Advanced Placement (AP) Chemistry Practice Exam

What is measured by a dipole moment?

1. The total number of atoms in a molecule

2. The polarity of a molecule

3. The molecular weight of a substance

4. The energy level of electrons in an atom

The correct answer is: The polarity of a molecule

The dipole moment is a quantitative measure of the polarity of a molecule. It reflects the separation of positive and negative charges within the molecule, indicating how much of an electric charge is unevenly distributed across its structure. A molecule with a dipole moment has areas of partial positive and partial negative charge due to differences in electronegativity between atoms. For instance, in a molecule like water (H2O), the oxygen atom is more electronegative than the hydrogen atoms, resulting in an imbalance of charge. This creates a dipole moment pointing toward the oxygen atom, signifying that the molecule behaves as a polar entity. Therefore, the dipole moment serves as a crucial parameter in understanding molecular interactions, solubility, and boiling/melting points, being a fundamental concept in molecular chemistry and a key indicator of whether a substance is polar or nonpolar.