we understand that two molecules could have the same empirical formula but different molecular formulas these molecules both have the empirical formula ch2 but when we write the molecular formula one is c2h4 and the other is c6h12 but what we also must understand is that two molecules might have the same empirical formula and the same molecular formula but still be different molecules that is because molecules can have the same molecular formula but different structural formulas these two molecules are both c4h10 but the way that those carbon and hydrogen atoms are connected to one another differs that makes these structural isomers of one another different structural isomers may have completely different properties even though they have the same number of the same type of atoms now let's take the molecular formula c4h9br there are different structural formulas that can correspond to this molecular formula in other words there are different ways that we could connect these four carbon atoms nine hydrogen atoms and one bromine atom how many can you draw take a moment and see what you can come up with here are three possible structural formulas that correspond to this molecular formula notice that they all have four carbon atoms nine hydrogen atoms and one bromine atom so while empirical formulas describe the ratio of the atoms of each element in the compound and the molecular formula describes the precise number of atoms of each element in the compound the structural formula takes the precise number of atoms and shows precisely how they are connected to one another this hierarchy is important to understand because we will want to know the different structures that are possible given the information we have about a molecule