Question

Classify each of the following molecules as polar or nonpolar: (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{Cl}_{2}\) (c) \(\mathrm{OF}_{2}\) (d) \(\mathrm{PBr}_{3}\)

Short answer

(a) Polarity: Polar. (b) Polarity: Nonpolar. (c) Polarity: Polar. (d) Polarity: Polar.

Step-by-step solution

- Understand Polarity

The polarity of a molecule is determined by the shape of the molecule and the electronegativity differences between its atoms. If the molecule has symmetrical geometry and the atoms have equal electronegativity, it is nonpolar. If there is an uneven distribution of electron density, the molecule is polar.

- Analyze \(\mathrm{CH}_{3}\mathrm{Cl}\)

Methane chloride (\(\mathrm{CH}_{3}\mathrm{Cl}\)) has a tetrahedral shape with a chlorine atom more electronegative than the hydrogen and carbon atoms. This difference in electronegativity creates a dipole moment, making the molecule polar.

- Analyze \(\mathrm{Cl}_{2}\)

The molecule is composed of two chlorine atoms with the same electronegativity and a linear shape, resulting in a nonpolar molecule due to symmetry as there is no net dipole moment.

- Analyze \(\mathrm{OF}_{2}\)

Oxygen difluoride (\(\mathrm{OF}_{2}\)) has a bent structure due to two lone pairs on the oxygen atom. The fluorine atoms are more electronegative than the oxygen, creating a dipole. Thus, \mathrm{OF}_{2}\ is a polar molecule.

- Analyze \(\mathrm{PBr}_{3}\)

Phosphorus tribromide (\(\mathrm{PBr}_{3}\)) has a trigonal pyramidal shape due to the lone pair on the phosphorus atom. With different electronegativities and an asymmetric shape, \mathrm{PBr}_{3}\ is polar.

Key concepts

Polar Molecules

Polar molecules are those that exhibit a variation in the electrical charge distribution around them. This occurs due to differences in electronegativity among the atoms within the molecule, which leads to an uneven distribution of electrons. As a result, one part of the molecule carries a slight positive charge, while another part carries a slight negative charge, resulting in a dipole moment.

For instance, in the molecule methane chloride (CH₃Cl), the chlorine atom is more electronegative than hydrogen and carbon atoms. This causes the electrons to be drawn more towards the chlorine atom, creating a partial negative charge at that end and a corresponding partial positive charge at the opposite end, which makes CH₃Cl a polar molecule. Similarly, oxygen difluoride (OF₂) is polar because its bent shape with two lone pairs on oxygen and more electronegative fluorine atoms leads to a pronounced separation of charges.

Nonpolar Molecules

Nonpolar molecules, on the other hand, do not have regions with different charges. This is because the molecule either comprises atoms with similar electronegativities that share electrons fairly equally, or any polar bonds present are arranged symmetrically so that the dipoles cancel each other out.

An example is dichlorine (Cl₂), which consists of two chlorine atoms sharing electrons evenly due to their identical electronegativity. Since there is no difference in charge across the molecule, it remains nonpolar. Such molecules, when placed in an electric field, do not align with the field as polar molecules do, since they have no side that is more positive or negative than the other.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons when forming a chemical bond. Differences in electronegativity among atoms in a molecule lead to polar or nonpolar bonds, and consequently, polar or nonpolar molecules.

If the atoms in a bond have similar electronegativity values, the electrons are shared more or less equally, and the bond is nonpolar. If there is a significant difference in electronegativity, the electrons are not shared equally. The more electronegative atom will attract the shared electrons closer to itself, leading to a polar bond. In molecules like phosphorus tribromide (PBr₃), the difference in electronegativity between phosphorus and bromine creates polar bonds, contributing to the overall polarity of the molecule.

Molecular Geometry

Molecular geometry plays a crucial role in determining the polarity of molecules. The 3D arrangement of atoms in a molecule influences how dipole moments, if any, combine to give the molecule its overall polarity.

Linear, trigonal planar, tetrahedral structures, and more can either support or negate dipole moments depending on the molecule's symmetry. For example, phosphorus tribromide (PBr₃) has a trigonal pyramidal geometry due to a lone pair on the phosphorus atom, making it polar because the shape does not permit the complete cancellation of dipole moments. Contrastingly, symmetrical structures like that of dichlorine (Cl₂) lead to a nonpolar molecule due to the equal and opposite distribution of charge.