In this video, we're going to begin our lesson on the element carbon. Now, of all the bulk elements, which recall from our previous lesson videos are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur or CHNOPS. If we exclude water then it's actually the element carbon that is the most abundant element in living systems. And so that goes to show how important the element carbon is to living things. Now, carbon is the main component of organic molecules, and really there are 4 classes of organic molecules that we're going to talk about moving forward in our course in separate videos, and those 4 classes of organic molecules are carbohydrates, proteins, nucleic acids, and lipids. And once again, carbon is the main component of these molecules. Now the terms organic molecules are really just referring to any molecule with covalently linked carbon and hydrogen atoms. And so if the molecule contains covalently linked carbon and hydrogen atoms at some point within the molecule, then we refer to it as an organic molecule. Now the term hydrocarbons, on the other hand, as its name implies, are organic molecules themselves that are only made up of carbon and hydrogen atoms, and so they contain no other elements. Again, hydrocarbons shows that they only have hydrogens and carbons, whereas organic molecules that are not hydrocarbons, they can have other types of atoms. So just to get a better feel for this, let's take a look at our example down below, which is asking us to circle the organic molecules in green and then highlight the hydrocarbons in yellow. And so when we take a look at these structures that we have down below, notice that this molecule over here has a carbon covalently bound to a hydrogen, so that makes it an organic molecule. So we'll go ahead and circle this one in green right here. Then notice this one over here has an oxygen bonded to hydrogens, and this is a water molecule. But water molecules are not organic molecules because they don't have covalently linked carbon and hydrogen atoms. So this is not going to be an organic molecule. And notice this over here is CO₂ or carbon dioxide, and once again, CO₂, it does not have covalently linked carbon and hydrogen atoms, so this is not going to be an organic molecule. Now notice that this one over here does have carbon and hydrogen covalently linked together so that makes this an organic molecule and we can circle it in green. And then last but not least over here, notice that this one also has carbon and hydrogen bound, and so that also makes it an organic molecule. We can go ahead and circle it. So now to identify the hydrocarbons, the hydrocarbons are going to be organic molecules that are only made of carbon and hydrogen atoms. So if we take a look at option a over here, notice that this is methane, which is only made up of carbon and hydrogen atoms. So that makes this a hydrocarbon, and so we can go ahead and highlight it in yellow. And really the same goes for option d over here. It is only containing carbon and hydrogen atoms. So once again, that makes this a hydrocarbon as well. But then notice that this last one over here has an oxygen atom over here, and so it is not only made up of carbon and hydrogen atoms if it has an oxygen in it. And so this one is an organic molecule but it is not a hydrocarbon. And so, as we see here, this is the correct answer to this example. And so this here concludes our introduction to carbon and we'll get to talk a little bit more about the element carbon in our next lesson video. So, I'll see you all there.