The correct answer is Both 1 and 2. Key Points Isotopes: Isotopes are those elements that have the same atomic no. but different mass no. The mass number is the sum total of protons and neutrons. For example, 612C , 613C, 614C Other examples are Hydrogen isotopes named Protium, deuterium, and tritium having mass numbers 1,2,3 respectively. In order to identify isotopes, one must use methods to determine the mass number. Isobars: The nuclei which have the same mass number (A) but a different atomic number (Z) are called isobars. Examples of Isobars are 14 C 6, 14 N 7 From above it is clear that the nuclei which have the same mass number (A) but a different atomic number (Z) are called isobars. Isobars occupy different positions in the periodic table so all isobars have different chemical properties, but the same physical properties.