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Chemical classification systems attempt to classify elements or compounds according to certain chemical functional or structural properties. Whereas the structural properties are largely intrinsic, functional properties and the derived classifications depend to a certain degree on the type of chemical interaction partners on which the function is exerted. Sometimes other criteria like purely physical ones (e.g. molecular weight) or – on the other hand – functional properties above the chemical level are also used for building chemical taxonomies.
Some systems mix the various levels, resulting in hierarchies where the domains are slightly confused, for example having structural and functional aspects end up on the same level. Whereas chemical function is closely dependent on chemical structure, the situation becomes more involved when e.g. pharmacological function is integrated, because the QSAR can usually not be directly computed from structural qualities.