2.5 Skeletons in the Closet
What if we want to represent a compound like vanillin, artificial vanilla flavor, where three of the hydrogens of a benzene ring are substituted? We could write it as CH3OC6H3(OH)CHO (note CHO means CH=O), but that doesn't tell us which three places on the ring are bonded to these other groups. We could write out a 2 dimensional representation of a ball-and-stick model:
HC--C-OCH3
// \\
O=CH-C C-OH
\ /
HC==CH
But that's tedious, and for large complex molecules, it would be a nightmare. It's also not very intuitive; it's hard to see the symmetry of the hexagon shape. What if we simply omit the carbon atoms and most of the hydrogens? Then we can write something like this:
This is called a skeletal formula, and it takes advantage of two facts; one that organic molecules are made of a heavy atom skeleton onto which hydrogen atoms are bonded to satisfy valences, and two, that the skeleton is made mostly of carbon. The hexagon with the three lines is such a universal symbol for a benzene ring that it even has its own unicode encoding: ⌬, and drawing it out like that enables us to show exacty where its substitutions are attached.
In a skeletal formula, each line represents a bond between two atoms. Double and triple lines indicate double and triple bonds, respectively. Any atom that isn't specified is assumed to be carbon; the corners of the figure and all line endings are carbon atoms. Hydrogens of these carbon atoms are implied, but when a heavy atom is written, its hydrogens (if any) are also written out.
While skeletal formulas work great for complex molecules, they fall short for molecules like methane, which would just be a dot. Therefore you will often see structural formulas that are hybrids of the skeletal and condensed formats, combining elements of both. This is just done for clarity and readability.
By the way, you will sometimes also see 3 dimensional structures that resemble skeletal formulas. Hydrogens are omitted in this format and elements other than carbon are indicated by colors. It is sometimes called sticks or licorice, and is very common in depictions of molecules interacting with proteins. Double bonds may or may not be simplified to single lines in this mode. Here's vanillin in a licorice representation: