Here's a video for A-level. That's about isotopes. I would just tell the definition first of ox. This is what you need to know. Two marks in the exam. Is an isotope. Is an atom with the same number of protons but different number of neutrons. Or the same atomic number, but different mass number. So to explain that, we need to start off with the most fundamental of nuclei, which is a proton. This is just a nucleus of hydrogen atom. Now you're going to electron whizzing around in a cloud outside. Okay, we're just going to discount that for now. We're going to focus on just the nucleus. That's where the isotopes really count. Now, a hydrogen-on-zone is just one. It's got a mass number of one and atomic number of one. However, if we get a neutron and we attach that neutron to it, it becomes hydrogen too. Now this is an isotope. Same protons, different amounts of neutrons. We can also make hydrogen-3. Now these are the only atoms that really have a different name. We call them deuterium for hydrogen-2 and tritium for hydrogen-3. Now, if we just add an extra proton, then this no longer becomes an isotope, it becomes helium, because we have a different amount of protons. So that's it. This is just called a helium nucleus. So two protons, two neutrons. Now the thing is with these isotopes, they are actually stable if, especially lower down the periodic table, they have equal amounts of protons and neutrons. This is pretty stable. Now we can make lithium here, which is just three protons and two neutrons. That's an isotope of lithium, lithium-5. So we've got lithium-6, which is the most stable version. And I really like to take this point to say that nuclei are not like this. These nucleons, the protons and neutrons, are constantly moving and flowing around. So we're wishing past each other. So here, this shouldn't make sense. You've got three protons close together. They've got a positive charges. They should repel by the electromagnetic force. You've got three neutrons, which are actually neutral. So they have no charge, but they're attracted. So what is going on? Now, if there's one force pushing apart, then there must be another force attracting. And there is an incredibly short ranges. We have this attractive force called the strong nuclear force. Now you need to know the range of the strong nuclear force, because it's in about three to 0.5 femtometers. And the reason it's that range, protons and neutrons, are actually, you know, we can think of it as a model, as they're actually touching each other. It's not really like this, because some of you may know you have quarks inside, and there's something else going off there. But this is a model, and it's good to understand it like this. So 0.5 femtometers, this force becomes strongly repulsive. But at three femtometers, it becomes strongly attractive, even more attractive, the electromagnetic force. Now, if you look at this graph here, this will show you to crossover points. So as the electromagnetic force, as the nucleons, as the protons get closer and closer and closer together, they're feeling more and more repulsion, until this point here. So as you can see, where the strong nuclear force, and the electromagnetic are equal, that is when, and atom, is just stable. When, just like on this here, the Velcro represents a strong force. We have to get within that short distance before these feel any attraction. So, if it's beyond that, there's no attraction, if it's closer than that, there is attraction, but until we compress it, and we can no longer compress it anymore. So that is why we have an attractive force, called the strong nuclear force, between three and 0.5 femtometers. Now, where does this come from? Well, it's one of the four fundamental forces of nature. Electromagnetism, gravity, the strong nuclear force, and the weak nuclear force. Now, weak nuclear force is due with radioactive decay. The strong nuclear force has an exchange particle, just like the electromagnetic force has photons, the strong nuclear force has gluons, because it glues the nucleus together. Anything with quarks inside it, is going to be subject to the strong nuclear force. Like, stable atoms. Why do we have stable atoms? Well, lower down the periodic table, it's because we equal amounts of protons and neutrons. So, I can quite happily build this up. But this is not a stable atom. I can have, there we go, I'm going to have six protons. So, this would be an isotope of carbon. It will be very short-lived, but it has three neutrons there. So, in these six protons, six neutrons, we have to be in its most stable. This is carbon 12. Six protons, six neutrons, so it's equal. Now, remember, these are all constantly flowing around each other. That becomes the problem. Now, what happens is, we think about three femtometers. It's not between this nuclear and this nuclear. Because that's further away than three femtometers. And these two protons here, they're going to push the part. Right, so, what happens here then, if we add an extra neutron? Now, overall, it just becomes carbon 13. No, that's not a problem, we think. But this breaks down the kind of symmetry of the atoms. Now, the symmetry of the atoms is that it wants to be roughly spherical. Every single atom wants to be its lowest possible energy state. And what happens here is then, we end up with something that's obsessed with the part that we've got carbon 13. Now, this is unstable if we're radioactive. Even more radioactive is carbon 14. Now, what happens if we're carbon 14? This is what we use for carbon dating. Now, because it's unstable, it wants to become more stable. And it does that. How does it do that? Well, one of the neutrons changes into a proton. And it does this by decaying through the weak force. Now, the weak force, what happens is, this neutron here changes into a proton and it meets an electron, an anti-electron nutrient. This then becomes nitrogen 14. It's got seven protons and seven neutrons. So that balance in between the extra strong nuclear force and the neutrons with no repulsion, and the balancing out are the positive charges. So we get equal distances between the protons, and it smooths everything out, and we come back to this beautiful symmetry of seven and seven. Now, this doesn't happen higher up. Atoms like uranium, for example, they are radioactive, but they're particularly stable. A half-life of four and a half billion years. That's because there are extra neutrons in there, and they fill the gaps in between the protons and bring back a symmetry. Now, you may have seen in some of the latest research of pear-shaped nuclei. This is predicted for some time now. Please go away and research it. You don't need to know anything about it at the moment. Ask me questions. It's fantastic. It's the latest research. Please come and ask me. But you don't need to know for this. Just focus on lower down, equal numbers of protons with neutrons stable, uneven numbers of protons and neutrons, usually unstable. This is what we call an isotope. The same number of protons, different number of neutrons. If you have any questions, please leave a comment. Thank you.