1.12 calculate the relative atomic mass of an element from the relative abundances of its isotopes

Relative atomic mass of an atom is the average of all the weights of its different isotopes.

It is measured using this formula:

(% of isotope 1 x its mass) + (% of isotope 2 x its mass) + (% of isotope 3 x its mass) etc. ÷ 100

e.g. Hydrogen has 3 isotopes: Protium, Deuterium and Tritium

                        Percentage                         RAM

Protium =         99.98%                                1

Deuterium =     0.018%                                2

Tritium =          0.002%                                3

(99.98 x 1) + (0.018 x 2) +  (0.002 x 3) = 0.99842 ≈ 1