 Now before we go any further we need to look at a couple of ways of drawing these molecules because they become quite large. So the simplest way of representing a molecule is the molecular formula that you already know. This has the advantage of being easy to write but it gives you no information about how the atoms are joined to each other. So butane for instance would be written as C4H10. You've encountered this problem before and the way we solved it was with Lewis structures or structural formulae. Here each line represents a covalent bond with two electrons shared between the two atoms. This is the most explicit way of drawing the structure of an organic molecule. However sometimes you don't want to be bothered drawing out all the bonds but you do still want to show which atoms are bonded to which. In this case you might use a condensed structural formula. Here you show each carbon individually and you group the hydrogens that are attached to it. So you could write CH3 or CH2 for instance. The bonds between the carbons can be shown explicitly or not depending on how condensed you want your formula to be. If you're interested mainly in the carbon skeleton of the molecule and not in the hydrogens you could use a skeletal or stick formula. Here only the bonds between the carbon atoms are shown. The hydrogens are implicit that is you assume that the reader knows that any missing or invisible bonds are actually to hydrogens. Skeletal formulae are mostly used for more complicated molecules when drawing a structural or condensed formulae would not clearly show the molecule's shape. Lastly never draw a structure like this. It's a crazy combination of a skeletal and a structural formula and it doesn't mean anything. If you draw the carbon atoms explicitly then you must draw the other atoms as well.