Understanding Permanent Dipole Moment

A molecule has a permanent dipole moment if it has polar bonds (due to electronegativity differences) and an asymmetric shape (net dipole doesn't cancel out). Symmetric molecules with polar bonds may have zero dipole moment.

Step 1: Analyze Each Option

Option 1: BeCl₂, CO₂, BCl₃, CHCl₃

• BeCl₂: Linear (symmetrical), $\mu =0$
• CO₂: Linear (symmetrical), $\mu =0$
• BCl₃: Trigonal planar (symmetrical), $\mu =0$
• CHCl₃: Tetrahedral (asymmetric), $\mu \ne 0$

Not all have permanent dipole.

Option 2: SO₂, C₆H₅Cl, H₂Se, BrF₅

• SO₂: Bent shape (asymmetric), $\mu \ne 0$
• C₆H₅Cl (Chlorobenzene): Polar C-Cl bond, asymmetric, $\mu \ne 0$
• H₂Se: Bent shape (asymmetric), $\mu \ne 0$
• BrF₅: Square pyramidal (asymmetric), $\mu \ne 0$

All have permanent dipole.

Option 3: BF₃, O₃, SF₆, XeF₆

• BF₃: Trigonal planar (symmetrical), $\mu =0$
• O₃: Bent shape (asymmetric), $\mu \ne 0$
• SF₆: Octahedral (symmetrical), $\mu =0$
• XeF₆: Distorted octahedral (asymmetric), $\mu \ne 0$

Not all have permanent dipole.

Option 4: NO₂, NH₃, POCl₃, CH₃Cl

• NO₂: Bent shape (asymmetric), $\mu \ne 0$
• NH₃: Pyramidal (asymmetric), $\mu \ne 0$
• POCl₃: Tetrahedral (asymmetric due to different atoms), $\mu \ne 0$
• CH₃Cl: Tetrahedral (asymmetric), $\mu \ne 0$

All have permanent dipole.

Step 2: Final Answer

Options where all four molecules possess permanent dipole moment are: Option 2 and Option 4.

Related Topics & Formulae

Key Concepts:

• Polar Bonds: Occur when atoms with different electronegativities bond. Electronegativity difference $\Delta \chi >0.4$ usually indicates polarity.
• Molecular Geometry: Determines if bond dipoles cancel. Symmetrical shapes (linear, trigonal planar, tetrahedral with identical atoms, etc.) often have $\mu =0$.
• Dipole Moment ($\mu$): Vector sum of individual bond dipoles. $\mu =q\times d$, where $q$ is charge magnitude and $d$ is distance.

Common Symmetric (Zero Dipole) Molecules: BeCl₂, CO₂, BF₃, CH₄, CCl₄, SF₆, etc.

Common Asymmetric (Non-zero Dipole) Molecules: H₂O, SO₂, NH₃, CHCl₃, CH₃Cl, O₃, etc.