Portions of this entry contributed by Leonardo Motta

An atom is an electrically neutral body consisting of a nucleus (consisting of protons and neutrons) and a cloud of electrons equal in number to the number of protons in the nucleus. The number of protons in an atom is called its atomic number, and uniquely identifies a chemical element . The atom is smallest unit of matter which can take part in a chemical reaction. The simplest atom is the hydrogen atom, whose nucleus consists of a single proton. There are roughly 10⁷⁸ atoms in the observable universe.

The concept of the atom was first introduced by the Greek philosophers Democritus and Leucippus around 450-420 B.C. The word "atom" derives from the Greek for "indivisible," which was the original concept of atom considered by the Greek atomist philosophers (although it is now known that atoms are composed of a number of different elementary particles). Dalton (1803) made several discoveries about atoms in the course of his study of gases. In particular, he proposed that elements consist of collections of identical atoms that differ from the atoms of other elements, and also that atoms can chemically combine with one another to form a "strong union."

In April 1897, J. J. Thomson reported the discovery of a radiation from a cathode ray tube that was different from Röntgen's X-rays and was composed by very tiny particles, smaller than an atom and negative charged. In the same year as Thomson's discovery, Heinrich Hertz found that zinc negatively charged emits negative charged radiation when exposed to an ultraviolet radiation. Based on his experiments, Thomson introduced the famous "plum pudding model" of the atom in which the negative charge particles are placed around a concentration of positively charged particles, called protons (corresponding to the "plums" in a "pudding").

However, the plum pudding model was soon rejected by Rutherford, who introduced the theory that electrons are actually in orbit around the nuclei, based on his famous scattering experiment (1911). In 1932, James Chadwick discovered a companion particle of the proton found in nuclei: the neutron. By the end of 1940, Maria Goeppert-Mayer, Eugene Wigner, and others proposed that neutrons and protons are in a successively "shells" in the nucleus, held together by a very strong force, appropriately dubbed the "strong force."

Subsequently, the development of quantum mechanics by of Niels Bohr, Sommerfeld, Heisenberg, and Schrödinger led to the understanding of an atom as a nucleus surrounded by a "cloud" (in quantum mechanical terminology, a superposition of probability density function) of electrons.

Bohr Model, Electron, Hydrogen Atom, Ion, Neutron, Nucleus, Plum Pudding Model, Positronium, Proton, Sommerfeld Model