Problem 64 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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 more 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

The correct statement is (c).

Step 1: Identify Electron Configurations

Understand that both \( \mathrm{NH}_3 \) and \( \mathrm{PH}_3 \) molecules have nitrogen and phosphorus atoms, respectively, with a lone pair of electrons. The nitrogen and phosphorus atoms in these molecules are sp³ hybridized, meaning that their lone pairs occupy an sp³ orbital.

Step 2: Assess Directional Nature of Orbitals

A sp³ orbital is more directional than a spherical s orbital because it is a hybrid orbital that emphasizes electron density along a particular axis, enhancing the overlap when forming bonds.

Step 3: Evaluate Electron Donor Ability

The better electron donor will have a lone pair in a more directional orbital because this allows for better overlap with the acceptor orbitals. Since sp³ orbitals are directional, \( \mathrm{NH}_3 \) as well as \( \mathrm{PH}_3 \) have their lone pairs in sp³ orbitals, making them more directional than a pure s orbital.

Step 4: Compare Properties of Nitrogen and Phosphorus

Nitrogen is more electronegative than phosphorus, meaning that it holds the lone pair more tightly. However, due to the smaller size and greater electron density focus in \( \mathrm{NH}_3 \), the \( \mathrm{NH}_3 \)'s lone pair is more available for donation compared to \( \mathrm{PH}_3 \).

Step 5: Determine the Correct Statement

Between options (a), (b), (c), and (d), evaluate previous insights. Options (a) and (d) wrongly describe orbital types. Option (c) correctly identifies \( \mathrm{NH}_3 \) as a better electron donor due to its sp³ orbital, which is more directional.

Step 6: Final Choice Confirmation

The correct choice is (c): Between \( \mathrm{NH}_3 \) and \( \mathrm{PH}_3 \), \( \mathrm{NH}_3 \) is a better electron donor because the lone pair of electrons occupies sp³ orbital and is more directional.

Key Concepts

Hybridization

Hybridization is a fundamental concept in chemistry that helps explain the shapes and bonding properties of molecules.
It involves the mixing of atomic orbitals to form new hybrid orbitals that are suitable for the pairing of electrons to form chemical bonds.
For example, in ammonia (\(\mathrm{NH}_3\)), nitrogen undergoes \(sp^3\) hybridization. This means that one \(s\) orbital and three \(p\) orbitals mix to create four equivalent \(sp^3\) hybrid orbitals.
These orbitals are arranged in a tetrahedral fashion, which provides a shape where electron pairs can be more directional.
In molecules like phosphine (\(\mathrm{PH}_3\)), phosphorus also shows \(sp^3\) hybridization.
Despite similarities in hybridization with \(\mathrm{NH}_3\), there are differences due to the size and electronegativity of the atom involved.
Understanding hybridization allows us to predict the shape and bond angles in molecules, which are critical for determining functionality in chemical reactions.

sp³ orbitals

\(sp^3\) orbitals result from the mixing of one \(s\) orbital and three \(p\) orbitals in an atom.
This results in four new equivalent orbitals that are oriented in a tetrahedral geometry.
These orbitals are highly directional, promoting a better overlap with other atoms when forming bonds.
The directionality is important because it allows lone pairs of electrons, such as those in \(\mathrm{NH}_3\) and \(\mathrm{PH}_3\), to effectively overlap and form strong bonds with electron acceptors.
The highly directional characteristic is one reason why \(\mathrm{NH}_3\) is a more effective electron donor than \(\mathrm{PH}_3\).
The lone pair in \(\mathrm{NH}_3\) resides in an \(sp^3\) orbital, making it readily available for bonding interactions.
This concept is particularly significant in understanding molecular interactions and reactivity.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons.
In the context of this exercise, nitrogen and phosphorus have different electronegativities, affecting their electron donor capabilities.
Nitrogen is more electronegative than phosphorus, meaning it tends to hold electrons more tightly.
However, despite its high electronegativity, the smaller size and focused electron density of \(\mathrm{NH}_3\) provide its lone pair greater accessibility compared to \(\mathrm{PH}_3\).
This relates back to its ability as an electron donor and emphasizes the importance of considering both electronegativity and atomic size in chemical bonding.
Understanding electronegativity helps predict how different atoms will interact in a molecule.
For instance, because nitrogen attracts electrons more strongly, the electron pair in \(\mathrm{NH}_3\) is more tightly held, yet still more directional due to \(sp^3\) hybridization, enhancing its donor qualities compared to \(\mathrm{PH}_3\).
Consequently, knowledge of electronegativity aids in anticipating how molecules like \(\mathrm{NH}_3\) and \(\mathrm{PH}_3\) function as electron donors in various chemical contexts.