We've already seen in other videos that nonmetals can join together by covalent bonds, in which they share electrons so that all of the atoms have full outer shells. Sometimes this results in small molecules, such as chlorine or ammonia, and we call these simple molecular substances. In other cases, nonmetals bond covalently to form giant covalent structures, like diamond, graphite or silicon dioxide. In today's video we're going to look at the respective properties of each of these, and then finish up by taking a closer look at the structure of silicon dioxide. The first thing to know is that covalent bonds are really strong, which means that a lot of energy is going to be needed to break apart any atoms. So if we consider a simple molecular substance like chlorine, the atoms within each molecule will be strongly bonded together. However in order to melt or boil chlorine, we don't break these strong covalent bonds. Instead we only need to break the weak forces that exist between different molecules, which we call intermolecular forces. Because of this we only need very low temperatures to melt the covalent bonds. This is why we call these covalent bonds, or covalent bonds, the weak forces. They are the weak forces that exist between different molecules, which we call intermolecular forces. We need to take into account that these forces are very hard to melt. For example, chlorine boils at minus 34 degrees Celcius. The more of them that a molecule has, the stronger the overall attraction is going to be. This is why we try to make sure that the energy is not in the form of a solid molecule. In this case, we take into account that the energy in the form of a solid molecule is very weak. We add some more of them, and we obtain a mixture of chlorine, bromine, and iodine. So we have a mixture of chlorine and bromine. The more of these intermolecular forces are individually quite weak, the stronger the overall attraction is going to be. For example, let's compare the halogens. Chlorine, bromine, and iodine. Because we're going down the group, the atoms and thus molecules are getting bigger. so there will be more intermolecular forces between them. This in turn means that more energy will be required to break them all, so the melting and boiling points should increase as you go down the group, which they do, as bromine has a boiling point of 59 degrees and iodine doesn't boil until 184 degrees. There's no need to remember these specific numbers, you just need to understand that the boiling point increases as you go down the group because the molecules get larger, so there are more intermolecular forces between them. Another property to note is that simple molecular substances don't conduct electricity, because there are no free electrons, and the molecules themselves have no electric charge. You'll see this point again and again in chemistry. In order to conduct electricity or heat, substances have to have some electrons or ions that are free to move about. So, moving on to giant covalent structures, these are made of huge numbers of non-metal atoms that are all bonded to each other by covalent bonds, and they're generally arranged into regular repeating lattices, which just means that their structure kind of repeats over and over. The three important examples are diamond, graphite, and silicon dioxide. The main thing to remember about these structures is that they're very strong, and they have main thing to note is that the covalent bonds are not always the same, and they are often the same. The covalent bonds are usually the same, but they are often the same. The high melting and boiling points, because we'd have to break all of these strong covalent bonds in order to melt them. The other property to note is that they generally don't conduct electricity, because they don't contain any charged particles, even when they're molten. An exception to this though is graphite, which we take a closer look at in another video along with diamond, which are both made of carbon atoms. Silicon dioxide, on the other hand, is made of silicon and oxygen atoms in a ratio of one to two. It's also known as silica, and is the main component of sand. You won't have to draw it, but you do need to be able to recognise giant covalent structures like this. Now the key thing to take away from this video is that simple molecular substances are small molecules that are made up of just a few covalently bonded atoms. And the separate molecules are only joined together by weak intermolecular forces. Meanwhile in giant covalent structures, all of the atoms are covalently bonded in regular repeating lattices, which makes them much stronger, and gives them much greater melting and boiling points. Anyway, that's it for now, so hope you enjoyed it, and we'll see you next time. If you haven't heard yet, you can find all of our videos on our website cognito.org. You'll also find questions, flashcards, exam style questions and past papers, and we track all of your progress so that you always know what to study next. So sign up for free by clicking here or browse our playlist here on YouTube.