Which of the following molecules would you expect to have a dipole moment of zero?
a. CH₃CH₃
b. H₂C═O
c. CH₂Cl₂

Verified step by step guidance

1. Step 1: Understand the concept of dipole moment. A dipole moment arises when there is an uneven distribution of electron density in a molecule due to differences in electronegativity between bonded atoms. A molecule will have a dipole moment of zero if it is symmetrical and the bond dipoles cancel each other out.
2. Step 2: Analyze molecule (a) CH3CH3 (ethane). Ethane is a nonpolar molecule because it consists of two identical CH3 groups connected by a single bond. The molecule is symmetrical, and there is no net dipole moment as the bond dipoles cancel out.
3. Step 3: Analyze molecule (b) H2C═O (formaldehyde). Formaldehyde contains a polar C═O bond due to the significant electronegativity difference between carbon and oxygen. The molecule is not symmetrical, so the dipole moment does not cancel out, and the molecule has a net dipole moment.
4. Step 4: Analyze molecule (c) CH2Cl2 (dichloromethane). Dichloromethane has two polar C-Cl bonds and two C-H bonds. The molecule is not symmetrical because the chlorine atoms are more electronegative than hydrogen, leading to an uneven distribution of electron density. Therefore, CH2Cl2 has a net dipole moment.
5. Step 5: Conclude that the molecule with a dipole moment of zero is (a) CH3CH3, as it is the only molecule in the list that is symmetrical and nonpolar.

Key Concepts

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge across a molecule. A molecule is polar if it has a net dipole moment due to differences in electronegativity between atoms, leading to uneven charge distribution. Conversely, nonpolar molecules have symmetrical charge distributions, resulting in a dipole moment of zero.

Dipole Moment

The dipole moment is a vector quantity that measures the separation of positive and negative charges in a molecule. It is calculated as the product of the charge and the distance between the charges. A dipole moment of zero indicates that the molecule is nonpolar, meaning that the individual bond dipoles cancel each other out due to symmetry.

Symmetry in Molecules

Symmetry plays a crucial role in determining the polarity of a molecule. Molecules with symmetrical shapes, such as linear or tetrahedral arrangements, often have their dipole moments cancel out, resulting in nonpolar characteristics. In contrast, asymmetrical molecules typically exhibit a net dipole moment, indicating polarity.