in this video we're going to learn how to calculate the relative atomic mass of an element from its isotopic abundances if we consider carbon its relative atomic mass is very close to 12 atomic mass units this is because in nature 99 atoms out of 100 atoms of a carbon sample are carbon2 isotopes another way of saying this is that carbon 12 has a 99% isotopic abundance however not all relative atomic masses of elements are weighted in this way chlorine for instance exists as two isotopes chlorine 35 and chlorine 37 how many protons and neutrons do you think each chlorine isotope has remember the mass number for an isotope is equal to the number of protons it has plus the number of neutrons pause and select from one of the following options use the chemical symbols to help you work this out both chlorine 35 and chlorine 37 have 17 protons but the chlorine 35 isotope has 18 neutrons whereas the chlorine 37 isotope has 20 neutrons the answer was a did you get it right in nature out of 100 atoms of chlorine sample 75 of those atoms are chlorine 35 isotopes and 25 of those atoms are chlorine 37 Isotopes for you to be able to work out the relative atomic mass for chlorine you have to multiply the mass number of the Isotopes with its abundance percentage and add these products together this gives the total mass number of the Isotopes using this data can you calculate the total mass of chlorine Isotopes using their abundance and mass numbers pause think choose and continue when you're ready the answer is B you have to multiply a 75% abundance of the chlorine 35 isotope by its mass number 35 you then have to multiply a 25% abundance of the chlorine isotope by its mass number 37 when added together this gives a total of 3,550 the final step in calcul in the relative atomic mass is to divide your answer by 100 this is because the abundances are given as percentages this means chlorine has a relative atomic mass of 35.5 here's a challenge from the following data calculate the relative atomic mass of uranium pause and continue when ready make sure you write your working out down as you do the calculation the correct answer and working out is displayed on the screen remember you then have to divide this value by 100 that gave you 23799 did you get it right in summary the relative atomic mass is the average weighted mass of the for the Isotopes of a particular element to calculate this you need to multiply the relative abundances by the mass number of the isotope add these products together for each isotope and then divide the total by 100 and that's how you calculate the relative atomic mass of any element from its Isotopes in the periodic table