An atom is made of a tiny, dense nucleus (protons and neutrons) in the center, with very light electrons moving around it in an extended “cloud” of negative charge. Almost all the mass is in the nucleus, while most of the atom’s size is empty space occupied by the electron cloud.

What an atom is

  • An atom is the smallest unit of a chemical element that still has that element’s properties.
  • Every object, including your body and everything around you, is made of unimaginably huge numbers of atoms.

The three main parts

  • Protons
    • Positively charged particles in the nucleus.
* The number of protons defines which element the atom is (6 = carbon, 8 = oxygen, 11 = sodium, etc.).
  • Neutrons
    • Neutral particles (no charge) in the nucleus, about the same mass as protons.
* Atoms of the same element can have different numbers of neutrons; these different “versions” are called isotopes.
  • Electrons
    • Negatively charged, very light particles that move in a cloud around the nucleus.
* They occupy “shells” or regions around the nucleus and are responsible for chemical bonding and reactions.

How these parts fit together

  • The nucleus (protons + neutrons) is incredibly small but contains nearly all the atom’s mass.
  • Electrons form a widespread cloud, so the atom is mostly empty space even though matter feels solid at our scale.
  • Overall, a neutral atom has equal numbers of protons and electrons, so the positive and negative charges balance out.

Extra: beyond the basics

  • Protons and neutrons themselves are made of even smaller particles called quarks, held together by gluons in the framework of the Standard Model of particle physics.
  • There are also many other exotic particles, but in ordinary matter the long‑lived, stable building blocks are protons, neutrons, and electrons.

TL;DR: What makes up an atom?
Three main subatomic particles—protons and neutrons in a tiny central nucleus, and electrons in a surrounding cloud—together form the basic building block of all matter.

Information gathered from public forums or data available on the internet and portrayed here.