Skeletal Formula - Implicit Carbon and Hydrogen Atoms

Implicit Carbon and Hydrogen Atoms

For example, in the image below, the skeletal formula of hexane is shown. The carbon atom labeled C₁ appears to have only one bond, so there must also be three hydrogens bonded to it, in order to make its total number of bonds four. The carbon atom labelled C₃ has two bonds to other carbons and is therefore bonded to two hydrogen atoms as well. A ball-and-stick model of the actual molecular structure of hexane, as determined by X-ray crystallography, is shown for comparison, in which carbon atoms are depicted as black balls and hydrogen atoms as white ones.

Any hydrogen atoms bonded to non-carbon atoms are drawn explicitly. In ethanol, C₂H₅OH, for instance, the hydrogen atom bonded to oxygen is denoted by the symbol H, whereas the hydrogen atoms which are bonded to carbon atoms are not shown directly. Lines representing heteroatom-hydrogen bonds are usually omitted for clarity and compactness, so a functional group like the hydroxyl group is most often written −OH instead of −O−H. These bonds are sometimes drawn out in full in order to accentuate their presence when they participate in reaction mechanisms.

Shown below for comparison are a ball-and-stick model of the actual three-dimensional structure of the ethanol molecule in the gas phase (determined by microwave spectroscopy, left), the Lewis structure (centre) and the skeletal formula (right).