In this lesson, we'll introduce a very important number on the periodic table, the atomic mass. This number is the average mass of an atom of that element. Recall from the last lesson, the protons and neutrons provide mass to an atom, the each way 1amu. The atomic mass, often called the atomic weight, is one of the most important numbers on the periodic table. It gives the average mass of an atom of that element. For example, helium has two protons and two neutrons, which each way one atomic mass unit, giving it an atomic mass of 4amu. Looking at the atomic masses on the periodic table, you might wonder why are most of them decimals instead of whole numbers? Well, the answer is that the atomic mass is the average mass of an atom of that element. Recall that isotopes of an element have the same number of protons, but different masses due to a different number of neutrons. For example, this slide shows two isotopes of chlorine, one was with a mass of 35amu and the other with a mass of 37amu. All the elements in our universe are a mix of many different isotopes, and every sample of chlorine contains atoms of both chlorine 35 and chlorine 37. The atomic mass is the average mass of an atom of that element. The average mass of chlorine is 35.45, which is in between the masses of the two isotopes of chlorine. Specifically, the atomic mass is the weighted average, which means that we need to take into account how much of each isotope there are in a sample. For example, carbon has two main isotopes, carbon 12 and carbon 13. By far, the most common form of carbon is carbon 12, which has a mass of 12amu, but 1% of the carbon on the earth and in your body has an extra neutron and weighs 13amu. The atomic mass of carbon on the periodic table reflects the two isotopes of carbon and their abundance. Since 99% of carbon is carbon 12, the atomic mass is very close to 12amu, but since 1% of carbon is carbon 13, the mass is just a little bit more than 12 atomic mass units. There is a formula for calculating atomic mass from elemental abundance, which is shown at the bottom of this slide along with a practice problem. I suggest you pause the video right here and try to calculate silver's atomic mass on your own. The solution to this problem is to multiply each isotope's mass by its abundance and add them together as shown on the solution to this problem is to multiply each isotope's mass by its abundance and add them together as shown in the calculation in the middle. Now, you might still be wondering why the mass of each silver isotope is not a whole number. For example, the mass of silver 107 is slightly less than 107 atomic mass units. Well, the reason for this is that protons and neutrons don't weigh exactly 1amu, but it's convenient to round their mass to 1amu during our calculations in this class. But the real reason is even crazier than that. And I don't think you're ready for it yet. It might have a little something to do with E equals MC squared if you've heard of it. Anyway, the way scientists measure atomic mass is what the sick instrument called a mass spectrometer. A mass spectrometer is like a gun that shoots charged particles down a curved barrel. As the particle enters a magnetic field, their trajectory starts to curve, but heavy particles don't curve quite as much as light particles do. Scientists can use the amount of curvature in the trajectory to calculate the particle's mass. Similarly, the abundance of each element is given by the height of the peak on the detector. Take a look at this mass spectrum of chlorine, which shows the two isotopes chlorine 35 and chlorine 37. Now, look at the atomic mass of chlorine 35.453. Does it make sense that the peak for chlorine 35 is about three times as large as the peak for chlorine 37?