atom

American  

[at-uhm] / ˈæt əm /

noun

atoms plural

1. Physics.

   1. 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.

   2. 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.

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

4. anything extremely small; a minute quantity.

   Synonyms:

   whit, jot, iota, scintilla, speck, shred

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atom British  

/ ˈætəm /

noun

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

   2. this entity as a source of nuclear energy See also atomic structure

      the power of the atom

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

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

4. 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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atom Scientific  

/ ăt′əm /

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.

2. Compare compound See also ion isotope orbital

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atom Cultural  

1. 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.)

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Discover More

Atoms link together to form molecules.

Other Word Forms

Noun Inflected Forms

• atoms
  plural

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.

Example Sentences

See Examples For:

atom atoms

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

The approach relies on a diamond containing a microscopic magnetic defect formed by a nitrogen atom and a missing neighboring carbon atom.

From Science Daily ● Jun. 23, 2026

So when my Google searches treat my request for info about sitcom intros of years past like splitting the atom, I get suspicious.

From Salon ● Jun. 10, 2026

For an atom, uranium 235 has a pretty enormous nucleus, packed with 92 protons and 143 neutrons.

From "Meltdown" by Deirdre Langeland

Researchers from the University of Toronto, L'École Normale Supérieure in Paris and Lehigh University in Pennsylvania reached the finding by studying ultracold potassium atoms cooled to nearly absolute zero.

From Science Daily ● Jul. 31, 2026

Dislocations are microscopic irregularities in a crystal where atoms are no longer aligned in a perfectly ordered pattern.

From Science Daily ● Jul. 31, 2026

Using rapid X-ray flashes produced at the European XFEL, the researchers followed changes at specific atoms as the molecule released and redistributed the absorbed energy.

From Science Daily ● Jul. 30, 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

“Even though only about fifteen atoms were involved, they kept falling out of the awkward pincers set up to hold them.”

From "The Gene" by Siddhartha Mukherjee