Hello there! How are you today? The answer you are looking for is "atomic number." The atomic number represents an elements identity and the amount of protons it has. For example; Magnesium (Mg) is an element on the periodic table that has an atomic number of 12. This means that Magnesium has 12 protons, leaving it in Group 2 of the periodic table. I hope this helps!
The_,or the number of protons an atom has determines what element it is.