Chapter 4: Problem 68
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
Expert verified
\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
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
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.
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.
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.
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.
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.
