Elements of the same mass number but of different atomic numbers are known as isobars. The mass number of an atom is defined as the sum of its protons and neutrons, which gives the total number of nucleons in the atom's nucleus. Isobars have equal mass numbers, meaning they have the same total number of protons and neutrons, but they differ in their atomic numbers, which indicates they have different numbers of protons. This difference in proton number means isobars are different elements. As they have different numbers of neutrons as well, their chemical properties can vary significantly.