Melting point of hydrogen of nitrogen family follows the order

Options

A. `NH_3 lt PH_3 lt AsH_3 lt SbH_3`
B. `PH_3 lt NH_3 lt AsH_3 lt SbH_3 `
C. `PH_3 lt AsH_3 lt NH_3 lt SbH_3 `
D. `PH_3 lt AsH_3 lt SbH_3 lt NH_3 `

Answer

To determine the melting point of hydrogen compounds in the nitrogen family (Group 15), we need to analyze the compounds formed with hydrogen: NH3 (ammonia), PH3 (phosphine), AsH3 (arsine), and SbH3 (stibine). Here’s a step-by-step solution:

### Step 1: Identify the Compounds
The hydrogen compounds of the nitrogen family are:
- NH3 (Ammonia)
- PH3 (Phosphine)
- AsH3 (Arsine)
- SbH3 (Stibine)

### Step 2: Analyze the Melting Points
We need to look at the melting points of these compounds:
- NH3 has a melting point of -77.7 °C.
- PH3 has a melting point of -133.8 °C.
- AsH3 has a melting point of -62.4 °C.
- SbH3 has a melting point of -55.8 °C.

### Step 3: Consider Molar Mass
Generally, the melting point tends to increase with molar mass. The molar masses of the compounds are:
- NH3: 17 g/mol
- PH3: 34 g/mol
- AsH3: 77 g/mol
- SbH3: 121 g/mol

### Step 4: Identify the Exception
While the trend suggests that melting points should increase with molar mass, NH3 is an exception. This is due to the presence of hydrogen bonding in NH3, which is a strong intermolecular force that significantly raises its melting point compared to the other compounds.

### Step 5: Establish the Order
Considering the melting points and the effect of hydrogen bonding, the order of melting points from highest to lowest is:
1. NH3 (highest due to hydrogen bonding)
2. AsH3
3. SbH3
4. PH3 (lowest)

### Final Order
Thus, the order of melting points for the hydrogen compounds of the nitrogen family is:
**NH3 > AsH3 > SbH3 > PH3**