Explainer: The particle zoo

Various subatomic particle types are compared to how they oddly relate to animals in a zoo

An atom was once considered the smallest unit of matter, but even smaller particles have taken the atom's place as the littlest bits that exist in the universe. This model offers a simplistic view of protons and neutrons in an atom’s nucleus, with electrons orbiting about them.


Particles are the building blocks of matter, and matter makes up everything you can see. The Earth and moon are matter. So is your body, your computer’s screen, even the air you breathe. Which means they’re all made of particles. Lots and lots of particles, of all different kinds, stuck together.

Atoms, which used to be considered the smallest unit of matter, are made from particles too. Just how small is an atom? That’s a tricky question, since different atoms have different sizes and atoms are mostly empty space. But here’s one way to think about it: Let’s say you wanted to fill up a baseball-sized bowl with gold atoms. You’d need roughly twice as many of these atoms as it would take to fill an Earth-sized bowl with baseballs.

Particles that are even smaller than an atom are called “subatomic.” The main subatomic particles that make up atoms are protons, electrons and neutrons. But some of these particles are also made of even smaller particles. Protons and neutrons, for example, are made of subatomic particles called “quarks.” There are six kinds of quarks, each with a weird name: up, down, strange, charm, top and bottom.

Dozens of types of subatomic particle exist, and scientists suspect there may be still more to discover. When a new type emerges, scientists tend to give them pretty odd-sounding names. To date, we’ve got bosons, fermions, leptons, (which include muons and neutrinos), pions, photons, gluons and gravitons.

Neutrinos are unusually weird because they have almost no mass and they fly through space at almost the speed of light. Three types exist: muon neutrinos, electron neutrinos and tau neutrinos.

And the strangest particle of all: the tachyon. It’s considered “hypothetical,” which means it might not even exist. If it does, it can go faster than the speed of light and travel back in time.

No wonder some physicists refer to these — the smallest inhabitants of our universe — as their “particle zoo.”

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News for Students since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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