An atom is the smallest unit of matter that has the properties of a chemical element. The size of an atom is around 100 picometers.

The word "Atom" is derived from the Greek word "Atomos" which translates to something "uncuttable"("a" = not, "tomos"= cut).

The earliest recorded discussions of the basic structure of matter come from ancient Greece and continue until the current years. For a summary, you can read about the evolution of atomic theory.

The atom contains 3 subatomic particles, known as neutrons, protons, and electrons, neutrally, positively and negatively charged respectively. The neutrons and protons exist in the nucleus of the atom, and the electrons orbit around the nucleus. The nucleus is a small dense area in the center of the atom.

The number of protons represents the Atomic Number of an atom, and the sum of both neutrons and protons reflects the Mass number of an atom. Since protons and electrons have an opposite charge, the difference in their number reflects their charge. Charged atoms are called ions.

On the left, 5 yellow neutrons shown as circles and 4 red protons shown as circles are clumped together in the nucleus. Two orbitals surround the nucleus, both containing 2 blue electrons. On the right, a clump of yellow neutron circles plus a clump of red proton circles equals clump of mixed red and yellow circles labeled mass number.

Figure 1 The components of an atom