Can atoms of two different elements have the same atomic number or mass number? Why or why not?
Atoms of two different elements, by the definition of an element, MUST have different atomic numbers. We define the element helium, for example, to be the atoms that have 2 protons, or atomic number (Z) = 2. Two atoms of different elements can certainly have the same mass number, though it is much more common in larger atoms. For example, the isotope 3He has 2 protons and 1 neutron, mass number = 3. 3H has 1 proton and 2 neutrons, mass number = 3. One important note is that two different atoms, even if they have the same mass number, are extremely unlikely to have the exact same “mass” – this is because they have different binding energies, and invoking the classic “e=mc^2”, different binding energies have different masses.
