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An atom is said to be the smallest particle of any substance or piece of matter that still retains the properties of that substance. An atom usually consists of protons, electrons, and neutrons, but in one solitary case, the lightest isotope of hydrogen has none.

An isotope is defined by how many neutrons there are in the atom, and atoms of different elements can have varying amounts of neutrons. A certain amount of any element will usually contain the same ratio of each isotope.

In an atom there is a large amount of empty space. Protons and neutrons are clustered together, with an electron cloud surrounding it. Electrons are 1840 times smaller than protons, and orbit the protons and neutrons (together they are called a nucleon). In an atom, for every proton there is an electron, as a proton contains a positive charge, and an electron a negative one. If an atom loses electrons, usually by bonding with other atoms, it becomes an ion. Ions are less stable than atoms and will try to bond more vigorously with other atoms if they have not already.

Atoms are the building blocks of materials. The form the elements, they bond with other atoms in certain structures to create new materials. If they bond in different ways with other atoms of the same type, and result in different substances, they are allotropes, for example, graphite and diamond are both allotropes of carbon. When they bond with different atoms, they form molecules, and molecules make up compounds, for example, when hydrogen and oxygen atoms bond, they can form a structure where there are two hydrogen atoms and one oxygen atom, forming H2O or water.

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