The_,or the number of protons an atom has determines what element it is.

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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.

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