Determine whether the following molecules are polar or nonpolar: (a) SF4, (b) SiCl4.
The Correct Answer and Explanation is:
Thought for a few seconds
Answer:
- (a) SF₄: Polar
- (b) SiCl₄: Nonpolar
Explanation
The polarity of a molecule depends on both the individual bond polarities and the overall molecular geometry.
1. SF₄ (Sulfur Tetrafluoride)
- Electron‑pair arrangement: Sulfur has five regions of electron density (four S–F bonds and one lone pair). According to VSEPR theory, these regions adopt a trigonal‑bipyramidal electron‑pair geometry.
- Molecular shape: With one lone pair occupying an equatorial position, the resulting molecular shape is called a “seesaw.”
- Bond polarities: Each S–F bond is highly polar because fluorine is much more electronegative than sulfur.
- Dipole moment: In a perfect trigonal bipyramid with no lone pairs, some bond dipoles would cancel. However, in the seesaw shape the lone pair creates an asymmetry. The four bond dipoles cannot fully cancel because the lone pair pushes the surrounding fluorine atoms into positions that break the symmetry. As a result, there is a net dipole moment pointing from the sulfur toward the region occupied by the lone pair.
- Conclusion: SF₄ is polar.
2. SiCl₄ (Silicon Tetrachloride)
- Electron‑pair arrangement: Silicon has four bonding pairs and no lone pairs. The four Si–Cl bonds arrange themselves in a tetrahedral electron‑pair geometry.
- Molecular shape: The shape is a regular tetrahedron, with bond angles of about 109.5°.
- Bond polarities: Each Si–Cl bond is polar because chlorine is more electronegative than silicon.
- Dipole cancellation: In a perfect tetrahedron, the four bond dipole vectors are symmetrically arranged and cancel one another out exactly. There is no remaining net dipole.
- Conclusion: SiCl₄ is nonpolar.
Key points to remember:
- Geometry controls dipole cancellation. Even strongly polar bonds can yield a nonpolar molecule if the shape is highly symmetric.
- Lone pairs introduce asymmetry. When a central atom has lone pairs, the molecular shape is distorted from perfect symmetry, typically resulting in a polar molecule if the bond polarities do not symmetrically oppose each other.
