Which of the following statements correctly describes the dipole moment of the AlCl_3 molecule?

• A. AlCl_3 has a dipole moment due to the presence of lone pairs on the aluminum atom.
• B. AlCl_3 has a dipole moment because the Al–Cl bonds are polar.
• C. AlCl_3 has a dipole moment because it is an ionic compound.
• D. AlCl_3 has no dipole moment because it is a symmetric, trigonal planar molecule.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the molecular geometry of AlCl_3. Aluminum trichloride (AlCl_3) has a trigonal planar shape, where the aluminum atom is at the center and three chlorine atoms are symmetrically arranged around it at 120° angles.
2. Step 2: Consider the polarity of the Al–Cl bonds. Each Al–Cl bond is polar because chlorine is more electronegative than aluminum, creating bond dipoles pointing from Al to Cl.
3. Step 3: Analyze the overall molecular dipole moment by vector addition of the individual bond dipoles. In a trigonal planar molecule with symmetrical bond angles, the bond dipoles cancel each other out due to their equal magnitude and symmetrical arrangement.
4. Step 4: Evaluate the presence of lone pairs on the central atom. Aluminum in AlCl_3 does not have lone pairs; it uses all three valence electrons to form bonds with chlorine atoms, so lone pairs do not contribute to any dipole moment.
5. Step 5: Conclude that despite polar bonds, the symmetry of the trigonal planar shape causes the dipole moments to cancel, resulting in no net dipole moment for AlCl_3.