 How do we find the mass of an atom? Well, the mass of an atom is approximately equal to the mass of protons in the atom, plus the mass of the neutrons in the atom. This is the mass of a proton, and this is the mass of a neutron. These numbers are very awkward to use because of the exponents. Fortunately, chemists use a different unit of measurement called the atomic mass unit. One atomic mass unit, or 1AMU, is approximately the mass of a proton or a neutron. The mass of electrons is so small that we can ignore it when we talk about the mass of an atom. If we want to know the atomic mass of a single atom, we add up the masses of the protons and the neutrons. For example, to calculate the approximate mass of a carbon atom, we add up six protons and six neutrons and get an atomic mass of 12 atomic mass units. You have probably noticed that the boxes in a periodic table have numbers on the bottom that have decimals rather than nice even numbers, and that's because these numbers represent average atomic masses. In nature, you don't usually just have one type of atom. You have several isotopes. Remember, isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are more common in nature than others. For example, a single chunk of carbon could have some carbon 12 in it with six protons and six neutrons. It might also have a little bit of carbon 13 with six protons and seven neutrons and maybe a tiny, tiny amount of carbon 14, which has six protons and eight neutrons. Carbon 12 is the most abundant isotope. The atomic mass reported in the periodic table is what we call a weighted average, and this means it accounts for the mass of each isotope as well as how much of each isotope there is. So because carbon 12 is most common in nature and because the other isotopes of carbon are present in very, very small amounts, the average atomic mass is 12.01 AMU, mostly carbon 12 with a little bit of the other isotopes. Just in case you're curious, average atomic mass is something that you can calculate if you know the relative abundance of the different isotopes in your sample. Relative abundance means the proportion. You also need to know the mass of each isotope, and you use this equation to figure out the average mass. This is beyond the scope of this video, but there are other videos on this YouTube channel that will teach you how to do this. In summary, the numbers you see at the bottom of the squares in the periodic table refer to the average atomic mass, which is a weighted average of the masses of the various isotopes in a sample found in nature.