Question

Which one of the following has zero dipole moment? (a) \(\mathrm{CIF}\) (b) \(\mathrm{PCl}_{3}\) (c) \(\mathrm{SiF}_{4}\) (d) \(\mathrm{CFCl}_{3}\)

Short answer

SiF_{4} has zero dipole moment.

Step-by-step solution

Understanding Dipole Moment

A dipole moment occurs when there is a separation of charge in a molecule, resulting in a molecule with a positive end and a negative end. Dipole moments arise from differences in electronegativity between atoms and the molecule's geometry. Molecules with symmetrical charge distribution may have zero dipole moment.

Examining Each Molecule

Let's evaluate the geometry and symmetry of each molecule. 1. **CIF**: This is a diatomic molecule, polar due to electronegativity difference. 2. **PCl_{3}**: This is a trigonal pyramidal molecule, not symmetrical, and has a dipole moment. 3. **SiF_{4}**: This molecule has a tetrahedral symmetric structure, leading to cancellation of dipole moments. 4. **CFCl_{3}**: This has a less symmetric trigonal pyramidal structure, resulting in a dipole moment.

Identifying the Molecule with Zero Dipole Moment

Among the options, **SiF_{4}** is the only molecule with a symmetrical tetrahedral shape. The individual Si-F bond dipoles cancel each other out due to the symmetry, resulting in a net dipole moment of zero.

Key concepts

Molecular geometry

Molecular geometry is all about the 3D arrangement of atoms within a molecule. It's like understanding how atoms are arranged in space, and this shape decides many properties of the molecule, including reactivity and phase of matter. Specific shapes arise from the spatial distribution of electrons around a central atom, which in turn influences how the molecule interacts with others.

When determining molecular geometry, it's essential to use the VSEPR (Valence Shell Electron Pair Repulsion) theory. This theory suggests that electron pairs around a central atom will repel each other, leading to specific geometrical configurations. A central atom surrounded by four substituents, such as in ewline\( \mathrm{SiF}_4 \), usually forms a tetrahedron. This arrangement helps in understanding the distribution of bonds and lone pairs, which is crucial in predicting the molecular shape.

Quick Tips:

• Apply the VSEPR theory to predict geometry.
• Look for lone pairs, as they affect angles and shape.
• Use molecular geometry to predict physical and chemical properties.

Symmetrical molecules

Symmetry in molecules is about balanced distribution, much like how a well-balanced diet keeps you healthy. A symmetrical molecule has identical halves or elements when split, which often results in the cancellation of dipoles. Let's look closely at tetrahedral molecules: \( \mathrm{SiF}_4 \), for example, has a central silicon atom with fluorine atoms evenly distributed, forming a symmetrical shape.

A molecule is symmetrical if its geometry allows it to be divided into mirrored halves. This balance means that individual bond dipoles can cancel out, leading to no overall dipole moment for the molecule. Visualizing symmetry is not always straightforward, but practicing with molecular models can be a useful strategy.

**Key Symmetry Traits:**

• Check for mirror planes or rotational symmetry.
• Assess if substituents are identical and evenly spaced.
• Use symmetry to determine potential polarity and reactivity.

Electronegativity differences

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. When two atoms bond, differences in their electronegativity values determine whether electrons are shared equally or unequally. This difference is the main driver for dipole moments in molecules.

Atoms with high electronegativity, like fluorine or oxygen, attract electrons more strongly than those with lower electronegativity, such as hydrogen or silicon. In a molecule like \( \mathrm{SiF}_4 \), the Si-F bond sees fluorine exerting a stronger pull on the shared electrons compared to silicon. However, because \( \mathrm{SiF}_4 \) is symmetrical, these individual bond dipoles cancel out each other, resulting in no net dipole moment.

**Core Concepts in Electronegativity:**

• Higher differences in electronegativity increase bond polarity.
• The periodic table trend: Electronegativity increases across a period and decreases down a group.
• Polarity impacts molecular interactions like solubility and boiling points.