In simple terms
An atom is the smallest piece of matter that still behaves like a particular chemical element. Split it further and you no longer have gold, or oxygen, or carbon.
IUPAC defines it as the smallest particle still characterizing a chemical element, made of a nucleus carrying almost all its mass and the electrons that determine its size.
How it works
The nucleus sits at the center. It holds protons, which carry positive charge, and neutrons, which carry none, bound together by the strongest known fundamental force.
The number of protons decides which element the atom is. Electrons, negatively charged, occupy the space around the nucleus and determine how the atom bonds to others.
The scale inside is startling. The US Department of Energy notes that the nucleus holds more than 99.9 percent of the atom’s mass while taking up much less than 0.01 percent of its volume, and by a tighter estimate less than one ten trillionth of it.
Atoms of the same element can carry different numbers of neutrons. Those variants are isotopes, and some are stable while others decay.
Why it matters
Nearly all of chemistry follows from how electrons are arranged. Bonding, reactivity and the layout of the periodic table trace back to atomic structure.
The nucleus matters in a different way. Changes there release energy on a far larger scale, which is the basis of nuclear power, nuclear medicine and radiometric dating.
Where you’ll see it
- The periodic table and its atomic numbers
- Chemical formulas such as H2O and CO2
- Medical imaging and radiotherapy
- Materials science and semiconductor design
Example
A carbon atom has six protons. Add two neutrons to the usual six and it becomes carbon 14, the isotope used to date organic remains.
Often confused with
An atom is not a molecule. A molecule is two or more atoms bonded together, such as the two hydrogen atoms and one oxygen atom in water.
Key facts
- IUPAC defines an atom as the smallest particle still characterizing a chemical element, consisting of a positively charged nucleus carrying more than 99.9 percent of its mass and the electrons that determine its size.1
- Atomic nuclei consist of electrically positive protons and electrically neutral neutrons.2
- The nucleus makes up much less than 0.01 percent of the volume of the atom but typically contains more than 99.9 percent of its mass.2
- By a tighter estimate the nucleus accounts for more than 99.9994 percent of total atomic mass while occupying less than one ten trillionth of the atomic volume.2
- Protons and neutrons are held together by the strongest known fundamental force, called the strong force.2
Related concepts
In the news
Quick checkWhat decides which chemical element an atom is?Show answer
The number of protons in its nucleus.
Sources
- International Union of Pure and Applied Chemistry. atom, IUPAC Compendium of Chemical Terminology (Gold Book), entry A00493. Undated (accessed 22 September 2026)
- U.S. Department of Energy, Office of Science. DOE Explains Nuclei. Undated (accessed 22 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 22, 2026Researched and drafted with AI assistanceReport an issue