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Forming a covalent bond

A covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals.

These shared electrons are found in the outer shells of the atoms. In general, each atom contributes one electron to the shared pair of electrons.

The slideshow shows how a covalent bond forms between a hydrogen atom and a chlorine atom, making hydrogen chloride.

Molecules

A molecule consists of a group of two or more atoms joined together by covalent bonds. Molecules of the same element or compound will have a set size - in other words, they will always contain the same number of atoms of each element. For example, a molecule of methane, CH₄, will always contain one carbon atom and four hydrogen atoms.

Sizes of atoms and simple molecules

A simple molecule contains only a few atoms, so the sizes of atoms and simple molecules have a similar range of sizes - they are very small, typically around 0.1 nm or 10^-10 m across.

Individual atoms and molecules are too small to see even with the strongest light microscope. Some electron microscopes can produce images of atoms and simple molecules.