The atomic number of an element is determined by the number of protons in the nucleus. In this case, both atoms have 93 protons, which means they are both atoms of the same element. The mass number, on the other hand, is the sum of the number of protons and neutrons in the nucleus. Since the number of protons is the same for both atoms, the difference in their mass numbers must be due to a different number of neutrons. This means that atoms A and B are isotopes of the same element.