[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe what's meant by relative atomic mass you should then be able to calculate the relative atomic mass for an element in the last video we looked at Isotopes remember that Isotopes are atoms of the same element with different numbers of neutrons I'm showing you here the Isotopes for the element Chlorine both Isotopes have an atomic number of 17 this means that both Isotopes have 17 protons in their nucleus however as you can see the Isotopes have different Mass numbers telling us that they have different numbers of neutrons the isotope on the left has 18 neutrons whereas the isotope on the right has 20 neutrons now there is a problem here which mass number do we use for chlorine on the periodic table looking on the periodic table we can see that the mass number for chlorine is actually 35.5 so how do we get 35.5 from two isotopes with mass numbers of 35 and 37 well the answer is that we take an average of the mass numbers for each isotope however the average is weighted for the abundance of each isotope and the word abundance means how common each isotope is scientists call this weighted average the relative atomic mass looking at chlorine the isotope with a mass number of 35 is three times more abundant than the isotope with a mass number of 37 and this explains why the relative atomic mass of chlorine is much closer to 35 than to 37 in the exam you could be asked to calculate the relative atomic mass for an element based on its Isotopes to calculate relative atomic mass we use this equation the relative atomic mass is the mass number of isotope one multip by the percent abundance of isotope 1 added to the mass number of isotope 2 multiplied by the percent abundance of isotope 2 and we divide all of this by 100 here's an example for you to try boron has two common Isotopes the first isotope has a mass number of 10 and an abundance of 20% the second isotope has a mass number of 11 and an abundance of 80% I'd like you to calculate the relative atomic mass of boron so pause the video now and try this yourself okay putting these numbers into the equation gives us a relative atomic mass for Boron of 10.8 now in the gcsc periodic table the relative atomic mass for boron has been rounded up to 11 and we can see that here here's another example for you to try magnesium has three common Isotopes isotope one has a m number of 24 and an abundance of 79% isotope 2 has a mass number of 25 and an abundance of 10% and isotope 3 has a mass number of 26 and an abundance of 11% I'd like you to calculate the relative atomic mass of magnesium so pause the video now and try this yourself okay putting these numbers into the equation gives us a relative atomic mass for magnesium of 24.3 however in the GCS periodic table the world of atomic mass of magnesium has been rounded down to 24 you'll find plent of questions on this topic in our vision workbook which you can get by clicking on the link [Music] above