How many of the following molecules are polar?
BrF3, CS2, SiF4, SO3
The Correct Answer and Explanation is:
To determine how many of the molecules BrF₃, CS₂, SiF₄, and SO₃ are polar, we must examine both molecular geometry and electronegativity differences. A molecule is polar if it has a net dipole moment, meaning the distribution of electron density is uneven across the molecule due to its shape and the nature of its bonds.
1. BrF₃ (Bromine trifluoride)
- Lewis Structure: Br is the central atom with three bonding pairs and two lone pairs.
- Molecular Geometry: T-shaped (based on a trigonal bipyramidal electron geometry).
- Polarity: Polar, because the shape causes an asymmetrical distribution of electron density, leading to a net dipole moment.
2. CS₂ (Carbon disulfide)
- Lewis Structure: Linear molecule with double bonds between C and S atoms.
- Molecular Geometry: Linear.
- Polarity: Nonpolar, because the two polar C=S bonds are 180° apart and cancel out each other’s dipole moments.
3. SiF₄ (Silicon tetrafluoride)
- Lewis Structure: Si is bonded to four fluorine atoms.
- Molecular Geometry: Tetrahedral.
- Polarity: Nonpolar, due to its symmetrical shape, the dipoles cancel out even though Si–F bonds are polar.
4. SO₃ (Sulfur trioxide)
- Lewis Structure: S bonded to three O atoms with double bonds (resonance structures).
- Molecular Geometry: Trigonal planar.
- Polarity: Nonpolar, because the molecular symmetry causes dipoles to cancel out.
Final Answer:
Only one molecule, BrF₃, is polar.
Correct Answer: 1
Understanding molecular polarity is crucial in predicting a substance’s physical properties, such as solubility and boiling point. Despite the presence of polar bonds, overall molecular symmetry often determines whether a molecule is polar. BrF₃’s asymmetrical shape leads to a net dipole moment, making it the only polar molecule among the four.
