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

Question

The decreasing values of bond angles from \(\mathrm{NH}_{3}\) \(\left(106^{\circ}\right)\) 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 p orbital character in sp \(^{3}\) (c) decreasing lp-bp repulsion (d) decreasing electronegativity

Step-by-Step Solution

Answer

The correct cause is (d) decreasing electronegativity.

Step 1: Understanding Bond Angles in Group 15 Hydrides

The bond angles in hydrides like \(\mathrm{NH}_3\) to \(\mathrm{SbH}_3\) are affected by electronic and geometrical factors. It's important to note that as you move down the group, the central element changes, which influences these factors.

Step 2: Assessing Electronegativity Down Group 15

Electronegativity decreases as you move down a group in the periodic table due to the increased atomic size and electron shielding. This results in less efficient bonding, affecting bond angles.

Step 3: Examining Lone Pair-Bond Pair Repulsion

The lone pair (lp)-bond pair (bp) repulsion affects bond angles. As the molecule size increases down the group, the effective repulsion decreases because the lone pairs are less localized, thereby decreasing bond angles.

Step 4: Analyzing Orbital Hybridization

Hybridization affects bond angles through the percentage of s and p character. The increase in p character influences the angles. As the central atom becomes larger and more electropositive, p orbital character in hybrid orbitals can increase.

Step 5: Evaluating Options

(a) Increasing bp-bp repulsion would increase angles, but bond angles decrease. (b) Increasing p character aligns with observed angle decrease due to decreased s character strength. (c) Decreasing lp-bp repulsion is a result, not a cause. (d) Decreasing electronegativity aligns with angle decrease as larger atoms form weaker bonds.

Key Concepts

Electronegativity Trends

Electronegativity is a measure of how strongly an atom attracts shared electrons in a chemical bond. In the periodic table, electronegativity tends to decrease as you move down a group. This occurs due to the increase in atomic size and electron shielding, which diminishes the attractive force exerted on bonding electrons by the nucleus. In the case of group 15 hydrides, from ammonia (\(\mathrm{NH}_3\)) to stibine (\(\mathrm{SbH}_3\)), the decrease in electronegativity as you descend the group means that the central atom holds the shared bonding electrons less tightly. This can lead to weaker bonds and, consequently, smaller bond angles. The weaker the attraction between the central atom and the hydrogen atoms, the more likely the bond angle will reduce due to less repulsion between the bonding pairs.

Lone Pair-Bond Pair Repulsion

The concept of lone pair-bond pair (lp-bp) repulsion is crucial in understanding the geometry of molecules, especially those within group 15. Lone pairs are pairs of valence electrons not shared with another atom, and they take up more space than bonding pairs. In group 15 hydrides, as you move down the group from nitrogen to antimony, the central atom increases in size. This increase in size results in lone pairs being less localized on the central atom. As a result, the repulsion between lone pairs and bonding pairs decreases, causing the bond angles to decrease as well. It's the decreased repulsion that allows the bond angles to become smaller compared to those molecules with more localized lone pairs.

Orbital Hybridization Effects

Orbital hybridization plays a significant role in determining molecular geometry and bond angles. In group 15 hydrides, the central atoms exhibit \(sp^3\) hybridization, combining one \(s\) orbital and three \(p\) orbitals. The hybrid orbital's characteristics depend on the proportion of \(s\) and \(p\) orbitals. As you move down group 15, the atoms undergo changes in their hybrid orbital composition. The proportion of p character increases compared to s character. Higher p character typically results in smaller bond angles since \(p\) orbitals are oriented differently than \(s\) orbitals. Therefore, the increased \(p\) character in the hybridization of heavier central atoms down the group is responsible for the reduction in bond angles.

Periodic Table Group Trends

Understanding periodic table group trends is key to explaining the behavior of elements as you move down a group, including group 15. Within a group, elements show predictable patterns in properties such as atomic size, electronegativity, and chemical reactivity. In group 15, which includes elements like nitrogen and antimony, atomic size increases as you move down the group. This increase in atomic size leads to changes in chemical bonding and molecular geometry. For hydrides, as atoms get larger, their electronegativity decreases, and lone pair repulsion lessens, both contributing to smaller bond angles. These group trends help explain why bond angles decrease in hydrides from \(\mathrm{NH}_3\) to \(\mathrm{SbH}_3\), as the structural and electronic attributes change predictably with the group trend.