 In this video, we are going to learn more about atoms, and in particular isotopes, and how you can find the mass numbers of elements from the periodic table. In the periodic table, elements are written with a small number and a bigger number next to the symbol. The smaller number is the atomic number, which is unique to the element. It tells us how many protons there are. The bigger number is the atomic mass number. It depends on the weight of the nucleus, that is, the number of protons plus the number of neutrons, because the mass of an electron is so small we ignore it. The atomic number of sodium is 11, and mass number 23. How many protons, electrons and neutrons does it have? Pause the video whilst you count them. Time for the answer. There are 11 protons in sodium, because this is the atomic number, so there must be 11 electrons too. There are 23 minus 11 neutrons, which makes 12. The atomic mass number in the periodic table is not always a whole number. It is an average of different isotopes, weighted according to their abundance. For example, the atomic mass of chlorine is 35.5. There are two common isotopes of chlorine, chlorine 37 and chlorine 35. Chlorine 35 is three times as common as chlorine 37, so the average weight is three quarters times 35, plus one quarter times 37, which gives 35.5. Look at the periodic table. Which other elements have mass numbers that are not whole values? The lighter elements have roughly the same number of neutrons as protons, but heavier elements tend to have more neutrons to glue the nucleus together. Electrons surrounding the nucleus of an atom aren't completely free. They are restricted to moving in fixed orbitals or energy levels. Each energy level can take a certain number of electrons before it is full, and any more electrons have to occupy a level further out. We call these electron shells and draw them as concentric circles, working outwards from the nucleus. The number of electrons each shell can take can be seen by looking at the rows of the periodic table. The first row has just two elements in it, hydrogen and helium. The first shell can take just two electrons. Hydrogen has one shell with one electron in it, and helium has one shell with two electrons in it. Look at some of our other videos to learn more about how electrons arrange themselves. By now, you should be comfortable with the idea that atoms are made up of protons, neutrons and electrons. You should remember that electrons weigh essentially nothing, and that protons and electrons are charged, but that neutrons are not. You should remember what an isotope is, and know how to use atomic and mass numbers to find out how many fundamental particles there are in an element. To learn more, you may want to watch our video on how the elements are laid out in the periodic table.