Simple Molecules

Summary: Simple molecules are small, discrete groups of atoms held together internally by strong covalent bonds, existing as separate units with only weak intermolecular forces between them — in contrast to giant structures where bonding extends continuously throughout. Tags: igcse chemistry bonding molecules structures Created: 2026-07-18

A simple molecule is a small, discrete group of a fixed number of atoms held together by covalent bonds. The atoms within the molecule are joined by strong shared-electron attractions, but the molecules themselves exist as separate, independent units with only weak intermolecular forces (van der Waals’ forces) attracting one molecule to another. Common examples include the diatomic elements H2, O2, N2, Cl2, and compounds such as H2O, CO2, CH4, NH3, and HCl. Simple molecules are fundamentally different from giant structures (ionic, covalent, or metallic), where bonding extends continuously in all directions and the entire crystal is essentially one enormous bonded network rather than a collection of individual units. The term “simple molecular” describes the structure type; the resulting substances are discussed in detail under Simple Molecular Compounds.

What Defines a Simple Molecule?

A simple molecule has three defining characteristics:

1. Fixed number of atoms per unit: Each molecule has a definite atomic composition (e.g., H2O always has 2 H and 1 O). The Molecule is the smallest unit of the substance that retains its chemical properties.

2. Strong covalent bonds within the molecule: The atoms are held together by shared electron pairs. These intramolecular bonds require substantial energy to break (typically 150—800 kJ/mol).

3. Weak intermolecular forces between molecules: Once formed, each molecule interacts only weakly with neighbouring molecules. These are van der Waals’ (London dispersion) forces, or in some cases hydrogen bonds (as in H2O, NH3). These intermolecular forces are typically 1—40 kJ/mol — orders of magnitude weaker than covalent bonds.

Common Simple Molecules at IGCSE

All of these substances exist as individual molecules with the formula accurately representing one discrete unit. This is in contrast to giant structures where the formula (e.g., NaCl, SiO2) represents only the ratio of atoms or ions in the lattice.

Simple Molecules vs Giant Structures

The table below contrasts simple molecular substances with the three types of giant structures:

The key conceptual distinction: in simple molecular substances, the forces that must be overcome during melting or boiling are the weak intermolecular forces, not the strong covalent bonds. The molecules remain intact as separate units; they are merely separated from each other. By contrast, melting a giant structure requires breaking the strong bonds that extend throughout the lattice.

Within the Molecule: Covalent Bonds

The covalent bonds inside a simple molecule follow the same principles described in Covalent Bond:

• Atoms share electrons to achieve noble gas electronic configurations.
• Each atom in the molecule (except H) is surrounded by 8 outer electrons counting both shared and lone pairs.
• Hydrogen follows the duplet rule (2 electrons).
• The molecular shape is determined by the number of bonding pairs and lone pairs around the central atom (VSEPR theory).

For example, in a water molecule (H2O), oxygen shares one electron pair with each hydrogen (two single bonds) and retains two lone pairs. The O—H bonds are strong covalent bonds. The molecule is bent (V-shaped) due to the repulsion between the two lone pairs and the two bonding pairs.

Between the Molecules: Intermolecular Forces

Between separate simple molecules, only weak intermolecular forces act. These include:

• London dispersion forces (instantaneous dipole-induced dipole): present between all molecules. Strength increases with molecular size (more electrons = stronger forces).
• Permanent dipole-dipole forces: present when molecules are polar (e.g., HCl, where Cl is more electronegative than H).
• Hydrogen bonds: a particularly strong type of intermolecular force that occurs when H is bonded to N, O, or F (e.g., in H2O, NH3).

These forces are responsible for the physical properties of simple molecular substances — see Simple Molecular Compounds for a detailed discussion of melting points, boiling points, and electrical conductivity.

Sources

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

Common Misconceptions