Question

The correct decreasing order of basicity of hydrides of Group - 15 elements is

• A. SbH₃ > AsH₃ > PH₃ > NH₃
• B. PH₃ > AsH₃ > SbH₃ > NH₃
• C. AsH₃ > SbH₃ > NH₃ > PH₃
• D. NH₃ > PH₃ > AsH₃ > SbH₃

Answer 1

The Correct Option is D

Understanding Basicity:

• Basicity refers to the ability of a substance to donate a lone pair of electrons.
• In the case of Group 15 hydrides (NH3, PH3, AsH3, SbH3), basicity is related to the availability of the lone pair on the central atom.

Factors Affecting Basicity:

• Electronegativity: As we move down the group, the electronegativity of the central atom decreases. This means the lone pair becomes more available for donation, which would seem to increase basicity. However, this is not the dominant factor.
• Bond Dissociation Enthalpy: As we move down the group, the size of the central atom increases, and the bond length increases. This results in a decrease in bond dissociation enthalpy, making it harder for the central atom to hold onto its lone pair.
• Size: The size of the central atom increases as we move down the group. The larger the atom, the more diffuse the lone pair becomes, and the less available it is for donation.

Trend:

• The dominant factor is the size of the central atom.
• Therefore, as we move down the group, the basicity of the hydrides decreases.

Correct Order:

• The correct decreasing order of basicity is: NH3 > PH3 > AsH3 > SbH3.

Conclusion:

The correct answer is Option 4.

Answer 2

Basicity of Group 15 Hydrides 

In general, the basicity of group 15 hydrides decreases on moving down the group from NH₃ to SbH₃. This trend is due to the increasing size of the central atom and the decreasing availability of the lone pair for donation. Thus, the correct order of basicity of the hydrides is:

\[ \text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 \]

This order is explained by the fact that the nitrogen atom in NH₃ is smaller and more capable of donating its lone pair compared to the larger atoms like phosphorus (PH₃), arsenic (AsH₃