Atoms are the basic structural units of all materials. The constitution of the atoms and the bonds between them determine the chemical, physical and mechanical properties of materials.

Atoms are mainly composed of three subatomic particles:

• protons
• electrons
• neutrons.

Protons have a positive electric charge, electrons have an equal negative electric charge, and neutrons have no electric charge; that is, they are electrically neutral. The atoms of each element contain an equal number of protons and electrons and are therefore electrically neutral.

The simplified model most frequently used today to describe atomic behaviour depicts the atom as consisting of a nucleus, where protons and neutrons are located, surrounded by electrons.

In the quantum model, the location of electrons in the atom can only be specified in probabilistic terms. The regions around the nucleus where electrons are most likely to be found are called orbitals, often represented by electron clouds of variable density.

Electrons, particularly those outermost ones – valence electrons – play a very important role in the bonds that atoms establish.