The replacement of the nondescript term ‘heavy metals’ by a biologically and chemically significant classification of metal ions

It is proposed that the term ‘heavy metals’ be abandoned in favour of a classification which separates metal ions into class A (oxygen-seeking), class B (nitrogen/sulphur-seeking) and borderline (or intermediate). A survey of the co-ordination chemistry of metal ions in biological systems (mostly X-ray crystallographic data) demonstrates the potential for grouping metal ions according to their binding preferences (i.e. whether they seek out O-, N- or S-containing ligands). This classification is related to atomic properties and the solution chemistry of metal ions. A convenient graphical display of the metals in each of the three categories is achieved by a plot of a covalent-bonding index versus an ionic-bonding index. A review of the roles of metal ions in biological systems demonstrates the potential of the proposed classification for interpreting the biochemical basis for metal-ion toxicity and its use in the rational selection of metal ions in toxicity studies.