Remember, elements are the most basic form of matter. Elements are matter made of only one kind of atom. It is when you start to combine elements together that you form compounds. A chemical compound is matter created when two or more elements bond together.
Chemical compounds have names that describe what they are made out of. They can also be expressed in chemical formulas that tell you which elements are in that compound and the number of each element that makes it up.
For example, water is a common chemical compound. When written as a chemical formula, it is expressed as H₂O. The H shows that water is made up of hydrogen and the O shows that is also made out of oxygen atoms. The smaller 2 after the H tells you there are two hydrogen atoms. Oxygen does not have a smaller number after it because there is only one oxygen atom. If we were to put a number after the O, it would be a 1, but scientists don’t write in the 1, the letter alone informs you that there is only one.
When there is a number in front of the chemical formula, it tells you how many of that chemical compound there are. If there are 3 molecules of water, it would be written as 3H₂O. That means there are six hydrogen and three oxygen (because there are 2 hydrogen for every water molecule, and there are 3 molecules, so 2 x 3 = 6).