A student submits the following work on the mass number, but he has made a few mistakes. Select the sentences that are incorrect. Calculating the mass number for an atom requires that we know the atomic number and the number of protons in the atom’s nucleus. The mass number then gives us the average weight of atoms of a given element. Specifically, these units are calibrated by setting the mass number for carbon-12 as 12 atomic mass units. Neutrons and protons have the same masses. However, as long as the number of protons equals the number of neutrons, the values balance out and we always obtain a whole number for the mass number.