Forms or species of an element that has the same numbers of protons (hence the same atomic number), but different numbers of neutrons, thus different atomic masses. Technically speaking if an element has only one form that is there is no variability in the number of neutrons that exist in the element, it has no isotopes. There are 21 elements that do not vary in the number of neutrons in their nuclei. More information While the terms nuclide and isotope are often used interchangeably, there is a subtle difference in useage. Nuclide refers to a specific isotope of an element that has a specific atomic number Z and mass number A. The term isotope can be used to compare 2 different nuclides of the same element. They are isotopes of each other not nuclides of each other, because while they have the same atomic number, they have different atomic masses.