Atomic number is the number of protons in an atom. Since these are all Hydrogen, the number of protons remains the same, and the Atomic Number for each is 1. Mass number is the number of Protons and Neutrons in an atom. So, Protium is 1, Deuterium is 2, Tritium is 3. The number of neutrons is stated in the question (Protium 0, Deuterium 1, Tritium 2). Hope this helps, if you wouldn’t mind marking brandies if it did!
Because mass is not always the same for objects made from the same material. Just because an object has the same mass as another doesn’t mean they are the same material.
