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. In the case of niobium, 93 minus 41 is 52, which means that a niobium atom has 52 neutrons.The atomic mass is an experimental number determined from all of the naturally occuring isotopes of an element. Finally, subtract the number of protons from the rounded up atomic weight to find the number of neutrons in the atom. Niobium has an atomic weight of 92.906, so you would round it up to 93. Round up the atomic weight to the nearest whole number. Next, find the atomic weight of the element, which is usually underneath the element symbol. For instance, the atomic number of niobium (Nb) is 41, meaning that a niobium atom has 41 protons. This number represents the number of protons in the atom. It’s usually located somewhere above the element symbol. Then, find the atomic number for the element. First, locate the elemental symbol for your atom on the periodic table. To find the number of neutrons in an atom, you just need a periodic table that lists the atomic number as well as the atomic weight of each element.
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