[ at-uhm ]
See synonyms for atom on
  1. Physics.

    • the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.

    • an atom with one of the electrons replaced by some other particle: muonic atom;kaonic atom.

  2. Energy. this component as the source of nuclear energy.

  1. a hypothetical particle of matter so minute as to admit of no division.

  2. anything extremely small; a minute quantity.

Origin of atom

First recorded in 1350–1400; Middle English attomos, athomus, from Latin atomus, from Greek átomos, noun use of átomos “undivided,” equivalent to a- a-6 + tomós “divided,” from témnein “to cut”

Other words for atom

Words Nearby atom Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use atom in a sentence

British Dictionary definitions for atom


/ (ˈætəm) /

    • the smallest quantity of an element that can take part in a chemical reaction

    • this entity as a source of nuclear energy: the power of the atom See also atomic structure

  1. any entity regarded as the indivisible building block of a theory

  1. the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter: See also atomism

  2. a very small amount or quantity; minute fragment: to smash something to atoms; there is not an atom of truth in his allegations

Origin of atom

C16: via Old French and Latin, from Greek atomos (n), from atomos (adj) that cannot be divided, from a- 1 + temnein to cut

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for atom


[ ătəm ]

  1. The smallest unit of an element, consisting of at least one proton and (for all elements except hydrogen) one or more neutrons in a dense central nucleus, surrounded by one or more shells of electrons. In electrically neutral atoms, the number of protons equals the number of electrons. Atoms remain intact in chemical reactions except for the removal, transfer, or exchange of certain electrons. Compare compound. See also ion isotope orbital.

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for atom


A unit of matter; the smallest unit of a chemical element. Each atom consists of a nucleus, which has a positive charge, and a set of electrons that move around the nucleus. (See Bohr atom.)

Notes for atom

Atoms link together to form molecules.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.