Skeletal structures

In skeletal drawings, chemists usually omit hydrogens bonded to carbon; heteroatoms and their attached hydrogens are normally shown. A skeletal structure shows carbon skeletons as zig-zag lines: every vertex and every line-end is a carbon, and each carbon silently carries as many hydrogens as it needs to reach four bonds.

It looks like information has been thrown away. It has not — the hydrogens are recoverable exactly, which is why the convention works.

Degrees of unsaturation turn a molecular formula into structural information before you know the structure:

> DoU = (2C + 2 + N − H − X) / 2

Each ring counts 1, each double bond 1, each triple bond 2. DoU = 4 is compatible with a benzene ring (three double bonds plus one ring), but also with many non-aromatic combinations; other evidence must decide.