What three particles make up an atom?

• A. Proton, neutron, and electron.
• B. Electron, photon, and neutrino.
• C. Proton, neutron, and positron.
• D. Proton, electron, and quark.

Answer: (A)

At the most basic level, an atom is made of three kinds of particles: protons and neutrons in the nucleus, and electrons that orbit around the nucleus. Protons are positively charged, neutrons have no charge, and electrons carry a negative charge; the nucleus defines the atom’s identity through the number of protons, while the total number of protons and neutrons gives the mass. The electrons balance the charge so the atom can be neutral overall.

The other particles listed don’t form the atom’s basic structure. Photons are particles of light, neutrinos are extremely light particles that rarely interact, and a positron is the antimatter partner of an electron, not a normal atomic constituent. Quarks exist inside protons and neutrons but aren’t counted as separate, free components of the atom in introductory descriptions. So the three constituents are protons, neutrons, and electrons.