm{XeO}_{2} \mathrm{F}_{2} \)

\( \mathrm{PCl}_{5} \) has a trigonal bipyramidal shape, \( \mathrm{IF}_{5} \) has a square pyramidal shape, and \( \mathrm{XeO}_{2} \mathrm{F}_{2} \) has a see-saw shape. Thus, these molecules do not have identical shapes.

Step 2: Analyze molecular shape of \( \mathrm{BF}_{3} \), \( \mathrm{PCl}_{3} \), \( \mathrm{XeO}_{3} \)

\( \mathrm{BF}_{3} \) is trigonal planar, \( \mathrm{PCl}_{3} \) is trigonal pyramidal, and \( \mathrm{XeO}_{3} \) is trigonal pyramidal. Notice that \( \mathrm{BF}_{3} \) is different while \( \mathrm{PCl}_{3} \) and \( \mathrm{XeO}_{3} \) share the trigonal pyramidal shape.

Step 3: Analyze molecular shape of \( \mathrm{SF}_{4} \), \( \mathrm{XeF}_{4} \), \( \mathrm{CCl}_{4} \)

\( \mathrm{SF}_{4} \) has a see-saw shape, \( \mathrm{XeF}_{4} \) has a square planar shape, and \( \mathrm{CCl}_{4} \) has a tetrahedral shape. These molecules do not share the same shape.

Step 4: Analyze molecular shape of \( \mathrm{ClF}_{3} \), \( \mathrm{XeOF}_{2} \), \( \mathrm{XeF}_{3}^{+} \)

All three molecules, \( \mathrm{ClF}_{3} \), \( \mathrm{XeOF}_{2} \), and \( \mathrm{XeF}_{3}^{+} \), have a T-shaped geometry due to the presence of lone pairs on the central atom that adjust their shape accordingly.

Key Concepts

VSEPR theory

The VSEPR theory, or Valence Shell Electron Pair Repulsion theory, is a handy way for predicting the geometry of molecules. This theory assumes that electron pairs around a central atom will arrange themselves to be as far apart as possible. The goal is to minimize repulsion between them, which determines the shape of the molecule.
Key points about VSEPR include:

• Electron pairs include both bonding pairs (in chemical bonds) and lone pairs (non-bonding pairs).
• Lone pairs occupy more space than bonding pairs, leading to adjustments in bond angles and molecular shapes.

VSEPR helps predict shapes like linear, trigonal planar, tetrahedral, trigonal bipyramidal, square planar, and more, based on how many electron pairs surround the central atom. For example, if there are five electron pairs, the shape could be trigonal bipyramidal or square pyramidal, depending on how many are bonding versus lone pairs. Understanding these shapes is crucial for explaining the properties and reactions of molecules.

Trigonal bipyramidal

Trigonal bipyramidal geometry is a molecular shape that is encountered when there are five electron pairs around the central atom. This geometric arrangement includes two types of positions: axial and equatorial.
In a trigonal bipyramidal shape:

• There are three atoms in the equatorial plane, forming the shape of an equilateral triangle.
• Two additional atoms are above and below this plane, occupying axial positions.

This configuration results in bond angles of 90° between the equatorial and axial positions and 120° between equatorial positions. An example of a molecule with this geometry is phosphorus pentachloride ( PCl}_{5} ).
Understanding this geometry helps predict chemical behavior. For instance, in reactions involving such molecules, the equatorial position is more reactive due to angle strain and steric factors.

Square pyramidal

The square pyramidal geometry emerges when there are five bonding pairs and one lone pair of electrons around a central atom. It is a common shape for octahedral electronic arrangements in which one position is occupied by a lone pair.
Key characteristics include:

• Four atoms form a square base around the central atom.
• There is one atom above the square plane creating a pyramid-like structure.

An example of a molecule with square pyramidal geometry is IF}_{5} , where iodine is the central atom with five bonds and one lone pair.
Knowing the square pyramidal structure is important as it helps infer the reactivity and polarity of these molecules. Typically, the presence of a lone pair affects the molecule’s symmetry and physical properties.

T-shaped geometry

T-shaped geometry occurs when a molecule has five electron domains comprised of three bonding pairs and two lone pairs on the central atom. These lone pairs occupy equatorial positions, minimizing repulsion and leading to a T-shaped configuration.
Some properties of T-shaped molecules include:

• The T-shape results from having three bonded atoms lie in a 90° angle plane from the central atom.
• The shape is non-linear and usually causes reduced symmetry.

Examples of molecules exhibiting T-shaped geometry are chlorine trifluoride ( ClF}_{3} ) and xenon difluoride oxide ( XeOF}_{2} ). The presence of lone pairs adjusts the typical angles expected in a trigonal bipyramidal arrangement (120° and 90°) to 90° and 180°, altering chemical properties such as polarity and reactivity. Knowing about T-shaped geometry helps interpret the spatial arrangement and chemical behavior of complex molecules.