Isotopes differ in mass because of the addition/removal of neutrons
Formula: (% of isotopes x Ar) + (% of isotopes x Ar) ) / 100
Q ) 75% of all chlorine has a mass of 35. The rest of chlorine atoms (25%) have an atomic mass of 37. Calculate the relative atomic mass.
( (75 x 35) + (25 x 37) ) / 100 = 35.5