Proton is a subatomic particle that is a building block of all atoms. Together with neutrons, protons make up atomic nucleus and are thus also called nucleons. The number of protons in a nucleus determines the atomic number of an element because the number of protons in nucleus is what differentiates one element from another. Unlike neutrons which are not present in the most common isotope of hydrogen atoms, protons are present in all atoms. Also, one element can have many isotopes depending on a number of neutrons it has, but as soon as it loses or gains another proton, it becomes another element entirely.
Protons have ½ spin, which makes them fermions. They have positive electromagnetic charge and in an atom they are bound to negatively charged electrons. Normally, the number of protons corresponds the number of electrons in an atom, but if an atom loses or gains extra electron, it is called an ion of an element. Again, if it loses or gains a proton, it is a different element altogether.
For more than four decades since its discovery in 1917, proton was thought to be an elementary particle, together with neutron, but we later found out that it is built from even smaller particles called quarks. Each proton is made of three quarks - two up quarks, and one down quark. Attributes “up” and “down” are two of the six flavours of quarks, a property of the strong force equivalent to negative and positive charges of the electromagnetic force. The strong force is what holds quarks together to form protons and then again it binds protons to neutrons in atomic nuclei. Protons react to other three fundamental forces as well.
Only about 1% of proton’s mass comes from these three quarks. The rest comes from quarks’ kinetic energy and the energy of gluon fields holding the quarks together, keeping in mind that mass equals energy divided by the squared speed of light. Proton’s mass is 1.672 x 10^-27 kg, or one atomic mass unit.
Unlike neutrons, protons are stable when not in an atomic nuclei. If temperatures are high enough, high-velocity protons can be found separated from electrons. This is the case in cosmic rays for example, 90% of which are made of free protons. When protons slow down and lose energy, they bind with electrons to make hydrogen atoms. It is possible that protons are also unstable, but it has never been observed and if they are, theorised time for their decay is 10^36 years.
Proton has been discovered by Ernest Rutherford as he proved that hydrogen nucleus is present in all other elements’ nuclei. As he suspected that it is the elementary particle that makes up all other atomic nuclei, he called it proton, after the Greek word for “first”.