atom

American  

[at-uhm] / ˈæt əm /

noun

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

---

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

---

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

---

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.)

---

Discover More

Atoms link together to form molecules.

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

These compounds feature a metal atom positioned between two carbon rings, giving them a distinctive "sandwich" structure.

From Science Daily • May 22, 2026

Extrapolating backward in time, Lemaître proposed the idea of a primeval atom: a singularity from which the universe emerged in a uniquely hot, dense state.

From The Wall Street Journal • May 19, 2026

That gap measures only about 0.14 nanometers, making it thinner than a single sulfur atom.

From Science Daily • May 9, 2026

The first phase involved determining if he predated the "nuclear age", which began in 1945 when the US first tested the atom bomb in New Mexico, the coroner heard.

From BBC • Apr. 14, 2026

Human beings would split the atom and invent television, nylon, and instant coffee before they could figure out the age of their own planet.

From "A Short History of Nearly Everything" by Bill Bryson