The result was a (good-natured) Twitterstorm, as other chemists pitched in with their views. The problems with these definitions were highlighted by a recent suggestion by Nature Chemistry’s editor Stuart Cantrill that catenanes and rotaxanes – entities consisting of distinct covalently bonded assemblies mechanically conjoined (threaded rings, for example) – must be considered molecular, not supramolecular.

And the idea that molecules are two or more atoms held together by chemical bonds gets us in to the murky business of defining a bond (which some consider to include van der Waals forces and ‘mechanical bonds’) and distinguishing molecules from, say, ionic crystals (as Gilbert Lewis first did a hundred years ago). Which properties? After all, most of those we focus on, such as melting point, are bulk properties. For example, it has been defined as: ‘The simplest unit of a chemical compound that can exist, consisting of two or more atoms held together by chemical bonds.’Īnother definition goes like this: ‘The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms a group of like or different atoms held together by chemical forces.’īut in this second definition, ‘the smallest particle of a substance that retains its properties’, is pretty meaningless.

Sentence with the word molecule