The bottom number on each element of a on the periodic table is called

On the periodic table, there are two numbers for every element: the atomic number and the atomic weight.

The numbers at the top of the square represents the number of protons present in the atom's nucleus of that element. This is called the atomic number.

For example, the atomic number for the element Krypton is 36. That means that there are 36 protons in the nucleus. If there were not 36 protons in the nucleus of that atom, that would create the atom of a completely different element. For example, if one proton was to be removed from the nucleus of the atom for the element of Krypton that atom will be an atom of the element bromine.