In the nucleus there are two main subatomic particles: The protons and the neutrons. The number of protons in the nucleus determines the element the atom is: 1 proton = hydrogen , 99 protons = Einsteinium , 6 protons = carbon etc.
The number of neutrons in the nucleus varies from atom to atom, even atoms of the same element. This causes the mass of the nucleus to vary slightly and can effect properties such as stability or type of nuclear decay. Atoms of the same element with different masses are called isotopes. They differ by the number of neutrons with the proton in te nucleus:
1 proton + 0 neutrons = 1 particle in the nucleus = hydrogen
1 proton + 1 neutron = 2 particles in the nucleus = deuterium
1 proton + 2 neutrons = 3 particles in the nucleus = tritium
Hope this helps
