The atoms of hydrogen, deuterium, and tritium all have the same number of protons, but differ in the number of neutrons they possess. these substances are:

Isotopes.

ISO means equal
Topo means position

Are substances that occupy the same position in the periodic table becomes they have the same number of protons but they differ on the number of neutrons and in consequence they have different mass

Copper, silver, aluminum, cobalt, chromium, mercury