In this module, we're going to look at a variety of ways that we can represent chemical compounds. In this unit, we're going to learn how to write both molecular and empirical formulas, and we're also going to look at varying ways of representing these models in three dimensions. Chemical formulas show us the composition of compounds or molecules in terms of chemical symbols and subscripts. Each of these formulas shows us the elements that are contained within that substance by using the chemical symbols that we found on the periodic table as well as subscripts to indicate the number of that type of atom in that unit. If we're looking at our first example of water, what we see is that we have H20. This tells us we have two atoms of hydrogen, and one atom of oxygen in each molecule of water. For sodium and chloride, I have one sodium and one chloride and I have that ratio in every sample of sodium chloride. When we get on into larger compounds such as organic compounds or hydrocarbons, we can see that we have two carbons and four hydrogens, we have more options, C4H8 C6H6. All of these represent the formula for a single molecule of those compounds. So, what we're used to seeing are chemical formulas, but sometimes, we actually don't have a chemical formula. We have what's called an empirical formula. This is kind of like a reduced fraction. Instead of saying six-eighths, I can also say three-fourths and it still shows me the same ratio between those numbers. The same thing is true for chemical formulas. If I look at something like water, I notice that the simplest form I could write is H2O. So my chemical formula and my empirical formula is the same. The same is true for sodium chloride and this is also true for most ionic compounds because we actually report the formulas of ionic compounds as their empirical formula. Remember that when we look at an ionic compound, we actually have an array of cations and anions and the size of that array can vary from sample to sample. All we're considered about for ionic compounds, is that we have the one cation to one anion ratio for sodium chloride, or whatever that ratio may be for the particular ionic compound. When I look at something like C2H4, that's what a single molecule of that compound has, but the empirical formula is actually somewhat simpler. It's CH2. Now this doesn't tell me the identity of the compound because I notice that C4H8 which is a single molecule of that particular compound has the same empirical formula. So these are two different compounds, but they have the same empirical formula and where this becomes useful is when we're looking at experimental results. Sometimes we can know the percent of carbon and the percent of hydrogen in the sample and therefore we can determine the empirical formula and with additional information about the sample, we can determine which sample it actually is, which particular compound we have. For C6H6, I'm left with CH and we know we're not going to getting compounds with carbon, that just have two hydrogens or just one hydrogen because that's not enough bonds for carbon. However, these empirical formulas are still a useful tool for us in our research and work on identifying compounds. Let's look at an example. Which one correctly pairs the chemical and empirical formulas? In this case, c is correct. We have C4H10. When I simplify that, or reduce it down to the simplest whole number ratio, I end up with C2H5. If I look at C3H8, that actually is also the empirical formula because I cannot reduce that down any smaller and keep whole numbers as my subscripts. For C6H12, this is a reduced form of that, but I can go one step further and say CH2 because there is one carbon for every two hydrogens. So each of these show the same ratio, but our empirical formula is the one with the lowest whole number subscripts. When I get down to d, I have C6H10, and so the empirical formula is actually going to be C3H5. I can divide both the six and the ten by two and get my smallest whole number ratio here. So we've looked at how we write formulas. How do we draw them? Well, one type of formula we can draw is called a structural formula. In this case, it shows all of the connections between atoms, and shows what type of bonds we have. For example, here we have a carbon carbon double bond, but elsewhere in the molecule, we have simply have carbon carbon single bonds. It also shows our hydrogens connected to each carbon atom. And so because of this structural formula, we can know both how many of each type of atoms are in the molecule but also how the atoms are connected and it depends on what we're doing on which type of formula is going to be the most useful to us. So ball and stick models are similar to structural compound. They still show the connectivity of the atoms. We still see our double bond, and our single bonds here and we see that our hydrogen is coming off of each of those carbon atoms. But what we also see is kind of something about the three dimensional shape of these molecules. Remember that molecules don't exist in two dimensions. These are three dimensional molecules, and so ball and stick models are just one way of representing that. We can also build physical models when we look at the ball and stick models, and so here's an example of a ball and stick model. When we look at this, we see we have a double bond, because we have our bond here and at the bottom. We show that's how our carbons are connected. These are the black atoms that represent our carbon atoms. We also see our hydrogens are connected to those carbons and this ball and stick model gives us some information about the structure of the molecule and let's us see what it kind of looks like in three-dimensional space. Now there are other molecules that we can look at as well, that shows other information and here's an example of this, would be something like methane, which is a carbon with four hydrogens attached. And, when we look at this molecule, what we see is that we can actually see those bond angles that are happening between the hydrogen to the carbon to a hydrogen, and we can see those angles, we can really see what the molecule looks like in three dimensions. And it's important to understand how these molecules look in three dimensions because what we're going to see is that's going to play a role in the reactivity and other properties of those compounds. Now when we look at the ball and stick models, we see these kind of bonds, we actually see a distance between those atoms. But what's really happening is it that those atoms are sharing electrons in these molecules. Remember, the covalent bonds are sharing electrons, and what we see happening is that there's atoms kind of morph into one another, and kind of meld into one another. And so instead of seeing specific bonds, like we saw with our ball and stick models, we actually see kind of a merging of these atoms, 'kay? So here we have our carbon atom here and a hydrogen atom here and these are called space-filling models. They're a little bit harder to use, to see, understand the geometry or the shape of a molecule, but they're a much more accurate representation of what a real molecule looks like. The next thing we're going to look at is the formulas of ionic compounds.
