There is essentially an infinity of molecular compounds that can be formed. In order to make them easier to keep track of, chemists apply a system of nomenclature that helps us determine the name for a given compound. The name has to convey what is in the compound, and in what quantities. We convey the quantities by a system of numerical prefixes. The bad news is there’s no trick to memorizing these, you just have to learn these prefixes. The good news is you may have seen many of these prefixes before. One is mono-, two is di-, three is tri-, four is tetra-, and five is penta-. These five prefixes will get you through the vast majority of compounds you’re likely to encounter in chemistry. But the system of prefixes does keep going. Six is hexa-, seven is hepta-, eight is octa-, nine is nona-, and ten is deca-. Sometimes, in order to make a word flow better, the last vowel will be dropped off. But usually, the prefix exists just as it’s written here.

Let’s look at a few other rules about naming these molecular compounds. First, when we’re reading the chemical formula, we read from left to right, using the numerical prefixes we just covered. For a compound that begins with a solitary element, we don’t begin compound names usually with the prefix mono-, we just drop that off. When you get to the last element in a compound, we drop off the last bit of the name, and replace it with -ide, just like we did with monatomic anions. And remember that when we write these down, these compounds are not proper nouns, so we write them in lower case.

As an example, let’s look at CO2. Reading from left to right, the first element we encounter is carbon, and there’s one of it, so we might want to put monocarbon. But remember that we never start a compound name with mono-, so we drop that off, and we just have carbon. Next, we have two oxygens, so we might want to put dioxygen, but since this is the last element in this compound, we drop off the end of the word, and replace it with -ide. So this compound is called carbon dioxide.

Let’s look at another few examples really quickly. First, CO, that’s a single carbon and a single oxygen. We don’t put the mono- prefix on the carbon, just on the oxygen. So this is called carbon monoxide. This is one of those words that flows better when we drop one of the Os, so it’s not “”mono-oxide.” Next, P4O10. That’s four phosphorous atoms and ten oxygen atoms. The prefix for four is tetra-, and the prefix for ten is deca-, so this is called tetraphosphorous decaoxide. Next one, AsBr3. That’s one arsenic atom and three bromine atoms. We don’t put the mono- prefix on the first one, so this is just arsenic tribromide. And lastly, SiI4, a single silicon and four iodine. Again, dropping the mono- prefix, we get silicon tetraiodide.