In this unit, we're going to explore the differences between elements and compounds. By the end of this module, you should be able to identify a substance as either being an element or a compound. There are a variety of properties which distinguish elements and compounds from one another. Elements are those substances that we find in the periodic table, whereas compounds are combinations of those elements. What we find is that we can actually get di, many different compounds forming from the same elements. For example, with nitrogen and oxygen, there are variety of compounds I can make simply by different combinations of nitrogen and oxygen, such as NO2, NO, N2O5, N2O4, and that has to do with how those bonds are forming. But what I can also see is that the properties of the elements going into the compound are very different than the properties of the resulting compound. For example, if I look at water, which I have here, it's composed of two hydrogen atoms and one oxygen atom. But if I look at the properties of hydrogen and oxygen, which compose this compound, I see very different sets of properties. For hydrogen and oxygen, they're both flammable, whereas water is used to put out flames. When I look at water at room temperature and pressure, I see it's a liquid, but both hydrogen and oxygen are gas phase at room temperature and pressure. So we need to be able to distinguish between elements and compounds so that we can further understand why these substances behave the way they do. So before we start distinguishing between elements and compounds, let's backtrack a little bit to look at matter. Now, we need to remember that matter is anything that has mass. And occupy space, which is essentially everything, whether it be something large, like an elephant, or something very small, like an electron. While the volume and mass may be very small, it's still considered matter. Now, once we look at matter, we can subdivide that down into pure substances or mixtures. And when we look at mixtures, what we see is we have two or more pure substances combined. So when we have two or more pure substances combined, we can have either a homogenous mixture or a heterogeneous mixture. But our main concern here is talking about the pure substances. When we have a pure substance, we can categorize it as either an element or a compound. And what I see in an element is that it has one type of atom. In other words, just hydrogen, just oxygen, just nitrogen, just sodium. Whereas if I look at a compound, I see that compounds have two or more elements in them, or two or more types of atoms, and these elements are chemically bonded. Now, this differs from a mixture because I could have a mixture of hydrogen and oxygen, and that would be a mixture because I have two separate substances there. I have hydrogen and oxygen. If I'm looking at a compound that contains hydrogen and oxygen, I'm looking at something such as H20 where the hydrogen and oxygen atoms are actually bonded together. Now, we can divide things as elements or compounds, but we can actually further divide them down into even smaller categories. For elements, we look at them as either atomic or molecular. And atomic means that these atoms exist as single atoms. So, helium, lithium, titanium, all of these are atomic elements. Molecular elements are elements that exist in nature as a group. We still only have one type of atom, but we have multiple of those atoms connected together. So, what we see are things like nitrogen, hydrogen, most of which are diatomic. We occasionally see something like S8 or P4, but those are very rare exceptions. And on the next slide, what we'll look at is which element form these molecular elements. Still only one type of atom, so they're an element, but there's more than one of them connected together, so it's a molecular element. Now, we also have molecular compounds and ionic compounds. And molecular compounds do contain bonds much like our molecular elements do. The difference is that we have two or more elements in those compounds. When we look at molecular versus ionic compounds, what we're looking at is the way that the atoms are bond together. And so, between these, there are different types of bonding. So we know they're bonded together because they're compounds, but we can have different types of bonding and therefore form different types of compounds. And we'll compare those in more detail in just a minute. So let's look at our molecular elements. The main ones we're going to be worried about recognizing are our diatomics. So, I see hydrogen, which is over here by itself, and then I see the remaining six. So, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. All of these elements exist in nature as diatomics. There are seven of them, including hydrogen, and we see we make a pattern of a seven if you look at what's highlighted in yellow. When we look at these in nature, what we see are molecules of N2. I'm not going to find nitrogen atoms in space. I'm going to find N2 molecules, O2 molecules, fluorine molecules. And that's how these species exist in nature. So, for example, if we say we have an sample of carbon, then we would simply say we have C because carbon is an atomic element. But if I say I have nitrogen, I need to know that that is N2 because it exists as a diatomic in nature. Notice there's nothing in the name that indicates whether something is an atomic element or a molecular element, so we're expected to know which elements exist as diatomic elements. So let's look back at the ionic versus molecular compounds. We talked about the bonding being different, and the reason that the bonding is different is because we have different types of species interacting with one another. When we have an ionic compound, what we see is that we have a cation or a positively charged ion, and an anion, a negatively charged ion. And they are attracted to each other because of the different charges on those ions. We also see that ionic compounds exist in arrays. So we have a line of sodium, and chlorine, sodium, and chlorine, sodium, and chlorine, and we see nice, order arrays of these ions. So if I look at any one row, I'm going to see a chlorine, a sodium, a chlorine, a sodium, chlorine, sodium, and so on. And so, when we talk about the formula for an ionic compound, we talk about a very simple formula. We write it as NaCL because when I look at an actual crystal of sodium chloride, it contains many, many, many sodium ions and many, many, many chloride ions. The only thing we're really worried about is the ratio, and we'll talk more about how we figure that out in a later unit, but we know that there's one sodium ion for every one chloride ion in this particular ionic compound. When we look at the types of elements that are involved here, if I look at sodium on the periodic table, I find that it's on the left side and that it's a metallic element. And if I look at chlorine, it's on the right side and it's a nonmetallic element. So, ionic compounds are formed between a metal atom that forms a so, an, a cation or positively charged ion, and a nonmetal, which forms a negatively charged anion. And together, they form an ionic compound. Now, when I look at molecular compounds, I see a couple of differences very quickly. One, notice I no longer have an array of ions like I did in my ionic compound, but I have separate molecules in the molecular compound. These are also called covalent compounds sometimes because they have covalent bonds. And in this case, this look, is looking at CH4, this is also known as methane or natural gas. And so, if you have gas line in your house for a stove, a lot of time it's burning natural gas. And so, that's what we have here. These molecules, these are methane molecules, and they exist discretely from one another. We don't see the array that we saw in our ionic compound. The other thing we notice when we look at our molecular compounds is the type of elements that make up these compounds. I see that carbon is a nonmetal and hydrogen is a nonmetal. Remember, hydrogen is our odd one out. Even though it's on the left side of the periodic table, it's really a nonmetal. And so, I get molecular compounds when two nonmetal atoms, or two or more nonmetal atoms bond to one another. We've been using the terms molecules and compounds a lot, but we want to take the time to distinguish between the two to make sure we understand the difference. Notice when we look at a molecule, it says at least two atoms in a definite arrangement held together by chemical forces. In other words, a bond, okay? So we have to have two atoms. When we look at a compound, notice that I have two or more elements held together by some chemical force or a bond. So the distinguishing factor here is two or more atoms versus two or more elements in order to be a compound. So let's look at our examples up here. This element up at the top is hydrogen, or H2, and this is a molecule because it has two or more atoms held together by a chemical force or a chemical bond. When I look at something like carbon tetrachloride, or CCl4, this is a molecule because it has two or more atoms, but it is also a compound because it has two or more elements, okay? Likewise for other examples here, this is both a molecule and a compound, as is HCN. And HCl. Now, we can also have compounds that are not molecules. If I look at something like sodium chloride, which is an ionic compound because we have a metal and a nonmetal, we have two ions that are attracted to one another, this is a compound, but it is not a molecule. Because we're not looking at discrete units of that substance. What we have in sodium chloride is a continuous array, so we call this a compound, but it's not a molecule. All of our covalently bonded compounds are also made up of molecules. For things like hydrogen and nitrogen, those diatomic species, they're molecules, but they're not compounds because they only have one type of element in them. So, let's look at an example. Which of the following is a molecular compound? So, carbon monoxide is a molecular compound. Remember our rules for a compound that we have to have two or more elements bonded together,. And to be a molecule or to be a molecular compound, they have to be between two nonmetals. So, carbon monoxide meets that definition. When I look at aluminum, I know it only has one type of atom. Therefore, it cannot be a compound, nor can it be a molecular element. And so, this is an atomic element. When I look at aluminum chloride, I notice that aluminum is a metal, chloride is a nonmetal. It's a compound because it has two or more elements bonded together. And so, what I see is this is an ionic compound because it has a metal and a nonmetal. Chlorine is not a compound because I only have one type of atom in there, but I do have two or more of them bonded together. And so, this is known as a molecular element. And then cobalt, Co, which is very different than CO of carbon monoxide, notice that capitalization matters, is also an atomic element because it's a single atom all by itself. In the next module, we're going to explore more about the different types of chemical bonds and how they form.