Atom

An atom is the fundamental unit of matter that defines the chemical elements. It is composed of three main types of subatomic particles: a central nucleus containing protons, which carry a positive electric charge, and neutrons, which have no charge; surrounding the nucleus is a cloud of electrons, negatively charged particles bound to the nucleus by the electromagnetic force. The nucleus contains protons and neutrons, collectively called nucleons, and accounts for nearly all of an atom’s mass. The overall size of an atom is about a tenth of a nanometer, while the nucleus itself is much smaller.

An atom’s identity is determined by its atomic number, Z, which is the number of protons in the nucleus. The total number of protons and neutrons gives the mass number, A; atoms with the same Z but different A are isotopes. Electrons occupy orbitals in a quantum-mechanical cloud around the nucleus, and their arrangement—electron configuration—determines an atom’s chemical properties and reactivity. In neutral atoms, the number of electrons equals the number of protons; adding or removing electrons produces ions with net electric charge.

Forces at work include the strong nuclear force, which binds protons and neutrons in the nucleus, and the electromagnetic force, which governs the attraction between negatively charged electrons and the positively charged nucleus. Atoms bond with other atoms to form molecules, the basis of all materials and life. The concept of the atom has evolved through history, from early simple models to the modern quantum mechanical description that explains observed spectra and chemical behavior.