Remember that mass number is not the same as the atomic mass or isotopic mass! The mass number is the number of protons + neutrons, while atomic mass (or isotopic mass) is the mass if you were to somehow weigh it on a balance. Neon has three naturally occuring isotopes. In other words, we will take the sum of the relative abundance of each isotope multipled by its mass. The formula to calculate the average atomic mass is:Īverage atomic mass = ∑(relative abundance x mass of isotope) Instead, we need to perform a weighted average. Since the abundances are not equal, we cannot do a typical simple average where we just add them up and divide by three. When these are averaged together you get the average atomic mass shown on the periodic table of 12.01 amu. In that case, then why is the atomic mass of carbon on the periodic table not exactly 12 amu? Because not all carbon in nature is 12C! Most of it is 12C, some of it is 13C, and a very tiny amount is 14C. So all of the other masses on the period table are relative to the mass of carbon-12. Carbon-12 was chosen as the basis for all of the masses on the periodic table and has been defined to be exactly 12 amu. This unit is based off the mass of the isotope 12C (carbon-12). Notice that the units were listed as amu, which stands for atomic mass units. So when you look on the periodic table and see that it has an atomic mass of 1.01 amu, that is the average of the masses of all three isotopes, not just one of them. For example, hydrogen has three different isotopes that occur in nature – 1H, 2H, 3H. As we saw in our lesson on atomic structure, not all atoms of an element are identical. The atomic mass is an experimental number determined from all of the naturally occuring isotopes of an element.
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