Question

Which Of the following orders is correct?

(1) $SbH_{3}>NH_{3}>AsH_{3}>PH_{3}$ - Boiling Point

(2) $NH_{3}>PH_{3}>AsH_{3}>SbH_{3}$ - Thermal Stability

(3) $NH_{3}>PH_{3}>AsH_{3}>SbH_{3}$ - Basic Character

(4) $NH_{3}>PH_{3}>AsH_{3}>SbH_{3}$ - Bond Angle

• A. (1), (2) and (3) only
• B. (2), (3) and (4) only
• C. (1), (3) and (4) only
• D. (1), (2), (3) and (4).

Answer 1

Answer: (D)

(1) $\text{N}\text{H}_{3}$ has higher boiling point than $\text{As}\text{H}_{3}$ and $\text{P}\text{H}_{3}$ on account $H$ ‐bonding. $\text{Sb}\text{H}_{3}$ has highest boiling point on account of highest molecular weight.

(2) As the size of element increases, the strength of M‐H bond decreases and thus stability decreases.

(3) Basicity decreases down the group. (Drago rule).

(4) $\mathrm{HEH}$ angle $\left({ }^{\circ}\right): \mathrm{NH}_3=107.8 ; \mathrm{PH}_3=93.6, \mathrm{AsH}_3=91.8 ; \mathrm{SbH}_3=91.3$.