Repeating unit

In chemistry, a repeating unit or more properly constitutional repeating unit,^[1] is a small piece of a large molecule like a polymer or crystal that connects to itself over and over to build the bigger structure. The elements and chemical bonds in the repeating unit are responsible for the chemical and physical properties of the larger molecules.

The repeating unit of polypropylene is −CH(CH₃)CH₂−.

More complicated polymers like polybenzimidazole have more complicated repeating units.

Table salt is a three-dimensional crystal with the repeating unit NaCl. This visualisation shows sodium as a blue ball and chlorine as a green ball.

Many minerals have very complicated crystal structures, but still have visible repeating units. Muscovite has the repeating unit KAl₂(AlSi₃O₁₀)(OH)₂.

By convention, polymers and ionic compounds are represented by their smallest repeating unit. One notable exception is polyethylene, which has a smallest repeating unit of (CH₂)_n but is frequently represented by (C₂H₄)_n which looks more like the repeating unit of the other vinyl polymers.