How do you know witch one is the mass number on the periodic table?

Answer 1

The mass will always be a larger or equal number to the atomic number. The only way it could even be equal is with Hydrogen, where there is only one proton to affect the mass. That is because the Atomic Mass is the combination of all protons and neutrons in that atom. Also, it is often written underneath the atomic symbol. In more advanced chemistry classes, the atomic number is a long string of digits after the decimal. This is because that is the average atomic mass between all of the known ions of that element

Here is an example using Carbon, as it is often shown, with the average atomic mass:

6

C

Carbon

12.0107

Answer 2

The mass number is typically located at the top left of the element's symbol on the Periodic Table. It represents the total number of protons and neutrons in the nucleus of the atom.