The Greek philosopher Democritus proposed that everything on Earth is made up of tiny particles called atomos, Greek for "indivisible," from which the modern term "atom" is derived. In the 19th century, John Dalton proposed the atomic theory that is still largely correct today. He put forth five postulates to explain how atoms made up the world around us. (1) All matter is composed of infinitely small particles or atoms. (2) All atoms of a given element are identical to one another and (3) are different from the atoms of all other elements. (4) Two or more elements can combine in a fixed ratio to form a compound. (5) Atoms cannot be created nor destroyed in a chemical reaction, but can rearrange to form new substances.

Dalton was only partially correct about the particles that make up matter. While atoms cannot be broken down further by ordinary chemical or physical processes, they are composed of three smaller subatomic particles. The first clue about the subatomic structure came when J.J. Thomson discovered the electron. Scientists knew that the overall charge of an atom was neutral, but Thomson's "plum pudding model" of the atom provided new information regarding the existence of a negatively-charged particle, suggesting that electrons were found studded throughout a positively charged area. Later, Ernest Rutherford performed an experiment showing that most of an atom's mass is concentrated in the nucleus, where protons account for an atom's positive charge, and the tiny negatively-charged electrons make up most of the space outside of the nucleus. This disproved Thomson's plum pudding model. In 1932, James Chadwick discovered that the neutron accounted for the mass present in an atom.

According to the currently accepted theory, protons are found in the nucleus of an atom, have a positive charge, and have a mass of one atomic mass unit (AMU) each. The number of protons equals the atomic number on the periodic table and determines the element's identity. Neutrons are also found in the nucleus. They have no charge but have the same mass as protons, contributing to an atom's atomic mass. Electrons orbit around the nucleus in clouds. They have a negative charge and negligible mass, so they contribute to an atom's overall charge but not its mass.