condensed structural formula

A condensed structural formula is a halfway house between a bare molecular formula and a full drawing. Instead of drawing every bond, you write the atoms in the order they are connected and let groups repeat. Ethanol becomes CH3CH2OH: a CH3 carbon joined to a CH2 carbon joined to an OH. You read it left to right like a string of beads, each chunk being one carbon and the hydrogens hanging off it.

The trick is that bonds to hydrogen are written but most other bonds are implied by left-to-right order. So propane, CH3CH2CH3, means CH3 bonded to CH2 bonded to CH3. Repeating groups can be collected in parentheses with a subscript: pentane is CH3(CH2)3CH3 or CH3CH2CH2CH2CH3. Branches go in parentheses too — isobutane is CH3CH(CH3)CH3 or (CH3)3CH, telling you a central CH carbon carries three methyl groups. A double bond is shown explicitly, as in CH2=CHCH3 for propene.

Condensed formulas are compact and easy to type in a line of text or an email, which is why chemists use them constantly for small molecules. Their weakness is that long or branched structures get hard to parse, and rings are awkward, so for anything bigger than a handful of carbons most chemists switch to a skeletal drawing, which shows the same connectivity at a glance.