How do you calculate the relative atomic mass of an element

The relative atomic mass (A_r ) is calculated from 2 things : mass numbers of its isotopes and abundance of these isotopes.Let's use Chlorine as an example. Chlorine naturally exists as two isotopes,  (chlorine-35) and  (chlorine-37). The abundance of chlorine-35 is 75% and the abundance of chlorine-37 is 25%. I.e. in every 100 chlorine atoms, 75 atoms have a mass number of 35, and 25 atoms have a mass number of 37.A_r, = total mass of atoms / total number of atoms A_r = (35 x 75) + (37 x 25) /(75+25) A_r = 35.50Note the number is closer to 35 rather than 37 because the 35-isotope is more abundant.