An atom is the smallest part of a simple body that can chemically combine with others to form matter.

For example, pure water is composed of two hydrogen atoms and an oxygen atom.

In a glass of water there are billions upon billions of atoms.

Atoms are the basic constituents of all solid, liquid and gaseous matter.

The physical and chemical properties of these materials are determined by the atoms that make them up as well as by the three-dimensional arrangement of these atoms.

The chemical properties of atoms are determined by their electronic configuration, which is derived from the number of protons in their nuclei.

This number, called the atomic number, defines the chemical element.

An atom is roughly a nucleus consisting of protons and neutrons around which electrons are attracted apart.

If we enlarge the nucleus to the size of a ping-pong ball, an electron smaller than a grain of sand will be attracted a few kilometers away from the ping-pong ball.

This makes most of the atom's volume empty