Covalent bonding is the primary mechanism by which non-metal atoms interact to form molecules. Unlike ionic bonding, which involves the complete transfer of electrons, covalent bonding is based on the mutual sharing of electron pairs. This sharing allows atoms to achieve a stable, full outer shell of electrons—the "noble gas configuration"—which is the driving force behind chemical stability. The IGCSE 0654 Extended syllabus requires a detailed description of covalent bond formation and an explanation of the physical properties of simple molecular substances.

A covalent bond is formed when a pair of electrons is shared between two atoms.

Mechanism and Dot-and-Cross Diagrams

When two non-metal atoms approach each other, their outer electron shells overlap. One or more pairs of electrons are shared such that both atoms effectively "count" the shared electrons as part of their own full outer shell.

• Hydrogen ($H_2$): Two hydrogen atoms share one pair of electrons (a single bond) to achieve the Helium configuration (2 electrons).
• Water ($H_2O$): One oxygen atom shares a pair of electrons with each of two hydrogen atoms.
• Methane ($CH_4$): One carbon atom shares four pairs of electrons with four hydrogen atoms.

Most covalent substances exist as simple molecules. In these substances, atoms are held together by very strong covalent bonds within the molecule (intramolecular forces). However, the individual molecules are only weakly attracted to their neighbors by intermolecular forces.

The macroscopic behavior of these substances is a direct consequence of the weak forces existing between the molecules.

Low Melting and Boiling Points

While the bonds holding the atoms together inside a molecule are strong, the intermolecular forces attracting one molecule to another are very weak. Consequently, very little thermal energy is required to overcome these weak forces and separate the molecules. This is why most simple molecular substances (like $O_2, CO_2,$ and $CH_4$) are gases or liquids at room temperature and have low melting and boiling points.

Electrical Conductivity

Electrical conductivity requires mobile charged particles (ions or delocalised electrons).

• Neutral Molecules: Covalent molecules are electrically neutral; they do not carry an overall charge.
• Lack of Mobility: There are no free ions or delocalised electrons to carry an electric current.

As a result, simple molecular compounds are poor conductors of electricity in both solid and liquid states.

Covalent bonding is the fundamental architect of the organic and atmospheric world. By sharing electron pairs, non-metal atoms create precise molecular geometries. While the internal bonds of these molecules are incredibly robust, the weak external attractions between them result in the volatile and non-conductive nature of the substances that sustain life and power the climate.