Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the correct statement is (a) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional (b) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(s p^{3}\) orbital and is more directional (c) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{NH}_{3}\) is a better electron donor because the lone pair of electrons occupies \(s p^{3}\) orbital and is less directional (d) Between \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}, \mathrm{PH}_{3}\) is a better electron donor because the lone pair of electrons occupies spherical 's' orbital and is less directional

Step-by-Step Solution

Answer

Option (c) is correct: NH_3 is better due to its more directional sp^3 orbital.

Step 1: Understand electron donation

Electron donation ability depends on the availability and directionality of lone pairs. A more directional lone pair in a hybridized orbital is generally more available for donation.

Step 2: Compare hybridization of NH3 and PH3

In ammonia (NH_3), the nitrogen is sp^3 hybridized, leading to a directional lone pair. In phosphine (PH_3), the phosphorus is also sp^3 hybridized but due to larger size and lower electronegativity, the lone pair is less directional.

Step 3: Assess lone pair availability

The lone pairs in NH_3 reside in a more directional sp^3 orbital due to nitrogen's higher electronegativity and smaller size, making it a better electron donor.

Step 4: Evaluate given statements

Evaluate each statement based on the hybridization and electron donation logic. Statement (c) suggests NH_3's lone pair is in sp^3 and is less directional, but NH_3's lone pair is actually quite directional. Therefore, statement (a) is correct where NH_3's lone pair is in sp^3 making it a better donor.

Key Concepts

Understanding Lone Pair Electrons

Lone pair electrons are a set of valence electrons that are not used in the bonding process of a molecule. These electrons reside on an atom and are not shared with other atoms, making them readily available for reactions. In the context of ammonia (NH3) and phosphine (PH3), lone pair electrons play a crucial role.

• In ammonia, there is a lone pair present on the nitrogen atom.
• In phosphine, the lone pair is found on phosphorus.

Lone pairs are critical because they influence molecular geometry and polarity. They also alter the chemical reactivity of a molecule. The donation of these electrons to other atoms or ions as a part of a reaction is why an understanding of lone pairs is essential in predicting a molecule's behavior.
Although both NH3 and PH3 have lone pairs, the effectiveness of these pairs as electron donors is affected by their hybridization and directionality.

Introduction to Hybridization

Hybridization involves the mixing of atomic orbitals to create new, hybridized orbitals. These orbitals can host the lone pair electrons and engage in bonding. Hybridization is an important concept as it dictates the shape and directional properties of the molecule.
For both nitrogen in NH3 and phosphorus in PH3, the hybridization is sp³. Here’s why hybridization matters:

• In sp³ hybridization, one s orbital mixes with three p orbitals to form four equivalent sp³ hybrid orbitals.
• This results in a tetrahedral shape, creating a distinct directional expression for electron pairs.

In NH3, the smaller size and higher electronegativity of nitrogen compared to phosphorus mean that the lone pair is held in a more compact and directional sp³ orbital, thus making it more effective for electron donation.

Role of Directional Electron Donation

Directional electron donation refers to the ability of lone pair electrons to participate in reactions, depending on how they are positioned in space.
In NH3, the lone pair on nitrogen is located in a well-defined, directional sp³ orbital. This means that the electrons can be donated to other molecules or ions more easily, as the directional nature allows for better overlap with neighboring orbitals.
In comparison, while PH3 also features sp³ hybridization, the lone pair on phosphorus is less directional due to the larger atomic size and lower electronegativity of phosphorus. This leads to reduced overlap and thus a lower electron donation capability. Directionality has significant implications for the molecular interactions and reactivity, playing a key role in chemical processes.

Comparing NH3 and PH3 in Terms of Electron Donation

When comparing the electron donation capabilities of NH3 (ammonia) and PH3 (phosphine), several factors come into play.

• Size and Electronegativity: Nitrogen is smaller and more electronegative than phosphorus, which results in a tighter hold and better directionality of the lone pair in NH3.
• Electron Geometries: Both molecules use sp³ hybridized orbitals, but the lone pairs in NH3 are more effective in engaging with other molecules due to nitrogen's specific properties.

These differences lead to NH3 being a more potent electron donor than PH3. This is summarised by the correct option in the original exercise, validating NH3's superior electron donation potential due to the directional and compact nature of its sp³ orbitals.