Which of the following molecules would you expect to have a dipole moment of zero?
d. NH₃
e. H₂C═CH₂
f. H₂C═CHBr

Verified step by step guidance

1. Analyze the molecular geometry of each molecule. The dipole moment depends on the shape of the molecule and the distribution of electron density. For NH₃, the geometry is trigonal pyramidal due to the lone pair on nitrogen, which creates an asymmetrical distribution of charge.
2. For H₂C═CH₂ (ethylene), examine the molecular geometry. The molecule is planar, and the two carbon atoms are sp² hybridized. The symmetry of the molecule causes the dipole moments of the C-H bonds to cancel out.
3. For H₂C═CHBr, consider the geometry and the presence of the bromine atom. Bromine is more electronegative than hydrogen, creating a polar bond. The molecule is not symmetrical, so the dipole moments do not cancel out.
4. Compare the symmetry and electronegativity differences in each molecule. Molecules with symmetrical geometries and equal distribution of electronegativity (like H₂C═CH₂) will have a dipole moment of zero, while asymmetrical molecules (like NH₃ and H₂C═CHBr) will have a net dipole moment.
5. Conclude that H₂C═CH₂ is the molecule with a dipole moment of zero due to its symmetrical planar geometry and equal distribution of charge.

Key Concepts

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It occurs when there is an uneven distribution of electron density, leading to a polar bond. Molecules with a dipole moment have a positive end and a negative end, while those with a dipole moment of zero are nonpolar, meaning their charge distribution is symmetrical.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The shape of a molecule significantly influences its dipole moment. For example, molecules with symmetrical shapes, such as linear or tetrahedral arrangements, often have dipole moments of zero, as the individual bond dipoles cancel each other out.

Polarity of Bonds

The polarity of bonds is determined by the difference in electronegativity between the atoms involved. When two atoms with different electronegativities form a bond, the more electronegative atom attracts the shared electrons more strongly, creating a polar bond. In contrast, bonds between identical atoms or atoms with similar electronegativities are nonpolar, contributing to the overall polarity of the molecule.