The hydrides of the first elements in groups 15-17, namely `NH_(3), H_(2)O` and HF respectively show abnormally high values for melting and boiling points. This is due to

Options

A. small size of N,O,F
B. the ability to form extensive intermolecular H-bonding
C. the ability to form extensive intramolecular
D. effective van der Waals interaction

Answer

To understand why the hydrides of the first elements in groups 15-17 (NH₃, H₂O, and HF) show abnormally high melting and boiling points, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Identify the Compounds**:
- The compounds in question are ammonia (NH₃), water (H₂O), and hydrogen fluoride (HF).

2. **Understand the Structure of the Compounds**:
- All three compounds consist of hydrogen bonded to highly electronegative elements: nitrogen (N) in NH₃, oxygen (O) in H₂O, and fluorine (F) in HF.

3. **Recognize the Role of Electronegativity**:
- The high electronegativity of N, O, and F leads to a significant difference in electronegativity between hydrogen and these elements. This results in a polar bond, where the hydrogen atom acquires a partial positive charge (δ+) and the electronegative atom acquires a partial negative charge (δ-).

4. **Explain Hydrogen Bonding**:
- Due to the polarity of the bonds, NH₃, H₂O, and HF can form hydrogen bonds. Hydrogen bonding is a strong type of dipole-dipole interaction that occurs between the hydrogen atom of one molecule and the electronegative atom of another molecule.

5. **Discuss the Impact of Hydrogen Bonding on Physical Properties**:
- The presence of hydrogen bonds in these compounds leads to stronger intermolecular forces. As a result, more energy (in the form of heat) is required to break these interactions, leading to higher melting and boiling points compared to other hydrides in their respective groups.

6. **Conclude the Explanation**:
- Therefore, the abnormally high melting and boiling points of NH₃, H₂O, and HF can be attributed to their ability to form extensive hydrogen bonding due to the small size and high electronegativity of nitrogen, oxygen, and fluorine.

### Final Answer:
The hydrides of the first elements in groups 15-17 (NH₃, H₂O, and HF) show abnormally high values for melting and boiling points due to their ability to form extensive intermolecular hydrogen bonding.

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