 In the first video, I described the structure of atoms, in particular the composition of the tiny atomic nuclei that light the heart of each atom. In this video, we will try and understand why nucleons can stick together to make stable nuclei. Behind me, you can see the chart of the nucleides. Every square on this chart is a nucleus, labelled by its proton number on the y-axis or its neutron number on the x-axis. What we will be looking at today is this line of nuclei through the middle of the chart along the diagonal. These are the stable nuclei that exist in the universe and on Earth naturally today. If you go away from the stable nuclei, these nuclei off to the side, these are the unstable nuclei that can decay from one to another through the process of nuclear decay. This will be the subject of the future videos. In our quest to investigate nuclear stability, we will start with two questions. Why do the nucleons and nuclei stick together? And why are some combinations of protons and neutrons stable, while others aren't? The reason is related to the forces that are exerted between nucleons. The first of these forces is the familiar electromagnetic force, in particular the Coulomb force that exists between charged particles. You probably know already that unlike charges, for example a positive and a negative charge attract each other, while like charges can repel each other, for example two positive charges. In the nucleus, the neutrons have no charge and experience no Coulomb force, while the positively charged protons will push each other apart. Why then do nuclear hold together? The answer is the nuclear forces that may be new to you. There is a weak nuclear force that doesn't actually hold nuclei together, but is very important in describing why some nuclei can decay, or in other words, can change into other nuclei. We will look at this in the next video. There is also a strong nuclear force that acts between any type of nucleon, for example between two neutrons, two protons, or a proton and a neutron. This nucleon independence is just one of the unusual properties that the strong force has when compared to the forces you might be more familiar with, such as gravity or the electric Coulomb force. The primary property of the strong force is that it is strangely enough strong. In fact, the strong force is strong enough to hold two protons close together, despite their positive charges and the Coulomb force wanting to drive them apart. Remember that I also said the strong nuclear force had some unusual properties.