Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of hybridization of the central atom in the following species \(\mathrm{NH}_{3}, \mathrm{PtCl}_{4}-2, \mathrm{PCl}_{5}\) and \(\mathrm{BCl}_{3}\) is (a) \(\mathrm{dsp}^{2}, \mathrm{sp}^{3} \mathrm{~d}, \mathrm{sp}^{2}\) and \(\mathrm{sp}^{3}\) (b) \(\mathrm{sp}^{3}, \mathrm{dsp}^{2}, \mathrm{sp}^{3} \mathrm{~d}, \mathrm{sp}^{2}\) (c) \(\mathrm{dsp}^{2}, \mathrm{sp}^{2}, \mathrm{sp}^{3}, \mathrm{sp}^{3} \mathrm{~d}\) (d) \(\mathrm{dsp}^{2}, \mathrm{sp}^{3}, \mathrm{sp}^{2}, \mathrm{sp}^{3} \mathrm{~d}\)

Step-by-Step Solution

Answer

The correct order is (b) sp3, dsp2, sp3d, sp2.

Step 1: Determine Hybridization of NH3

The central atom in NH3 is nitrogen (N). Nitrogen forms 3 sigma bonds with hydrogen and has 1 lone pair. Hence, the steric number is 3 + 1 = 4. According to VSEPR theory, a steric number of 4 corresponds to sp3 hybridization. Thus, NH3 is sp3 hybridized.

Step 2: Determine Hybridization of PtCl4-2

PtCl4-2 has a central atom of Platinum (Pt). In this complex, Pt forms 4 sigma bonds with chlorine atoms and carries a -2 charge. Additionally, Pt is a transition metal capable of forming dsp2 hybridization, common for square planar geometry as expected for this complex. Therefore, PtCl4-2 is dsp2 hybridized.

Step 3: Determine Hybridization of PCl5

The central atom phosphorus (P) in PCl5 forms 5 sigma bonds with chlorine atoms, resulting in a steric number of 5. According to VSEPR, this corresponds to sp3d hybridization, making use of one d orbital for bonding, consistent with a trigonal bipyramidal geometry. Hence, PCl5 is sp3d hybridized.

Step 4: Determine Hybridization of BCl3

The central atom boron (B) in BCl3 forms 3 sigma bonds with chlorine atoms and has no lone pairs, giving it a steric number of 3. This results in sp2 hybridization based on VSEPR theory, which corresponds to a trigonal planar geometry. Therefore, BCl3 is sp2 hybridized.

Step 5: Arrange in Correct Order

Now that we know the hybridization states, we can arrange them: NH3 is sp3, PtCl4-2 is dsp2, PCl5 is sp3d, and BCl3 is sp2. Placing them in the sequence requested, we have: sp3 (NH3), dsp2 (PtCl4-2), sp3d (PCl5), and sp2 (BCl3), matching option (b).

Key Concepts

VSEPR theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a simple model used to predict the geometry of molecular compounds. It helps us understand how molecules adopt shapes that minimize the repulsion between electron pairs around a central atom. In essence, VSEPR theory is built on the premise that electrons are negatively charged and repel each other.

According to this theory, electron pairs around a central atom will stay as far apart as possible to decrease the repulsion. For instance, if a central atom is surrounded by four regions of electron density (like in \(\text{NH}_3\)), these regions will arrange themselves into a tetrahedral shape. This minimizes the repulsion, leading to predictable molecular shapes based on the steric number.

It's important to note that VSEPR theory is particularly useful for determining the molecular geometry of non-transition elements. However, it can also provide a valuable perspective when looking at transition elements, especially when identifying potential shapes and hybridization states.

Steric number

The steric number is a crucial concept when trying to predict molecular geometry. It represents the total number of electron groups that interact around a central atom. This is determined by counting the number of sigma bonds and lone pair electrons around it.

For example, in the molecule \(\text{NH}_3\), the nitrogen atom forms three sigma bonds with hydrogen atoms and also has one lone pair of electrons. Therefore, the steric number is 4 (3 bonds + 1 lone pair).

This concept helps us quickly determine the hybridization and shape of a molecule. A steric number of 4 typically correlates with \(sp^3\) hybridization and a resulting tetrahedral geometry. This method allows chemists to predict and understand the spatial arrangement of atoms in a way that efficiently reduces electron pair repulsion.

Molecular geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is crucial for understanding the behavior and properties of molecules. The geometry is determined by considering the number of bonds and lone pairs around a central atom, as explained by VSEPR theory.

For example, \(\text{BCl}_3\) has a central boron atom with three sigma bonds and no lone pairs, resulting in a trigonal planar geometry. Each chlorine bond spreads equally around the boron atom, resulting in a 120-degree angle between the bonds.

Understanding such geometrical arrangements is essential because the shape of a molecule influences how it interacts with other molecules, affects its polarity, and dictates its physical and chemical properties. This makes mastering molecular geometry valuable for anyone studying or working in chemistry and related fields.