atom
Americannoun
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atoms
plural
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Physics.
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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.
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an atom with one of the electrons replaced by some other particle.
muonic atom;
kaonic atom.
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Energy. this component as the source of nuclear energy.
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a hypothetical particle of matter so minute as to admit of no division.
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anything extremely small; a minute quantity.
noun
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the smallest quantity of an element that can take part in a chemical reaction
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this entity as a source of nuclear energy See also atomic structure
the power of the atom
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any entity regarded as the indivisible building block of a theory
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the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter See also atomism
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a very small amount or quantity; minute fragment
to smash something to atoms
there is not an atom of truth in his allegations
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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.
Discover More
Atoms link together to form molecules.
Other Word Forms
Noun Inflected Forms
Etymology
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”
Explanation
An atom is the basic unit of an element. When you see the chemical formula for water, H2O, it's telling you that each molecule of water is made up of two atoms of hydrogen and one atom of oxygen. In science class, you've probably come across atoms, the bits that make up molecules. Less scientifically, the word atom can also mean a very small piece of anything at all. The Greek root of atom is atomos, which means "indivisible," since the scientists who first gave the atom its name imagined it couldn't be split or divided into smaller pieces. Even though we know there are smaller things that make up an atom (protons, neutrons and electrons), it's still a good way to think about the word atom.
Vocabulary lists containing atom
Quantum of Vocabulary: the Parlance of Particle Physics
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Word Generation Science - Introductory Chemistry Concepts
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Matter and Energy 1: Atoms and Molecules
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
See Examples For:
Before testing the first atom bomb, the Los Alamos crew asked Hans Bethe to confirm with math that it wouldn’t start a chain reaction and consume the atmosphere.
From The Wall Street Journal ● Sep. 18, 2026
Rather than treating the inductive effect as an influence that gradually travels along a chain of atoms, the team's interpretation confines it to the bond directly connected to the relevant atom in a neutral molecule.
From Science Daily ● Sep. 14, 2026
This effectively allowed each atom to follow two paths at once.
From Science Daily ● Sep. 8, 2026
Rydberg atom simulators are one possible platform for such an experiment and could provide a promising route toward finally detecting these elusive quantum quasiparticles.
From Science Daily ● Sep. 7, 2026
Eventually two other Cavendish scientists invented a more powerful proton-beam device, while in California Ernest Lawrence at Berkeley produced his famous and impressive cyclotron, or atom smasher, as such devices were long excitingly known.
From "A Short History of Nearly Everything" by Bill Bryson
"Superfluid helium is what is known as a quantum fluid, in which the individual helium atoms lose their identity, and which does not tolerate any impurities within it."
From Science Daily ● Sep. 18, 2026
The muonium atoms must pass through the quantum liquid without collisions and at a predictable speed.
From Science Daily ● Sep. 18, 2026
Those chlorine atoms then reacted with methane in the plume, helping destroy some of the methane released during the eruption.
From Science Daily ● Sep. 15, 2026
Organic chemists routinely study chains of atoms that form the molecular foundations of medicines, advanced materials, agrochemicals, polymers and many other technologies used in everyday life.
From Science Daily ● Sep. 14, 2026
Hold my hand, and the atoms on the surface of your skin interact with the atoms on mine.
From "Meltdown" by Deirdre Langeland
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
