What is the number of protons and neutrons in the nucleus of an atom?

Nuclei are conceived to contain #"protons"#, fundamental, massive charged particles, and #"neutrons"#, fundamental, massive particles of zero charge. On nuclear scales, the particles are so attracted by nuclear interactions that the electrostatic repulsion that operates between like charges is overcome.

The number of #"protons"# determines the identity of the element. #Z=1#, #"hydrogen"#; #Z=2#, #"helium"#; #Z=3#, #"lithium"#;...........................#Z=45#, #"rhodium"#. Of course, you would know this well.

Individual elements can have various numbers of neutrons, and this variability gives rise to the existence of isotopes; the same element that has different number of neutrons. For instance, most hydrogen nuclei contain the one nuclide, the proton, to give the protium isotope, #""^1H"#; a few nuclei contain a proton and a neutron to give the deuterium isotope, #""^2H#; and a smaller few nuclei contain a proton and 2 neutrons to give the tritium isotope, #""^3H#,

As the nucleus gets bigger, a greater range of isotopes becomes possible, and the atomic masses quoted on the Periodic Table are the weighted average of the individual isotopes.