Simple Molecular Compounds

Summary: Simple molecular compounds are substances composed of discrete molecules held together internally by strong covalent bonds; their low melting/boiling points and poor electrical conductivity are explained by weak intermolecular forces between molecules, not by the covalent bonds within them. Tags: igcse chemistry bonding structures properties Created: 2026-07-18

Simple molecular compounds are substances whose structures consist of individual Simple Molecules — small, discrete groups of atoms covalently bonded together, with only weak intermolecular forces acting between neighbouring molecules. These compounds include familiar substances such as water (H2O), carbon dioxide (CO2), methane (CH4), hydrogen (H2), oxygen (O2), nitrogen (N2), the hydrogen halides (HCl, HF, HBr), and many organic compounds such as those discussed in Alkanes, Alkenes, and Alcohols. The physical properties of simple molecular compounds are dominated not by the strong covalent bonds within each molecule, but by the much weaker forces between molecules. Because only these weak intermolecular forces must be overcome during melting, boiling, or evaporation, simple molecular compounds typically have low melting and boiling points and are often gases or volatile liquids at room temperature. This distinction — strong intramolecular bonds versus weak intermolecular forces — is the single most important concept for understanding why these substances behave as they do.

Properties of Simple Molecular Compounds

The physical properties of simple molecular compounds are determined by their structure: discrete molecules with strong covalent bonds inside and weak intermolecular forces outside.

Low Melting and Boiling Points

Simple molecular compounds have low melting points and low boiling points. Most are gases or volatile liquids at room temperature.

Explanation: When a simple molecular substance melts or boils, energy is used to overcome the weak intermolecular forces between neighbouring molecules. The strong covalent bonds within each molecule are not broken — the molecules remain intact as they separate from each other. Because intermolecular forces are weak (typically 1—40 kJ/mol compared to 150—800 kJ/mol for covalent bonds), only a small amount of energy is needed, resulting in low melting and boiling points.

In the case of water boiling, the hydrogen bonds (intermolecular forces) between H2O molecules are overcome. The O—H covalent bonds within each water molecule remain intact — steam is still H2O.

Trends within simple molecular compounds:

• Larger molecules generally have higher mp/bp because they have more electrons, creating stronger instantaneous dipole-induced dipole (London dispersion) forces. For example, in the alkane series, CH4 boils at -162 degrees C while C8H18 boils at 126 degrees C.
• Molecules capable of hydrogen bonding (H bonded to N, O, or F) have anomalously high mp/bp compared to similarly sized molecules without hydrogen bonding. Water (H2O, bp 100 degrees C) is a liquid at room temperature while the larger molecule methane (CH4, bp -162 degrees C) is a gas, because water molecules form strong hydrogen bonds between them.

Poor Electrical Conductivity

Simple molecular compounds do not conduct electricity in any state — solid, liquid, or gas.

Explanation: Electrical conductivity requires mobile charged particles — either free-moving ions (as in molten ionic compounds or aqueous solutions) or delocalised electrons (as in metals and graphite). Simple molecular compounds have neither:

• They contain no ions — the atoms within molecules are neutral and held by covalent bonds.
• They contain no delocalised electrons — all electrons are either in covalent bonds or in lone pairs, confined to individual molecules.

Even when dissolved in water, most simple molecular substances do not conduct because they do not dissociate into ions. Exceptions occur when a substance reacts with water to form ions — for example, hydrogen chloride (HCl) gas does not conduct, but when dissolved in water it ionises to form H+ and Cl- ions, and the resulting hydrochloric acid does conduct. In such cases, it is the aqueous solution that conducts, not the original molecular substance itself.

Other Properties

Solubility: Simple molecular compounds tend to be insoluble in water unless they can form hydrogen bonds (e.g., ethanol, ammonia, small alcohols) or react with water. They are often soluble in organic solvents such as hexane or tetrachloromethane. The general rule is “like dissolves like” — polar simple molecules dissolve in polar solvents; non-polar simple molecules dissolve in non-polar solvents.

Softness: Solid simple molecular compounds (e.g., iodine, I2) are typically soft and easily crushed. The weak intermolecular forces between molecules offer little resistance to deformation.

Comparison with Other Structure Types

The table below compares the characteristic properties of simple molecular compounds with those of ionic compounds and giant covalent structures:

This comparison makes clear that every distinctive property of simple molecular compounds — low mp/bp, non-conductivity, softness, volatility — traces back to a single structural fact: the particles are neutral molecules held together by weak intermolecular forces, not by the strong covalent bonds within them.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5 (Simple molecules and covalent bonds), Core 3 and Supplement 5, Cambridge Assessment International Education

Common Misconceptions