A Magnesium Atom Is 3 Times Heavier Than A Helium Atom.What Is The Relative Atomic Mass Of Mg?

A magnesium atom is not three times heavier than a helium atom!    The atomic number of helium is 2 and relative atomic mass of helium is 4, it has two protons and two neutrons in its nucleus.    Magnesium is in the third row of the periodic table, has an atomic number of 12 and has several naturally isotopes, these being Mg-24, Mg-25 and Mg-26.  The number after the symbol is the relative atomic mass of each isotope.  As you can see NONE of them are three times as heavy as helium. They are in fact 6, 6.25 and 6.5 times as heavy as helium atoms, respectively.    Mg-24 has 12 protons and 12 neutrons in its nucleus  Mg-25 has 12 protons but 13 neutrons  and Mg-26 has 12 protons and 14 neutrons    The number of protons present says which element is it. It affects the number of electrons in its shells and hence its chemistry.  All the isotopes react the same way as they have the same number of electrons and the same electronic configuration