Question

Which of the following property is true for the given sequence NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ ?

• A. Reducing property
• B. Thermal stability
• C. Bond angle
• D. Acidic character

Answer 1

The Correct Option is B, C

1. Analyze the Periodic Trends

The given sequence represents hydrides of Group 15 elements (Nitrogen family): $NH_3$, $PH_3$, $AsH_3$, $SbH_3$, $BiH_3$.

- Thermal Stability: As we move down the group, the size of the central atom increases. This leads to a longer and weaker M-H bond (where M is the Group 15 element). As the bond weakens, it becomes easier to break the bond, and hence the thermal stability decreases.

- Reducing Property: As the M-H bond weakens down the group, it becomes easier for the hydride to lose hydrogen atoms and act as a reducing agent. Therefore, the reducing property increases down the group.

- Bond Angle: As the electronegativity difference between the central atom and hydrogen decreases down the group, the bond pair electrons are less attracted to the central atom, and the bond angle decreases. This also affects the contribution of 's' character to bonding.

- Acidic Character: The acidic character increases down the group because the M-H bond strength decreases, making it easier to release $H^+$ ions.

2. Compare the trends with the given sequence
The given sequence shows a decreasing trend.
$NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$

Now, let's check which property matches this trend:

- (A) Reducing property: The reducing property increases down the group, which is opposite to the given trend.

- (B) Thermal stability: The thermal stability decreases down the group, which matches the given trend.

- (C) Bond angle: The bond angle decreases down the group, which matches the given trend.

- (D) Acidic character: The acidic character increases down the group, which is opposite to the given trend.

Final Answer:
The correct answer is both (B) : Thermal stability and (C) : Bond angle

Answer 2

Thermal Stability: As we move down the group from NH₃ to BiH₃, the thermal stability decreases due to the increasing size of the central atom, which weakens the bonds. The sequence of thermal stability is: 

NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

Bond Angle: The bond angle also decreases as we move down the group from NH₃ to BiH₃ due to the increase in the atomic size, which leads to less effective overlap of orbitals. Thus, the trend in bond angles is:

NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

The correct answer is both (B) : Thermal stability and (C) : Bond angle