Atoms of the same element can vary based on their atomic masses, basically the atomic mass units or amu. If there are isotopes of a given element present, then the atoms of all the elements posses the same atomic number and thus the same element, but can vary based on their atomic mass, due to the different number of neutrons that corresponding atom has. Isotopes are atoms of the same element, but with different number of neutrons and ultimately different atomic mass.
