Simple Molecular Structures

Simple molecular substances consist of small, discrete molecules held together by weak intermolecular forces. The properties of these substances are determined primarily by the strength of the intermolecular forces between molecules — not by the strength of the covalent bonds within them. Understanding this distinction is central to explaining the physical properties of simple molecular compounds.

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What Are Simple Molecular Substances?

A simple molecular substance is composed of individual molecules, each containing a fixed, small number of atoms covalently bonded together. Examples include water (H₂O), carbon dioxide (CO₂), methane (CH₄), oxygen (O₂), iodine (I₂), glucose (C₆H₁₂O₆), and sulfur (S₈).

The key structural feature is that strong covalent bonds exist within each molecule, but only weak intermolecular forces (London, dipole–dipole, or hydrogen bonding) act between molecules. It is the weak intermolecular forces that must be overcome during melting and boiling — not the covalent bonds.

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Low Melting and Boiling Points

Simple molecular substances generally have low melting and boiling points because the intermolecular forces between molecules are weak. It takes relatively little energy to separate the molecules from each other (to melt or boil the substance), even though the covalent bonds within each molecule are very strong.