Problem 168 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing values of bond angles from \(\mathrm{NH}_{3}\) (106 \(^{\circ}\) ) to \(\mathrm{SbH}_{3}\left(101^{\circ}\right)\) down group 15 of the periodic table is due to (a) increasing bp-bp repulsion (b) increasing \(\mathrm{p}\) orbital character in \(\mathrm{sp}^{3}\) (c) decreasing lp-bp repulsion (d) decreasing electronegativity

Step-by-Step Solution

Answer

The bond angle decreases due to decreasing electronegativity (option d).

Step 1: Understand the Trends in the Periodic Table

To answer this question, we first need to understand the trends in properties as we move down group 15 in the periodic table. Common elements in this group include nitrogen (N), phosphorus (P), arsenic (As), and antimony (Sb). One key trend is a decrease in electronegativity from nitrogen to antimony.

Step 2: Analyze Impact on Electronegativity

As we move down the group, the atoms become larger and the electronegativity decreases. This decrease in electronegativity affects the bond angle because less electronegative atoms hold less tightly to their bonding pairs, which allows more room for lone pair-bond pair repulsions.

Step 3: Evaluate Orbital Character in Bond Formation

Lower elements like Sb in group 15 exhibit more p-orbital character as opposed to hybrid sp^3 orbitals found in N. This change in orbital character affects the bond angle, leading to smaller bond angles due to the elongated shape of the p orbitals.

Step 4: Consider Effects of Lone Pair-Bond Pair Repulsions

Decreasing lp-bp (lone pair-bond pair) repulsions are observed as we go down the group. This is due to the fact that sp^3 hybridization in heavier elements like Sb becomes less pronounced, with more p-character, reducing the overall lp-bp repulsion.

Step 5: Identify Correct Explanation for Decreasing Bond Angles

Given the options, the explanation involving decreasing electronegativity and increasing p orbital character both contribute to a reduced bond angle as they move down the group. However, the primary reason that directly explains the change from NH3 to SbH3 is the decreasing electronegativity (option d).

Key Concepts

Electronegativity

Electronegativity is a key concept when studying periodic table trends, especially in relation to bond angles. It measures an atom's tendency to attract and hold onto electrons within a bond.
In Group 15 of the periodic table, electronegativity decreases from nitrogen (N) to antimony (Sb). This is a critical trend to recognize because it affects molecular geometry significantly.

• As atoms increase in size from top to bottom in Group 15, their ability to attract electrons diminishes.
• With decreased electronegativity, larger atoms tend not to pull bonding electrons as closely, increasing the likelihood of nearby electron repulsion.
• This reduced pull on electrons influences chemical bonds, resulting in variations of bond angles as we observe in molecules like ammonia (\(\mathrm{NH}_3\)) compared to stibine (\(\mathrm{SbH}_3\)).

Understanding these adjustments in bond interactions helps explain why bond angles decrease down the group, due primarily to the lower electronegativity.

Lone Pair-Bond Pair Repulsion

Lone pair-bond pair repulsion is a concept that explains how electron pairs around a central atom in a molecule arrange themselves to minimize repulsion.
Specifically, when considering bond angles in molecules like \(\mathrm{NH}_3\) and \(\mathrm{SbH}_3\), lone pair-bond pair interactions are influential.

• Lone pairs are non-bonding pairs of electrons that exert strong repulsive forces, stronger than those of bonding pairs.
• In elements down Group 15, lone pair-bond pair repulsion decreases due to the larger atomic size and increased p-orbital character.
• This decrease in repulsion ensures that bond pairs spread out slightly more, leading to smaller bond angles, such as the reduction seen from ammonia (\(106^{\circ}\)) to stibine (\(101^{\circ}\)).

This progressive reduction in repulsion is due to the accommodative nature of larger atoms and their electron arrangements, confirming the slow transformation in molecular geometry characteristics.

Orbital Hybridization

Orbital hybridization combines atomic orbitals to form new hybrid orbitals suitable for pairing electrons to create stable molecules.
In Group 15, from nitrogen to antimony, there's a noticeable shift in the character of these hybrid orbitals.

• Nitrogen primarily utilizes \(sp^3\) hybridization with significant s-character, forming tetrahedral geometries.
• As you move to heavier elements like antimony, p-character increases.
• This change leads to a less pronounced \(sp^3\) hybridization, resulting in smaller angles in molecular shapes due to elongated, less compact orbital configurations.

The influence of these orbital changes is evident in the decreasing bond angles from \(\mathrm{NH}_3\) to \(\mathrm{SbH}_3\). These differences underline how the type of orbital hybridization impacts molecular geometry across the group, offering a deeper understanding of periodic trends in such molecules.