Short answers, pulled from the story.
Ernest Rutherford described the discovery of the proton before the British Association for the Advancement of Science in August 1920. The meeting accepted proton as the standard term that same year after Oliver Lodge suggested avoiding confusion with neutral hydrogen atoms.
Protons are composite particles containing three valence quarks bound by gluons. Two up quarks carry a charge of plus e each while one down quark carries minus e, contributing only about 1 percent of the total mass.
The number of protons in an atomic nucleus defines the element itself through its atomic number. Chlorine possesses an atomic number of 17 meaning every chlorine atom contains exactly 17 protons regardless of neutron count or electron balance.
Spontaneous decay of free protons has never been observed according to current experimental evidence. Experiments at the Super-Kamiokande detector in Japan established lower limits for mean lifetime values around 10 to the power of 34 years for antimuon products.
The CODATA recommended value places the proton charge radius near 0.8 femtometers though conflicting data exists between scattering experiments and muonic hydrogen spectroscopy measurements. Muons are roughly 200 times heavier than electrons resulting in smaller atomic orbitals that allow much more precise determination of the internal size.