An atom is the basic unit that makes up all matter. There are many different types of atoms, each with its own name, mass and size. These different types of atoms are called chemical elements. The chemical elements are organized on the periodic table. Examples of elements are hydrogen and gold. Atoms are very small, but the exact size changes depending on the element. Atoms range from 0.1 to 0.5 nanometers in width. One nanometer is around 100,000 times smaller than the width of a human hair. This makes atoms impossible to see without special tools. Equations must be used to see the way they work and how they interact with other atoms.
Atoms come together to make molecules or particles: for example, two hydrogen atoms and one oxygen atom combine to make a water molecule, a form of a chemical reaction.

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