Question

which of the following lewis diagrams represents a molecule that is polar?
(a)
:f:
:f:b
:f:
(b)
ö::c::ö
(c)
h
:ci:c:h
:ci:
(d)
:n:::n:

Explanation:

To determine which Lewis diagram represents a polar molecule, we analyze the molecular geometry and electronegativity differences:

Step 1: Analyze Option A (BF₃)

• Molecular Geometry: Boron trifluoride (BF₃) has a trigonal planar geometry (120° bond angles).
• Electronegativity: B - F bonds are polar, but the symmetry of the trigonal planar shape causes the bond dipoles to cancel out. Thus, BF₃ is nonpolar.

Step 2: Analyze Option B (CO₂)

• Molecular Geometry: Carbon dioxide (CO₂) is linear (180° bond angle).
• Electronegativity: C - O bonds are polar, but the linear symmetry makes the bond dipoles cancel. CO₂ is nonpolar.

Step 3: Analyze Option C (CH₂Cl₂)

• Molecular Geometry: Dichloromethane (CH₂Cl₂) has a tetrahedral geometry, but the electronegativity of Cl (3.16) is higher than H (2.20) and C (2.55).
• Polarity: The C - Cl bonds are more polar than C - H bonds. The tetrahedral shape is not symmetric enough to cancel the bond dipoles (due to two Cl and two H atoms), resulting in a net dipole moment. Thus, CH₂Cl₂ is polar.

Step 4: Analyze Option D (N₂)

• Molecular Geometry: Nitrogen gas (N₂) is diatomic with a triple bond.
• Electronegativity: The bond is between two identical N atoms, so there is no electronegativity difference. N₂ is nonpolar.

Answer:

C. $\ce{:\overset{..}{Cl}:\overset{..}{Cl}C:H}$ (or the Lewis diagram for $\ce{CH2Cl2}$ as shown in option C)