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Welcome back everyone in this example we're filling in the blank. It says that the standard unit of atomic mass has been one 12th of the mass of an atom of the isotope blank. So what we should recall is our isotope for carbon known as carbon 12. So it's given as the following symbol because we recall that our isotope symbols are written in the left hand exponents. So we have carbon 12 given by this symbol here. And we should recall that this exponents number here in the left hand exponents is calculated as our mass number or atomic weight. And this is found by taking our number of neutrons and adding that to our number of protons in our nucleus of our atom. So recall that neutrons added to our number of protons in our atom gives us our nucleons. And so for carbon 12 we would recognize that we have six neutrons added to six protons which gives us nucleons as our weight of our nucleus for the carbon atom. And so we would say that therefore carbon has 12 atomic mass units am us. So we also want to recall that carbon 12 as an isotope is very abundant and it's also very stable. And because of these two facts, carbon 12 is a great reference for measuring our atomic weights of other elements. For example, when we recall hydrogen on our periodic table in group one A we see that it has an atomic weight of one a.m. U. And we would recognize that this is equal to 1/ multiplied by the mass Of one Carbon 12 Atom. Which is why more specifically, we can say that hydrogen has a weight of 1.008. And so to fill in our blank, we're simply going to recognize that this isotope is referring to carbon 12 isotope. And so and sorry, that should be clear. So that's a hyphen there, carbon hyphen 12. So for our final answer, we have this blank filled in as carbon 12 as the isotope that is used as the reference for measuring atomic weights of other elements. So I hope that everything that I reviewed was clear. If you have any questions, please leave them down below and I will see everyone in the next practice video.

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