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What is an atom?

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Quick answer

An atom is the smallest unit of an element that still has its chemical properties — the basic building block of all ordinary matter. Each atom has a dense central nucleus of protons and neutrons, surrounded by a cloud of much lighter electrons.

Key things to understand

  1. An atom has three main particles: positively charged protons and neutral neutrons in the nucleus, and negatively charged electrons around it.
  2. The number of protons (the atomic number) decides which element it is — 1 proton is hydrogen, 6 is carbon, 79 is gold.
  3. Atoms are mostly empty space: the nucleus is tiny compared with the electron cloud around it.
  4. Atoms join by sharing or swapping electrons to form molecules and compounds.
  5. Atoms are astonishingly small — millions would fit across the width of a human hair.

People also ask

What is an atom made of?
Protons and neutrons packed into a central nucleus, with electrons moving around it. Protons are positive, electrons negative, and neutrons carry no charge.
What's the difference between an atom and a molecule?
An atom is a single building block; a molecule is two or more atoms bonded together — like two hydrogen atoms and one oxygen atom forming a water molecule.
Can you see an atom?
Not with the naked eye or a normal microscope — atoms are far too small. Special instruments like scanning tunneling microscopes can image them indirectly.