Molecule which contains only bonded pairs of electrons on the central atom is

Options

A. `H_2O`
B. `NH_3`
C. `BeCl_2`
D. `BeF_3`

Explanation

To determine which molecule contains only bonded pairs of electrons on the central atom, we will analyze the molecular structures of the given options. We are looking for a molecule where the central atom has no lone pairs, meaning all its valence electrons are involved in bonding.

### Step-by-Step Solution:

1. **Identify the Central Atom and Its Valence Electrons:**
- For each molecule, identify the central atom and determine its group number in the periodic table to find out how many valence electrons it has.

2. **Analyze H2O (Water):**
- Central atom: Oxygen (O)
- Valence electrons: 6 (group 16)
- Bonding: Forms 2 bonds with hydrogen atoms, using 2 electrons.
- Remaining electrons: 4 electrons, which form 2 lone pairs.
- Conclusion: H2O has lone pairs on the central atom.

3. **Analyze NH3 (Ammonia):**
- Central atom: Nitrogen (N)
- Valence electrons: 5 (group 15)
- Bonding: Forms 3 bonds with hydrogen atoms, using 3 electrons.
- Remaining electrons: 2 electrons, which form 1 lone pair.
- Conclusion: NH3 has a lone pair on the central atom.

4. **Analyze BeCl2 (Beryllium Chloride):**
- Central atom: Beryllium (Be)
- Valence electrons: 2 (group 2)
- Bonding: Forms 2 bonds with chlorine atoms, using both electrons.
- Remaining electrons: 0 electrons, meaning no lone pairs.
- Conclusion: BeCl2 has only bonded pairs on the central atom.

5. **Analyze BeF3 (Beryllium Fluoride):**
- Central atom: Beryllium (Be)
- Valence electrons: 2 (group 2)
- Bonding: Forms 3 bonds with fluorine atoms. However, this results in an incomplete octet for beryllium.
- Remaining electrons: 0 electrons, meaning no lone pairs.
- Conclusion: BeF3 has only bonded pairs on the central atom, but it is not a stable molecule in the traditional sense.

6. **Final Conclusion:**
- Among the analyzed molecules, BeCl2 is the only one that contains only bonded pairs of electrons on the central atom without any lone pairs. Therefore, the answer to the question is BeCl2.

### Answer:
The molecule which contains only bonded pairs of electrons on the central atom is **BeCl2**.