• noun the smallest amount of a substance which can take part in a chemical reaction


  • noun a value or string that cannot be reduced to a simpler form


  • The smallest particle that retains all the properties of an element. Atoms are composed of protons, neutrons, and electrons, with protium, which is hydrogen-1, being the sole exception as it has no neutrons. The positively charged proton and neutral neutron are clustered together in the nucleus. The negatively charged electrons revolve in energy levels surrounding the nucleus. Atoms are overall neutral in charge, as the number of electrons equals the number of protons. If the number of electrons is not equal to the number of protons, you have a charged atom, which is a type of ion.
  • In computers, the smallest element within an information structure. For example, a character or a pixel.


  • noun a fundamental unit of a chemical element and the smallest part of an element that can exist independently


  • noun the smallest part into which an element can be divided and still keep its properties. It consists of a dense, positively charged nucleus surrounded by a system of electrons.


  • noun the smallest unit of a chemical element, which can be used as a source of nuclear energy

Origin & History of “atom”

Etymologically, atom means ‘not cut, indivisible’. Greek átomos ‘that which cannot be divided up any further’ was formed from the negative prefix a- ‘not’ and the base *tom-‘cut’ (source also of English anatomy and tome), and was applied in the middle Ages not just to the smallest imaginable particle of matter, but also to the smallest imaginable division of time; an hour contained 22,560 atoms. Its use by classical writers on physics and philosophy, such as Democritus and Epicurus, was sustained by medieval philosophers, and the word was ready and waiting for 19th-century chemists when they came to describe and name the smallest unit of an element, composed of a nucleus surrounded by electrons.