proton and neutron (nucleons)
/ NOO-klee-on /
Every atom has a tiny, dense core — the nucleus — and that core is built from just two kinds of particle: protons and neutrons. Because they are the building blocks of the nucleus, the two together are called nucleons. The number of protons decides which chemical element you have; the neutrons add mass and help hold the nucleus together. Essentially all the mass of everything you can touch is the mass of nucleons.
Both are baryons made of three quarks. The proton is two up quarks and one down (uud), giving it a charge of +1; the neutron is one up and two downs (udd), giving it a charge of exactly 0. They are nearly the same mass — the neutron is just a hair heavier — and each has spin one-half. That tiny mass difference matters enormously: it lets a lone neutron decay into a proton (plus an electron and an antineutrino) in about fifteen minutes, while a free proton, having no lighter baryon to turn into, appears completely stable.
Nucleons are the bridge between particle physics and the everyday world. Bound inside a nucleus by the residual strong force, they make up over 99.9 percent of the mass of ordinary matter. And almost none of that mass is the mass of the quarks themselves — it is the energy of the strong force seething inside each nucleon. The proton is also a workhorse of experiment: smashing protons together is exactly how the Large Hadron Collider hunts for new physics.
A free neutron decays: n -> p + electron + antineutrino, with a half-life of about ten minutes. A free proton has never been seen to decay.
Both nucleons are three-quark baryons; their tiny mass difference makes the neutron decay and the proton stable.
A neutron is stable inside most nuclei but unstable when free — the nucleus's binding energy can remove the energy advantage that would otherwise let the neutron decay. So "neutrons decay" is only true for free or loosely bound neutrons.