protons

• composite particle, a baryon: two up quarks and one down quark (uud), bound by the strong interaction
• charge $+e$, spin 1/2
• mass $1.673\times 10^{-27}$ kg (938.3 MeV/c²)
• number of protons in a nucleus = atomic number (which element it is)
• radius ≈ 0.84 fm (the “proton radius puzzle” now looks settled on this value, Maisenbacher et al., 2026)
• no decay observed: lifetime > $2.4\times 10^{34}$ yr for p → e⁺ + π⁰ (Takenaka et al., 2020)
• the three quarks’ own masses are ~1% of the proton mass, the rest is quark motion and gluon field energy