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Comparatively Speaking: Molecular Notation of Compounds vs. Compositions

Contact Author Anthony J. O'Lenick, Jr., Siltech LLC
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In this look at representative chemical structures, Tony O'Lenick compares the structural notation of molecules present in sodium chloride with those present in sodium polymethacrylate. A previous feature compared the number of molecules in compounds vs. compositions, and a future feature will discuss differences in their distribution.

Accuracy of the Structure

NaCl (Figure 1) is a simple structure that needs no explanation. Sodium polymethacrylate (Figure 2), however, is a different matter. Look at the structure and determine what is missing—or at least, unclear.

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Undergraduate chemists are told that all carbon molecules require four bonds around them. However, to which element are the carbon molecules in the parenthesis bonded? There is a bond outside the parenthesis that has no partner; (-) is not an element. Should it thus be concluded that the material is cyclic—i.e., that is connecting to the other (-)?

The terminal groups on the molecule, meaning those that are attached to the (-), relate to the initiator used to run the reaction and the type of post treatment used in making the polymer.

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Figure 1. Sodium chloride

Sodium chloride is a compound.

There is a salient difference between polymers and compounds. Pure compounds like NaCl are single molecules.

Figure 2. Sodium polymethacrylate

Sodium polymethacrylate

Polymers are complex oligomeric mixtures.

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