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:
Each slit functions like an artificial atom, or meta-atom.
From Science Daily ● Aug. 8, 2026
Their results show that exposure to light can make an atom more responsive to the movement of neighboring atoms.
From Science Daily ● Jul. 30, 2026
Of course, so much of what’s otherwise known points a finger directly back at Taylor Farms, false-positive atom splitting notwithstanding, and there’s only so much the company can do to deflect.
From Slate ● Jul. 22, 2026
This frustrated Marcus because he otherwise liked the innovative theory that had been proposed by Willard Libby, a physicist who had helped develop the atom bomb.
From Los Angeles Times ● Jul. 17, 2026
Using Avogadro’s mathematics, chemists were eventually able to work out, for instance, that a typical atom had a diameter of 0.00000008 centimeters, which is very little indeed.
From "A Short History of Nearly Everything" by Bill Bryson
"You can decompose a thought into subcomponents, like little atoms of logical propositions, and you can combine them in a hierarchical manner to make more complex structured rules, very akin to language."
From Science Daily ● Aug. 11, 2026
During oxygen evolution, they create conditions that make it easier for oxygen atoms to join together.
From Science Daily ● Aug. 10, 2026
During oxygen evolution, the silver instead helps oxygen atoms combine into oxygen molecules and then supports their release.
From Science Daily ● Aug. 10, 2026
At ordinary temperatures, heat causes atoms to vibrate constantly.
From Science Daily ● Aug. 8, 2026
However, the box the banana is in is made of atoms, too.
From "Zero: The Biography of a Dangerous Idea" by Charles Seife
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.
