# atom

/ˈætəm/ (US) · /ˈætəm/ (UK)

Source: https://englishreference.com/word/atom
Updated: 2026-06-20

## Senses

### 1. chemical unit

noun · CEFR B1

the smallest part of a chemical element that can exist.

For teachers: the basic unit of a chemical element, consisting of a nucleus of protons and neutrons typically surrounded by electrons.

- A water molecule is made of two hydrogen atoms and one oxygen atom.
- The researchers used a scanning tunneling microscope to manipulate individual atoms on the surface of the silicon crystal.

### 2. smallest unit of something

noun · CEFR C1 · Academic

the smallest possible piece or unit of an idea or a system.

For teachers: the smallest, indivisible constituent part or unit of a larger conceptual or physical system.

- In this theory, the individual person is the atom of society.
- Logical atomism suggests that the world consists of ultimate facts that cannot be further decomposed into simpler components.

### 3. a tiny amount

noun · CEFR C2 · Literary

a very small amount of something.

For teachers: an extremely small or negligible quantity of a quality or substance.

- There was not an atom of truth in the rumors spreading through the village.
- He faced the accusations with dignity, showing not an atom of fear even as the verdict was read aloud.

### 4. programming/math

noun · CEFR C1 · Technical

a single value or symbol that cannot be broken into a list.

For teachers: a primitive value or a scalar that is not a collection; in some languages, a unique integer representing a string.

- In the Lisp programming language, an atom is a basic element like a number or a symbol.
- The function is designed to process each atom within the nested list structure recursively until it reaches the base case.

## Examples

- A single drop of water contains billions of atoms.
- In chemistry class, the students learned how oxygen and hydrogen atoms bond together to form water.
- When a radioactive atom decays, its unstable nucleus releases energy and particles, eventually transforming into an entirely different element over time.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.