Which of the following molecules has zero dipole moment ?

Options

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

Answer

To determine which molecule has a zero dipole moment, we need to analyze the molecular geometry and the electronegativity of the atoms involved in each molecule. The dipole moment is a vector quantity that depends on both the magnitude and direction of the bond dipoles. Here’s a step-by-step solution:

### Step 1: Identify the molecules
Let's assume the molecules given in the question are:
1. BeCl2 (Beryllium chloride)
2. HCl (Hydrochloric acid)
3. NH3 (Ammonia)
4. H2O (Water)

### Step 2: Analyze BeCl2
- **Molecular Geometry**: BeCl2 has a linear shape due to the arrangement of its two chlorine atoms around the beryllium atom.
- **Electronegativity**: Chlorine is more electronegative than beryllium, which means the dipoles point towards the chlorine atoms.
- **Dipole Moment**: The dipoles from each Cl cancel each other out due to the linear geometry, resulting in a net dipole moment of zero.

### Step 3: Analyze HCl
- **Molecular Geometry**: HCl is a diatomic molecule.
- **Electronegativity**: Chlorine is more electronegative than hydrogen.
- **Dipole Moment**: The dipole points from hydrogen to chlorine, resulting in a net dipole moment that is not zero.

### Step 4: Analyze NH3
- **Molecular Geometry**: Ammonia has a trigonal pyramidal shape due to the presence of a lone pair on nitrogen.
- **Electronegativity**: Nitrogen is more electronegative than hydrogen.
- **Dipole Moment**: The dipoles from the three hydrogen atoms point towards the nitrogen, and the lone pair also contributes to the dipole moment, resulting in a net dipole moment that is not zero.

### Step 5: Analyze H2O
- **Molecular Geometry**: Water has a bent shape due to the two lone pairs on oxygen.
- **Electronegativity**: Oxygen is more electronegative than hydrogen.
- **Dipole Moment**: The dipoles from the two hydrogen atoms point towards the oxygen, and the lone pairs also contribute to the dipole moment, resulting in a net dipole moment that is not zero.

### Conclusion
After analyzing all the molecules:
- **BeCl2** has a zero dipole moment.
- **HCl**, **NH3**, and **H2O** all have non-zero dipole moments.

Thus, the molecule with zero dipole moment is **BeCl2**.