 Okay, so everything's made up of atoms or molecules and substances can be defined as either pure or mixtures. So now let's look at the distinction between elements and compounds. Both elements and compounds are pure substances. Remember that means that all the particles in these substances are identical. So the difference between elements and compounds has to do with the kinds of atoms that they're made from. An element is a substance that's made entirely of a single kind of atom. For instance, helium is an element and this means that it's made up only of one kind of atom, the helium atom. Oxygen's also an element. It happens to be made up of molecules, but each molecule consists of two identical oxygen atoms joined together. So in a container of oxygen gas, although it's full of molecules, there is still only one kind of atom present. They just happen to be hanging around in pairs. Note that on the periodic table the symbol for oxygen is just O. That's when we're talking about the oxygen atom by itself. But when you encounter oxygen gas in the real world, it always exists as molecules, so it's written as O2, to indicate that each molecule consists of two oxygen atoms joined together. Gold is also an element. At room temperature it's a solid, so the atoms are stuck together in a regular arrangement called a lattice. But if you were to melt the gold, the atoms would separate and move about independently. And if you heated the liquid gold further, you could eventually get gold gas consisting of individual gold atoms flying around, like with the helium. This is also why we write gold as simply AU. Although in a solid version the atoms are stuck together, it becomes clear when you heated up that they are still individual atoms and not bonded together in molecules. In contrast to elements, we have compounds. Remember compounds are also pure substances, they're made up of identical particles. But in the case of compounds, each particle is a molecule that's made up of more than one kind of atom bonded together. Water is a compound. All of its particles are identical, but each particle is a molecule that's made up of two hydrogen atoms bonded to one oxygen atom. So there are two different kinds of atoms in the one molecule, so it must be a compound. Water is a liquid at room temperature, so I've drawn the molecules close together. Carbon monoxide is another compound, but it's a gas. Each molecules are made of one carbon and one oxygen joined together, hence the name. And finally table salt, sodium chloride, is also a compound. It's made up of sodium and chlorine bonded together in a lattice. This is not strictly a molecule, it's known as an ionic compound, and later in this course we'll explore in more detail the difference between ionic compounds and molecular compounds such as water and carbon monoxide. For now, you just need to understand that because it involves more than one kind of atom bonded together, it's a compound.