The atom consists of a small but massive nucleus surrounded by a cloud of rapidly moving electrons. The atomic mass number determines especially the atomic mass of atoms. The average atomic mass of silver is 107.87 amu.The atomic mass number is the total number of protons and neutrons in the nucleus of an atom and is given the symbol A. Looking at the significant digits, both 55.2 40 amu and 52.4 49 amu are precise to the hundredths place so our answer must also be rounded off to the hundredths place. How can we figure it out? Well, both abundances have to total 100% so is Ag-107 is 51.84%, then Ag-109 must be 100 – 51.84 = 48.16%. But notice that while we have the abundance of Ag-107, 51.84%, we are not given the abundance of Ag-109.
#Equation for atomic mass number plus#
Just like before, we need to take the abundance of Ag-107 times the mass of Ag-107 plus the abundance of Ag-109 times the mass of Ag-109. If silver is 51.84% Ag-107 with a mass of 106.9051 amu and the rest Ag-109 with a mass of 108.9048 amu, calculate silver’s atomic mass. Based off this we would predict that the answer would be close to 20 amu, but slightly higher, which it is! According to the table, most neon (over 90%) has a mass of approximately 20 amu, a tiny bit has a mass of around 21 amu, and around 9% has a mass of 22 amu. Let’s just take a second and see if 20.18 amu seems like a reasonable answer, given the initial data. We would therefore round the answer off to 20.18 amu. So the answer can only be precise out to the least precise or most uncertain place, hundredths place. Looking at our significant digits, we see that 18.0 89 was precise to the hundredths place, 0.05 16 was precise to the thousandths place, and 2.0 34 was precise to the hundredths place. However, you might recall from your math courses that when you use a percentage in a calculation you always want to use the decimal form, meaning you must first divide the percentage by 100. To find the average atomic mass of neon, we will use the equation above and take the abundance of the first isotope times the mass of the first isotope plus the abundance of the second isotope times the mass of the second isotope plus the abundance of the third isotope times the mass of the third isotope. 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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