Protons are sturdy, positively charged particles that sit at the heart of every atom, defining the very elements that make up matter.
Protons belong to the baryon family, meaning they’re made of three quarks held together by the strong nuclear force. Specifically, a proton is composed of two up quarks and one down quark (uud), bound so tightly by gluons that they can never escape. This internal structure makes protons incredibly stable, and unlike neutrons, which can decay outside a nucleus, protons seem to last forever—at least, as far as we know. Some theories predict that protons might eventually decay after an absurdly long time (longer than the age of the universe!), but so far, they appear to be immortal.
Being positively charged, protons play a crucial role in determining an atom’s identity. The number of protons in an atom’s nucleus is what defines an element—one proton makes hydrogen, two make helium, six make carbon, and so on.
In the Sun, hydrogen nuclei (which are just single protons) smash together in nuclear fusion, creating helium and releasing the energy that powers our solar system. In particle accelerators, high-energy proton collisions help us uncover new physics, from quarks to exotic particles.
Despite their everyday presence, protons still hold mysteries. Their precise size is the subject of ongoing debate (the “proton radius puzzle”), and their deep internal structure is far more complex than just three quarks sitting inside. Instead, they’re a swirling sea of quarks, antiquarks, and gluons, constantly shifting in a dance dictated by quantum chromodynamics.