 You may have heard people sometimes refer to water as H2O. Well, what does that mean exactly? Where do the two come from? What are the H and the O? H2O is an example of a chemical formula and every single compound can be represented with one. The chemical formula of a compound includes the symbols for the chemical elements that make up the compound, as well as numbers called subscripts that show how many of each atom are in the compound. For example, H2O is the chemical formula for water. It consists of the chemical symbol for hydrogen and the chemical symbol for oxygen. The subscript 2 tells us that there are two hydrogen atoms in the compound. The subscript always refers to the element to the left of it, in this case, the H. If there is no subscript, we can assume that there is only one of that type of atom in the formula. Here are some examples of chemical formulas. C6H1206 is the formula for glucose. We have a little subscript on the right of the C. That means there are six carbon atoms. The subscript 12 is to the right of the H. That means there are 12 hydrogen atoms because H is the symbol for hydrogen. And then finally, we have that little six subscript to the right of the O. That means there are six oxygen atoms. We can add all of these atoms up, 6 plus 12 plus 6 equals 24. So we have 24 atoms total in this chemical formula. Now let's do another one. We have H202. This is different than H20. This is hydrogen peroxide. We have the subscript 2 in front of the H. That means we have two hydrogen atoms. And then we have a subscript 2 to the right of the O. Which means we have two oxygen atoms. If we want to count our total atoms, we will add these 2 plus 2 equals 4. Now let's do my favorite chemical sodium bicarbonate, otherwise known as baking soda. We don't have a subscript in front of the sodium Na. So that is one sodium atom. We also don't have a subscript in front of the H, so we have one hydrogen atom. We don't have a subscript in front of the C, so that means there's only one carbon atom. But we do have a subscript 3 in front of the oxygen. And that means we have three oxygen atoms. Often you will see chemical formulas like this one, where we have two or more elements that are in parentheses with a subscript on the outside of the parentheses. Now the reason the OH is in parentheses is because these OH atoms are bonded together, tightly, as a group. The type of bond that bonds them is called a covalent bond. And you will be learning more about these later. Now the subscripts outside these groups in parentheses tell us how many groups of those atoms we have. In the magnesium hydroxide example, we have one magnesium, because we see there's no subscript next to the magnesium. But the subscript to the right of the parentheses tells us we have two groups of these bonded OH atoms. Let's call this particular group a hydroxyl group. So each of the hydroxyl groups has two atoms in it. So if we have two of these groups, it means we have four atoms total from both of these groups together. Now let's total up all of our atoms. So we have magnesium, and then we have two hydroxyl groups here. And if we add all of these up, we get five atoms total. Now let's do another example with ammonium carbonate. Now the chemical formula for this, we see we have an NH4 in parentheses, and we have a subscript 2, which means we have two of these. And then we have a CO3. Now ammonium is the chemical name for NH4, and these atoms are tightly bonded together using these covalent bonds. We talked about, and we have two of these ammonium groups. And then we have one C, and then we have three O's. If we total up all our atoms, we have five atoms from the first ammonium group, five atoms from the second ammonium group. We have one carbon atom, and we have three oxygen atoms. And if we add all of these together, we get 14 atoms. In a chemical equation, you will often see numbers in front of a chemical formula. These numbers are called coefficients, and they tell us how many copies of the compound participate in each reaction. For example, here is an equation for the synthesis of water. We have two hydrogen molecules. Hydrogen happens to be an element that likes to travel in pairs, plus one molecule of oxygen, and that reacts to yield two molecules of H2O, or water. The two in front of the H2 tells us that there are two hydrogen molecules that are used up in each reaction. Likewise, the two in front of the water molecule tells us that there are two water molecules produced in each reaction. If we want to know how many of a certain type of atom are participating in a reaction, we must take both the subscripts and the coefficients into account. For example, let's say we have two molecules of sodium bicarbonate. There are several ways you could do this. If you're good at the distributive property in math, you could do it this way and effectively distribute the two or the molecular formula like this. Or you could write the molecule twice, in other words, the number of times that the coefficient specifies, and count the atoms. You could draw the molecule and count the atoms. It helps to do it in color. Or you could use a table. Here's an example of one way we could use a table. In the left column, we're going to write out all the elements in our formula. And then we're going to multiply these numbers by two to get the number of atoms. And we get two sodium atoms in one formula of sodium bicarbonate. We could also multiply the H by two and we get two hydrogen atoms. We can multiply the carbon by two and get two carbon atoms. And then notice that we have three oxygens in one formula of sodium bicarbonate. And we multiply that by two and we get six atoms of oxygen. If we add all of those up, we get 12 atoms. In summary, in chemistry, we often use chemical formulas to symbolize compounds. The chemical formula of a compound includes the symbols for the chemical elements that make up the compound, as well as numbers called subscripts that tell you how many of each atom are in the compound. If we can use the symbols and subscripts to count the number of each atom in the compound, we can add these up to find the total number of atoms in the compound. And if you have a coefficient in front of the formula, you must multiply each atom in that formula by the coefficient.