Which of the following have least dipole moment?

Options

A. `H_(2)O`
B. `BeF_(2)`
C. `NH_(3)`
D. `BF_(3).`

Explanation

To determine which of the given molecules has the least dipole moment, we will analyze the dipole moments of each molecule step by step.

### Step 1: Analyze H2O (Water)
- **Structure**: Water has a bent molecular geometry due to the two lone pairs on the oxygen atom and the two hydrogen atoms bonded to it.
- **Electronegativity**: Oxygen is more electronegative than hydrogen, creating a dipole moment directed towards the oxygen atom.
- **Conclusion**: The dipole moment of H2O is not zero because the dipoles do not cancel out.

**Hint**: Remember that the shape and electronegativity differences contribute to the dipole moment.

### Step 2: Analyze BeF2 (Beryllium Fluoride)
- **Structure**: BeF2 has a linear molecular geometry because beryllium has only two valence electrons.
- **Dipole Moment**: The dipoles from the two Be-F bonds are equal in magnitude but opposite in direction, leading to cancellation.
- **Conclusion**: The dipole moment of BeF2 is zero.

**Hint**: Linear molecules with symmetrical charge distribution often have a dipole moment of zero.

### Step 3: Analyze NH3 (Ammonia)
- **Structure**: Ammonia has a trigonal pyramidal shape due to one lone pair on the nitrogen atom and three hydrogen atoms.
- **Electronegativity**: Nitrogen is more electronegative than hydrogen, creating dipole moments directed towards nitrogen.
- **Conclusion**: The dipole moments do not cancel out, resulting in a net dipole moment for NH3.

**Hint**: Consider the molecular geometry and lone pairs when determining the overall dipole moment.

### Step 4: Analyze BF3 (Boron Trifluoride)
- **Structure**: BF3 has a trigonal planar geometry with boron at the center and three fluorine atoms.
- **Dipole Moment**: The dipoles from the three B-F bonds are equal in magnitude and symmetrically arranged, leading to cancellation.
- **Conclusion**: The dipole moment of BF3 is zero.

**Hint**: Symmetrical molecules with identical bonds can have a net dipole moment of zero.

### Step 5: Compare the Dipole Moments
- From our analysis:
- H2O has a non-zero dipole moment.
- BeF2 has a dipole moment of zero.
- NH3 has a non-zero dipole moment.
- BF3 has a dipole moment of zero.

- **Conclusion**: BeF2 and BF3 both have a dipole moment of zero. However, since the question asks for the least dipole moment, both BeF2 and BF3 qualify, but we can conclude that BeF2 is the least polar since it has no dipole moment due to its linear structure.

**Final Answer**: BeF2 has the least dipole moment.