Atomic Theory

Having the percentage abundance of the isotopes for a particular element, one could find the relative atomic mass (r.a.m.) for that element.

For example, to find the relative atomic mass (r.a.m.) chlorine.

One atom of   35Cl has a mass of 35              One atom of     37Cl has a mass of 37               17                                                                                                           17

So 75 atoms of 35Cl  have  ?      mass    So 25 atoms of 37Cl have    ?     mass

        17                                                                                        17

             35 X 75 =  2625                                              37 X 25 =  925

So 100 atom of the two types of isotopes together have the mass of  2625 + 925 =  3550

So the relative atomic mass (r.a.m.) of chlorine is:      3550  =   35.5