Isotopes are atoms that have the same number of protons but a different number of neutrons.
Three isotopes of hydrogen would all have the same number of protons and electrons but would have a different number of neutrons. These isotopes could have:
1 proton, 1 electron and 1 neutron
1 proton, 1 electron and 2 neutrons
1 proton, 1 electron and 3 neutrons
We name isotopes after their element name and their mass number. For example, chlorine-37 is an isotope of chlorine that has a mass number of 37.