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 \(\mathrm{p}\) orbital character in \(\mathrm{sp}^{3}\) (c) decreasing lp-bp repulsion (d) decreasing electronegativity

Step-by-Step Solution

Answer

The decrease in bond angles is due to decreasing electronegativity (option d).

Step 1: Understanding Bond Angles in the Group 15 Hydrides

To answer the question, recognize that as we move down Group 15 from nitrogen in NH₃ to antimony in SbH₃, the bond angles decrease due to changes in atomic and electronic properties that affect molecular geometry.

Step 2: Analyzing Bond Repulsions

In molecular geometry, the concept of lone pair (lp) - bond pair (bp) repulsion is important. For NH₃ and SbH₃, the lp-bp repulsion generally affects the bond angles since lone pairs occupy more space and push the bond pairs closer, thus affecting bond angles.

Step 3: Assessing the Role of Electronegativities

As we move down the group, the central atom's electronegativity decreases. This decrease means the bonding electron pairs are held less tightly, which influences the bond angle. Increased size and decreased electronegativity lead to a larger distance between bonding pairs, thus reducing bond angles.

Step 4: Evaluating the Orbital Hybridization Effect

The concept of hybridization is crucial. When moving down the group, the s-character in the sp³ hybrid orbital diminishes, thereby affecting the bond angles. Lower s-character means reduced lp-bp repulsion because bonding pairs are further apart.

Step 5: Conclusion - Determining the Primary Factor

The primary factor for the decrease in bond angles from NH₃ to SbH₃ is the decreasing electronegativity, which affects how tightly electron pairs are held. Thus, option (d) is the most appropriate explanation for the observed trend.

Key Concepts

Lone Pair-Bond Pair Repulsion

When atoms form a molecule, their shared electron pairs, known as bond pairs, strive to be as far apart as possible due to repulsive forces. Now, when a molecule also has lone pairs—unshared electrons on the central atom—these lone pairs introduce additional repulsive forces. Specifically, lone pairs occupy more space compared to bond pairs. This is because the electrons in lone pairs are only attracted to one nucleus, while bond pairs are drawn toward two nuclei.
This spatial occupation causes lone pair-bond pair (lp-bp) repulsion. Lone pairs push the bond pairs closer together, affecting the overall geometry of the molecule. In Group 15 hydrides like ammonia (NH₃), the presence of lone pairs results in a tetrahedral geometry adjustment where the bond angle is less than the ideal tetrahedral angle of 109.5°. As lone-pair repulsion increases, bond pairs are squeezed together, reducing the bond angle. Conversely, as we move down the group to elements like antimony hydride (SbH₃), the impact of lp-bp repulsion diminishes.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons within a chemical bond. In Group 15 hydrides, electronegativity differences significantly impact molecule geometry. As we descend the group from nitrogen to antimony, electronegativity values of the central atoms decrease.
This decrease in electronegativity means that the bonding electron pairs are not as tightly held by the larger, less electronegative atoms further down the group. Consequently, this looser hold allows the bonding electron pairs to spread out slightly more, increasing the distance between them and, as a result, reducing the bond angle. The trend of decreasing bond angles from NH₃ (ammonia) to SbH₃ (antimony hydride) is closely tied to this decrease in electronegativity.
The lessened electronegativity has a secondary effect, too. It alters the spatial distribution of electrons around the central atom, thus influencing the electron pair repulsions and overall molecular shape.

Orbital Hybridization

Orbital hybridization is the concept where atomic orbitals mix to form new, hybrid orbitals that facilitate bonding in molecules. For group 15 hydrides, the central atom adopts an sp³ hybridization state. As we proceed down the group, there is a change in the character of these hybrid orbitals.
In the sp³ hybridization of Group 15 hydrides, the degree of s-character in the orbitals affects molecular geometry. At the top of the group with nitrogen in NH₃, there is significant s-character contributing to a smaller bond angle of approximately 106°. As we move down to antimony in SbH₃, there is less s-character and more p-character in the orbitals. This shift causes the orbitals to become more accommodating, resulting in reduced lp-bp repulsion and an increased separation between bond pairs.
The shift in orbital character influences how tightly electron pairs are held and impacts the molecule's bond angles. Hence, changes in hybridization contribute to the pattern of decreasing bond angles observed in Group 15 hydrides.