In order to find the atomic number of any element, students can use a periodic table and look at the upper left-hand corner of a specific element. The atomic number is also the number of protons found in the nucleus of an element. For example, magnesium has 12 protons, so this is also its atomic number.
The atomic mass or weight is used to calculate an element's mass number from it. Magnesium has an atomic weight of 24.305. Rounding this to the nearest whole number, it is 24. This is the mass number for magnesium.
If students do not have access to a periodic table, but know the number of protons and neutron of an element, they can still calculate the mass number. The equation for mass number is given as mass number = number of protons + number of neutrons.