Concept: Intermolecular Forces

Intermolecular forces are attractive forces that exist between molecules. The type and strength of these forces determine many physical properties of substances, such as melting point, boiling point, and solubility.

Why (A) is correct:

Iodine (I₂) is a nonpolar molecule. It consists of two identical iodine atoms bonded covalently. In nonpolar molecules, the only type of intermolecular forces present are London dispersion forces (also known as instantaneous dipole-induced dipole forces or van der Waals forces). These forces arise from temporary fluctuations in electron distribution, creating transient dipoles that induce dipoles in neighboring molecules, leading to weak attractive forces. Since iodine is a large molecule with many electrons, its London forces are significant enough to hold it in a solid lattice at room temperature.

Option Analysis:

• A) London forces: Correct. Iodine is a nonpolar molecule, and London forces are the only intermolecular forces present.
• B) dipole-dipole interactions: Incorrect. Dipole-dipole interactions occur between polar molecules, which have permanent dipoles. Iodine is nonpolar.
• C) covalent bonds: Incorrect. Covalent bonds exist within the I₂ molecule (holding the two iodine atoms together), but they do not hold the individual I₂ molecules together in the solid lattice.
• D) coulombic force: Incorrect. Coulombic forces (electrostatic forces) are typically associated with ionic bonds (attraction between oppositely charged ions) or strong permanent dipoles. While London forces are ultimately electrostatic in nature, the term 'coulombic force' is generally not used to describe the weak, transient interactions between nonpolar molecules.

Correct Answer: (A)