Question

The molecules \(\mathrm{CH}_{4}\) and \(\mathrm{CH}_{3} \mathrm{Cl}\) both contain four bonds. Why is \(\mathrm{CH}_{4}\) nonpolar whereas \(\mathrm{CH}_{3} \mathrm{Cl}\) is polar?

Short answer

\text{CH}_{4} is nonpolar due to symmetrical tetrahedral arrangement, whereas \text{CH}_{3} \text{Cl} is polar due to the electronegativity of Cl creating a net dipole.

Step-by-step solution

Identify Molecular Structures

Draw the Lewis structures for both \(\text{CH}_{4}\) and \(\text{CH}_{3} \text{Cl}\). \( \text{CH}_{4} \) consists of a carbon atom bonded to four hydrogen atoms in a tetrahedral geometry. \( \text{CH}_{3} \text{Cl} \) consists of a carbon atom bonded to three hydrogen atoms and one chlorine atom in a similar tetrahedral geometry.

Analyze the Electronegativity

Electronegativity is the ability of an atom to attract electrons toward itself in a chemical bond. Hydrogen and carbon have similar electronegativities, but chlorine is much more electronegative than either hydrogen or carbon.

Determine Bond Polarities

In \(\text{CH}_{4}\), because all four atoms bonded to carbon are hydrogen atoms, the bond polarities cancel out due to the symmetric arrangement. In \(\text{CH}_{3} \text{Cl}\), the bond between carbon and chlorine is polar because of the significant difference in electronegativity.

Assess Molecular Polarity

A molecule is nonpolar if the dipole moments of the bonds cancel out, resulting in no net dipole moment. \(\text{CH}_{4}\) is nonpolar because the tetrahedral symmetry allows the bond polarities to cancel. \(\text{CH}_{3} \text{Cl}\), on the other hand, is polar because the C-Cl bond creates a net dipole moment in the molecule.

Key concepts

Electronegativity

Electronegativity is a measure of how strongly an atom can attract electrons in a chemical bond. In simpler terms, it indicates how 'hungry' an atom is for electrons. For example, chlorine is highly electronegative, meaning it strongly attracts electrons.
In contrast, hydrogen and carbon have similar, relatively lower electronegativities. This difference in electronegativity is crucial for understanding why some molecules are polar and others are not. When there is a big difference in electronegativity between two bonded atoms, the bond will be polar.
Such differences cause the electrons to be more attracted to the more electronegative atom, creating a slight charge difference between the atoms. This forms the basis for bond polarity.

Bond Polarity

Bond polarity occurs when there is an uneven distribution of electron density along the bond. This happens due to differences in electronegativity between the atoms.
For instance, in \({\text{CH}}_{4}\), all C-H bonds are between atoms with similar electronegativities, making these bonds nonpolar. The electrons are shared pretty equally. However, in \({\text{CH}}_{3} \text{Cl}\), the C-Cl bond is polar. This is because chlorine attracts the electrons more strongly than the carbon.
This difference causes a shift in electron density towards the chlorine atom, making it partially negative and leaving the carbon slightly positive. Thus, bond polarity is directly dependent on the atoms' electronegativities involved in the bond.

Lewis Structures

Lewis structures are simplified diagrams that show the bonding between atoms and the lone pairs of electrons in a molecule. They help us visualize the arrangement of atoms and the distribution of valence electrons.
For \({\text{CH}}_{4}\), the Lewis structure shows a carbon atom in the center with four hydrogen atoms symmetrically surrounding it. All these bonds are single bonds, meaning that each hydrogen shares one pair of electrons with carbon.
Meanwhile, the Lewis structure of \({\text{CH}}_{3} \text{Cl}\) places chlorine and hydrogen atoms around the central carbon. Chlorine, having more valence electrons, also shows three lone pairs in its Lewis structure. These diagrams are essential for visualizing how atoms are connected and help in predicting the molecule's shape and polarity.

Dipole Moments

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It results from the sum of all individual bond dipoles in a molecule and is a vector quantity, meaning it has both direction and magnitude.
In \({\text{CH}}_{4}\), the symmetry ensures that any dipole moments from the C-H bonds cancel each other out, leading to an overall nonpolar molecule. This means \({\text{CH}}_{4}\) has no net dipole moment.
On the other hand, in \({\text{CH}}_{3} \text{Cl}\), the C-Cl bond introduces a significant dipole moment due to the high electronegativity difference. This dipole doesn’t cancel out because of the asymmetry, resulting in an overall polar molecule. Thus, dipole moments are critical for determining the molecular polarity.