Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The molecular geometry of \(\mathrm{SF}_{6}\) is octahedral. What is the geometry of \(\mathrm{SF}_{4}\) (including lone pair(s) of electrons, if any)? (a) Tetrahedral (b) Trigonal bipyramidal (c) Pyramidal (d) Square planar

Step-by-Step Solution

Answer

The geometry of \( \mathrm{SF}_4 \) is see-saw (electron domain: trigonal bipyramidal).

Step 1: Count Valence Electrons

Sulfur (S) belongs to group 16 and has 6 valence electrons, while each fluorine (F) atom in SF4 contributes 1 valence electron. Therefore, SF4 has a total of 6 (from sulfur) + 4 * 7 (from four fluorines) = 34 valence electrons.

Step 2: Determine Electron Domain Geometry

In SF4, the sulfur atom forms bonds with four fluorine atoms, using 4 bonding pairs. Additionally, it has one lone pair of electrons because a total of 5 electron pairs (4 bonding pairs and 1 lone pair) surround the sulfur atom.

Step 3: Identify Molecular Geometry

The electron domain geometry around sulfur with 5 electron pairs is trigonal bipyramidal. However, the presence of the lone pair distorts this geometry, so the molecular geometry of SF4 is classified as 'see-saw' shaped.

Step 4: Consider Options and Select Answer

Out of the given multiple-choice options, none specify the 'see-saw' shape explicitly. However, considering the options, 'Trigonal Bipyramidal' relates to the electron domain geometry before taking the lone pair distortion into account. Therefore, option (b) Trigonal Bipyramidal is closest to describing SF4's electron arrangement.

Key Concepts

Trigonal Bipyramidal

In molecular geometry, **trigonal bipyramidal** is a five-sided figure that describes the arrangement of five electron pairs around a central atom. This is key in understanding the molecular structure of certain compounds. In this figure, three atoms are positioned in a plane (equatorial plane) 120 degrees apart, while two additional atoms are placed vertically above and below this plane (axial positions).
This arrangement allows for an efficient distribution of the electron pairs, minimizing the repulsion between them due to VSEPR (Valence Shell Electron Pair Repulsion) theory.
Under ideal circumstances without the presence of lone pairs, a molecule like phosphorus pentachloride (PCl extsubscript{5}) perfectly exhibits a trigonal bipyramidal molecular geometry.
However, it's important to note that actual observed geometries might slightly deviate due to lone pair interactions.

Electron Domain Geometry

**Electron domain geometry** is a description of the arrangement of electron pairs (bonding and lone pairs) around a central atom in a molecule. This arrangement dictates the overall shape of the molecule and is crucial in determining the molecular geometry.
For \(SF_4\), electron domain geometry is crucial in understanding its structure. It has five regions of electron density with six electrons from sulfur and one from each fluorine, forming a total of five electron pairs—four forming bonds and one as a lone pair.
The electron domain geometry for such an arrangement is initially trigonal bipyramidal. This is because when considering only the electron pairs (and not atom positions), every electron arrangement follows this five-point bipyramidal shape. This shapes how the molecule might look without considering lone pair effects.

Lone Pair Distortion

**Lone pair distortion** refers to the way the presence of a lone pair of electrons can alter the standard molecular geometry. Lone pairs occupy more space than bonding pairs, leading to adjustments in the observed shape of the molecule.
In the case of \(SF_4\), the central sulfur atom has one lone pair. This lone pair causes repulsion, distorting the original trigonal bipyramidal geometry. Instead of maintaining a perfect bipyramid, the geometry of \(SF_4\) is skewed to a 'see-saw' shape.
Lone pair electrons push the bonding electrons closer together because the lone pair takes up a larger spatial region. This leads to non-ideal bond angles and an observed geometry that deviates from the standard electron domain geometry. Therefore, understanding lone pair distortion is essential as it offers a deeper insight into molecular symmetries and the real-world geometry of compounds.