 Unlike metallic and ionic substances, covalent substances do not really have a set of characteristic properties. The fact that they can form molecules of many sizes and shapes gives them a wide range of properties. You can get a feel for this by looking around you right now. Most things in your world are made of covalently bonded substances. The computer or phone you're watching this on has a lot of plastic parts. Plastics are covalent substances. The ink in your pen and the paper of your notepad are covalent substances. Maybe you have a glass of water nearby. Glass is a covalent substance and so is water. The air you're breathing is made of covalent molecules. And of course you, you're almost entirely covalent. If you consider the properties of all of these substances, you'll see it's very difficult to find something that they all have in common. This is the beauty of covalent bonding. A simple sharing of electrons between individual atoms can lead to an almost limitless variety of materials with different properties. However, if we specifically consider covalent substances that are made of relatively small molecules, then we can identify some general properties. This includes the non-metal elements, which all form molecules, except the noble gases, which are single atoms. They tend to have low melting and boiling points. They tend to be weak and brittle when they're solids. They don't conduct electricity. Most of these properties can be explained by the fact that the substances consist of separate molecules, as opposed to lattices, like ionic and metallic substances. Although the atoms in covalent molecules are joined by strong covalent bonds, the separate molecules are not. This means it takes a relatively small amount of energy to separate them, which accounts for the low melting points and the weakness. There is also the fact that molecules are neutral. That is, they don't carry a charge. And that means that they can't conduct electricity. If we look at covalent lattice structures, however, they are no longer made of separate molecules. Instead, the atoms are all interlinked by strong covalent bonds. This means these substances, such as diamond and graphite and silicon, can have very high melting points and are much stronger, though they're still brittle. Even here there are exceptions, though, as you'll see when we talk about the allotropes of carbon in a different video. There's also the class of covalent compounds known as polymers. These are outside the scope of this part of our course, but you will encounter them later. And they're worth mentioning briefly here, if only because they're so common in our world. Polymers are covalent molecules that are made by joining atoms together in extremely long strings. And by long, I mean hundreds of thousands or even millions of atoms long. Man-made polymers include the soft, fluffy polypropylene that your fleece jumper is made of, the waterproof membrane in your hiking jacket, the silicone glue that bathroom tiles are sealed with, and Kevlar used to make armour. Then there are biopolymers, the proteins that every living thing relies on. Every part of your body relies on proteins in some way, and every protein has a different structure and different properties. So, what to take away from this video? Note down the general properties of small molecular covalent substances. They're worth knowing. But mostly, I really want you to start to build an appreciation for the fact that the covalent bond is responsible for most of the fascinating materials with their almost limitless properties that make up our world.