Concept: Molecular Dipole Moment

The dipole moment of a molecule is a measure of its overall polarity. It depends on two factors:

1. The magnitude of the individual bond dipoles (due to electronegativity differences).
2. The molecular geometry (how these bond dipoles are arranged in space).

A molecule has a zero dipole moment if it is symmetrical and the individual bond dipoles cancel each other out.

Step 1: Analyze the Geometry and Polarity of Each Option

Option 1: PCl₃

• Central atom: Phosphorus (P).
• Electron geometry: Tetrahedral (4 electron domains).
• Molecular geometry: Trigonal pyramidal (1 lone pair).
• Polar bonds: Yes (Cl is more electronegative than P).
• Symmetry: The bond dipoles do not cancel due to the asymmetrical shape. The molecule has a net dipole moment.
• Conclusion: Not zero.

Option 2: ICl

• This is a diatomic molecule.
• Polar bond: Yes (Cl is more electronegative than I).
• In any diatomic molecule with atoms of different electronegativities, the bond dipole is the molecular dipole. There is no symmetry to cancel it.
• Conclusion: Not zero.

Option 3: AsCl₃

• Central atom: Arsenic (As).
• Electron geometry: Tetrahedral (4 electron domains).
• Molecular geometry: Trigonal pyramidal (1 lone pair). This is analogous to PCl₃ and NH₃.
• Polar bonds: Yes (Cl is more electronegative than As).
• Symmetry: The bond dipoles do not cancel due to the asymmetrical shape. The molecule has a net dipole moment.
• Conclusion: Not zero.

Option 4: Ni(CO)₄

• This is nickel tetracarbonyl.
• Central atom: Nickel (Ni).
• Electron geometry: Tetrahedral (4 electron domains from 4 C-O bonds).
• Molecular geometry: Tetrahedral (no lone pairs on Ni).
• Polar bonds: The C-O bond is polar (O is more electronegative than C). However, the C atom is bonded to Ni, creating a Ni-C bond. The overall polarity of each Ni-CO unit is a more complex vector.
• Crucial Point: The molecule has a symmetrical tetrahedral geometry. The individual bond dipoles are symmetrical and point away from the center. In a perfect tetrahedron, these dipoles cancel each other out completely.
• Conclusion: Zero dipole moment.

Step 2: Final Answer

After evaluating all options based on molecular geometry and bond polarity, the molecule with a symmetrical shape that allows for the cancellation of all bond dipoles is Ni(CO)₄.

Final Answer: Ni(CO)₄

Related Topics & Formulae

Key Theory:

• Dipole Moment (μ): A vector quantity that measures the separation of positive and negative charges in a molecule. It is calculated as: $\mu =Q\times r$ where $Q$ is the magnitude of the charge and $r$ is the distance between the charges.
• VSEPR Theory: Valence Shell Electron Pair Repulsion theory is used to predict the 3D geometry of molecules, which is essential for determining the overall dipole moment.
• Electronegativity: The ability of an atom to attract a bonding pair of electrons. A difference in electronegativity between two atoms creates a polar bond with a bond dipole.

Common Symmetrical Shapes with Zero Dipole Moment:

• Linear (e.g., CO₂, BeCl₂)
• Trigonal Planar (e.g., BF₃, SO₃)
• Tetrahedral (e.g., CH₄, CCl₄, Ni(CO)₄)
• Square Planar (e.g., XeF₄)
• Octahedral (e.g., SF₆)

Common Asymmetrical Shapes with a Net Dipole Moment:

• Bent / Angular (e.g., H₂O, SO₂)
• Trigonal Pyramidal (e.g., NH₃, PCl₃)
• See-saw, T-shaped, Square pyramidal