In this module, we're going to look at how we name molecular compounds. So our objective is that by the end of this module, you will be able to name molecular compounds according to the IUPAC rules. So when we look at molecular compounds, we're going to see discrete molecular units, and these are between two non-metallic elements. Remember that when we talk about molecular compounds, we can't predict their formulas like we can for ionic compounds, because there are different ways for those electrons to be shared between atoms. And as a result, we can have different numbers of each type of atoms. We talked about our example with nitrogen and oxygen that we could have NO, NO2, N2O4, N2O5. Lots of different options for how to name those compounds, and so we have to have something in the name of molecular compounds that tells us how many of each type of atom we actually have in that unit. Now most of these are going to be binary compounds, meaning they contain two different elements. We could have many more than two atoms, but they'll only contain two elements. There are some exceptions but we're not going to be getting into those in this course. So now let's look at some examples of how we name molecular compounds. When we look at the formula, we see a cert, one element written first in the formula. And we're going to put that element first in the name as well. So we'll follow the exact same order as the chemical formula. For the first element we simply name it. The second element we change the ending to I-D-E. 'Kay? So much like we did with our anions and our ionic compounds. Now remember we said that there's lots of different ways that elements can be combined in order to form molecules. And so we have to include something in the name to show us how many of each type of element we have in the compound. So we use something called prefixes. And these prefixes tell us exactly how many of each element we have. So if I look at my example here of N2O4, I see that there are two nitrogen atoms. So I use the prefix di. And there are four oxygen atoms. So I use the prefix tetra. Now, we notice up here with the carbon monoxide that we have the, the mono prefix on our oxygen, but we don't have a prefix in front of carbon. And we'll talk about that in just a minute. That's one of our exceptions to the rules. There are always exceptions to the rules. And so what we need to know is to know those prefixes from one to ten, and we can then name any compound. It's also easier to get back to the formula from the name, because the name tells me exactly how many I have of each type of atom. Unlike with ionic compounds where we're getting a transfer of electrons and we had to learn about charges, electrons here are being shared. There are no charges, and therefore, there's no way to really predict what our formula will be, and so the name has to give that information. So, we have prefixes from one to ten. There are others that go greater than ten, but we're not going to be worrying about those, this is aplenty for us. Di is two. Tri is three, so we could think about a tricycle. Tetra is four. Penta is five. Hexa is six. Hepta is seven. Octa is eight, so if you see octagon-shaped stop signs, that's an oct, eight-sided figure. That's an octagon. Nona is nine, they kind of sound like each other. And deca is ten, so similar to decade, which means ten years. So these are the prefixes that we'll include in the name before each element to determine how many of each atom are in that compound, in that chemical formula. And as with any rule in chemistry, there are exceptions to that rule. And the first one we look at is about the prefix mono. And we can actually omit that mono, but only if it's on the first element. So on our previous page, we looked at carbon monoxide or CO. We can omit the mono in front of carbon, but we can't omit it in front of oxide, we have to keep it there. So we're not going to write this as monocarbon monoxide, it's simply carbon monoxide. Any other time we would need to include a prefix in front of that first element telling us how many of that particular atom we have. Just like we did here with our N2O4 we have dinitrogen tetraoxide. We also see that when we have oxides, we can also drop the A, so that we don't end up with these two vowels together. So we don't end up with tetraoxide, we end up with tetroxide. It just makes it a little bit easier to say, and that's why it's done. So let's look at some examples of how we get from a formula to a name, and from a name to a formula. So in this case we have two elements in our compound. We have sulfur and we have oxygen. And whenever we put the name oxygen in, we're going to change that suffix to ide, so we have sulfur and oxide. But because this is a molecular compound, because I have two non-metal elements here, in a molecular compound, I need to use prefixes when I'm writing the name. Now, sulfur does not have a subscript, which implies that it's one. The prefix that goes with one is mono. But because sulfur is my first element, I can omit that prefix. For oxygen, I have three as my subscript, which means my prefix is going to be tri. So what I end up with is sulfur, and there's a space between these two words, trioxide. Note that there's not a space between the prefix and the name of the element. So here we have sulfur trioxide. With our second example, we're actually looking at going from the name back to the formula. So again, when I look at the name, I'm going to look for the elements involved, and I see I've got carbon and I see chloride, so I know that's chlorine, so I have CCl. There's no prefix for carbon, which tells me that has a subscript of one. For chlorine, I see I have a prefix of tetra, and tetra equals four, so I have CCl4, or carbon tetrachloride. For our last example, we have P4S10. So I know I have phosphorous, and sulfur as my two elements. I'm going to leave the phosphorus name as it is, so right there, there's phosphorus. But I need to change the ending on sulfur to sulfide, the I-D-E ending there, now I need to put my prefixes in to tell how many I have of each type of atom. I see a four for my phosphorus, so I use tetra for phosphorus, sulfur has a sub, subscript of ten, so that's deca. So I'm going to have tetraphosphorus, all one word, decasulfide, and that's also all one word. The only space will be between those two elements. So what is the IUPAC name for SF6? The correct name is sulfur hexafluoride. So let's look at the other answer choices and determine what's wrong with those. We notice that there's no subscript on S, which means there's only one sulfur atom, and the prefix would be mono, but we don't ever use mono when it belongs with the first element. And so the problem with that one is if it's monosulfur hexafluoride. Likewise with answer choice B, we don't include the mono, but we also see that we have hepta, and that represents seven of an atom, so that can't be correct. And for the last one, we have sulfur, so we omitted the mono as a prefix which is great, but now we have hepta, which means seven. And so the formula for that would be SF7, not SF6. In the next module we'll look at how we name acids, both binary acids and oxy acids.
