Question

Classify each of the following as polar or nonpolar: a. \(\mathrm{CO}_{2}\) b. \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (octane) d. \(\mathrm{CH}_{3} \mathrm{CHF}_{2}\)

Short answer

\(\mathrm{CO}_{2}\) is nonpolar, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) is polar, octane (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)) is nonpolar, and \(\mathrm{CH}_{3} \mathrm{CHF}_{2}\) is polar.

Step-by-step solution

Determine the Molecular Geometry

Examine the molecular geometry to see if it's symmetric or asymmetric. Symmetrical molecules tend to be nonpolar while asymmetric molecules are usually polar due to an uneven distribution of charge.

Analyze Electronegativity Differences

Look at the differences in electronegativity between the atoms. A large difference generally indicates polar bonds, while small or no difference indicates nonpolar bonds.

Classify \(\mathrm{CO}_{2}\)

Carbon dioxide (\(\mathrm{CO}_{2}\)) is a linear molecule with two polar bonds. However, due to its symmetrical shape, the dipoles cancel each other out making \((\mathrm{CO}_{2}\)) nonpolar.

Classify \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)

\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (Dichloromethane) has a tetrahedral shape with two hydrogens and two chlorines attached to the carbon atom. The different electronegativities of hydrogen and chlorine create an asymmetrical charge distribution, making it polar.

Classify \((\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3})\) (Octane)

Octane is a hydrocarbon with a symmetrical and extended chain of carbon atoms. All the bonds are between carbons and hydrogens, which have similar electronegativities. This makes octane nonpolar.

Classify \(\mathrm{CH}_{3} \mathrm{CHF}_{2}\)

Fluorine has a high electronegativity, and its presence in \(\mathrm{CH}_{3} \mathrm{CHF}_{2}\) creates a significant dipole moment in the molecule. Despite the molecule having a similar shape to octane, the bonds are polar, making the molecule polar overall due to the asymmetrical charge distribution.

Key concepts

Molecular Geometry

Understanding the shape of a molecule, known as its molecular geometry, is crucial to determining its polarity. The geometry is defined by the positions of atoms around the central atom, influenced by the electron pairs that may be bonding or non-bonding. Symmetrical shapes like linear, tetrahedral, and trigonal planar often lead to nonpolar molecules if the surrounding atoms are of the same element.

However, even symmetrical molecules can become polar if the surrounding atoms differ and produce an uneven charge distribution. For example, water (H2O) has a bent shape due to the two lone pairs on the oxygen, leading to a polar molecule despite its symmetrical 'V' shape.

Electronegativity

Electronegativity is a property of an atom that indicates how strongly it attracts electrons in a bond. The greater the difference in electronegativity between two bonded atoms, the more polar the bond. Elements like fluorine, oxygen, and nitrogen have high electronegativity and form polar bonds with elements having lower electronegativity.

When you see a molecule with varied electronegativity, it's a clue to look closer to determine if the asymmetric arrangement leads to polar behavior as was the case in CH3CHF2 where the presence of highly electronegative fluorine atoms creates an uneven charge distribution.

Symmetrical Molecules

Symmetrical molecules are the ones where the components are arranged in such a way that the molecule looks the same from multiple perspectives. This usually leads to an even distribution of charge throughout the molecule, making it nonpolar. A great representative of this category is carbon dioxide (CO2), which is linear and the polarities of C=O bonds directly oppose each other, cancelling out, leaving us with a nonpolar molecule.

Examples of Symmetry:

• Carbon dioxide (CO2) - Linear and symmetrical
• Methane (CH4) - Tetrahedral and symmetrical

Asymmetrical Charge Distribution

When a molecule lacks symmetry in its shape or the atoms differ in electronegativity, it leads to asymmetrical charge distribution. This creates polar molecules as seen in dichloromethane (CH2Cl2). Despite its tetrahedral shape, which can be symmetrical, the presence of different atoms (hydrogen and chlorine) with different electronegativities disrupts that symmetry, causing a polar molecule with a net dipole moment.

Effects on Properties:

• An asymmetrical molecule like water (H2O) has a high boiling point due to strong intermolecular forces.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It quantifies the polarity of a molecule and is influenced by both the charge differences and the distance between the charges. A molecule with a high dipole moment is considered to be very polar.

For instance, the CHF3 molecule has a high dipole moment due to the highly electronegative fluorine atoms pulling electrons away from the less electronegative carbon and hydrogen atoms, resulting in a significant charge separation and thus, a polar molecule.