Proton ($p$)

The proton is a positively charged spin-$1/2$ baryon with ordinary valence quark content $uud$. It carries electric charge $+e$ and baryon number one. A hydrogen atomic nucleus is a single proton, while other ordinary atomic nuclei contain protons together with neutrons.

A proton is composite even when an atomic calculation treats it as a pointlike source. Electron scattering at increasing momentum transfer probes its spatial and magnetic response, whereas low-energy atomic behavior often needs only its charge, mass and magnetic moment. Its mass is about $1836$ electron masses; that ratio does not make it a bound collection of electrons.

The root mean square radius of the proton charge distribution. Finite nuclear size changes hydrogen atom energy levels and enters precision predictions for the Lamb shift.^[ref]