Answer the question on the basis of passage given below:
Nitrogen differs from rest of the members of group 15 due to its small size, high electronegativity, high ionization enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form pл - рл multiple bonds with itself and with other elements having small size and high electronegativity. Heavier elements of this group do not form \(р\pi -р\pi \) bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping.

Arrange the following in decreasing order of their basic strength :

A. \(PH_3\)

B. \(AsH_3\)

C. \(NH_3\)

D. \(SbH_3\)

E. \(BiH_3\)

Choose the correct answer from the options given below:

C > A > B > D > E

The correct answer is option 1. C > A > B > D > E.

Let us analyze the basic strength of the hydrides \( \text{NH}_3 \), \( \text{PH}_3 \), \( \text{AsH}_3 \), \( \text{SbH}_3 \), and \( \text{BiH}_3 \) in detail. Basic strength refers to the ability of a molecule to donate a lone pair of electrons to a proton \((H^+)\).

Basicity Trends in Group 15 Hydrides

These hydrides belong to Group 15 of the periodic table, where the general trend is that basicity decreases as you move down the group. Here's why:

Atomic Size: As you go down the group, the central atom (N, P, As, Sb, Bi) becomes larger. This increase in size means that the lone pair of electrons is further from the nucleus and more diffuse. This makes it less available to interact with protons.

Lone Pair Availability: In the smaller atoms, the lone pair is held more tightly and is more available to donate. In the larger atoms, the lone pair is less accessible because it is spread out over a larger volume.

Electronegativity: Smaller atoms like nitrogen have higher electronegativity compared to larger atoms like bismuth. Higher electronegativity means a stronger tendency to hold onto the lone pair, but since this lone pair is more localized, it is also more available for protonation compared to larger atoms where the lone pair is less effectively held.

Detailed Analysis

C. \( \text{NH}_3 \) (Ammonia)

Basic Strength: Highest among the group.

Reason: Nitrogen is small and has a high electronegativity. Its lone pair of electrons is relatively close to the nucleus and is available for bonding with protons. Additionally, nitrogen's high electronegativity means it can stabilize the positive charge that develops upon protonation

A. \( \text{PH}_3 \) (Phosphine)

Basic Strength: Weaker than \( \text{NH}_3 \) but stronger than the others.

Reason: Phosphorus is larger than nitrogen, so its lone pair of electrons is further from the nucleus and less concentrated. This makes it less available to donate to a proton, but it is still reasonably available compared to the larger atoms below it.

B. \( \text{AsH}_3 \) (Arsine)

Basic Strength: Weaker than \( \text{PH}_3 \).

Reason: Arsenic is even larger than phosphorus, making its lone pair of electrons even more diffuse. This decreases the lone pair's availability to bond with protons.

D. \( \text{SbH}_3 \) (Stibine)

Basic Strength: Weaker than \( \text{AsH}_3 \).

Reason: Antimony is larger than arsenic, and its lone pair of electrons is even further from the nucleus. This makes it less available for protonation.

E. \( \text{BiH}_3 \) (Bismuth Hydride)

Basic Strength: Weakest among the group.

Reason: Bismuth is the largest atom in this group, so its lone pair of electrons is very diffuse. The large size and the very low effective nuclear charge felt by the lone pair make it the least basic.

Thus, the correct order of basic strength is: \( \text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3 \)

So, the correct answer is: 1. C > A > B > D > E.