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What defines a “metal” is not always obvious and the differences between metallic and nonmetallic elements may be subtle (Vouk, 1986). Metals are typically defined by physical properties of the element in the solid state, but can vary widely with the metallic element. General metal properties include high reflectivity (luster), high electrical conductivity, high thermal conductivity, and mechanical ductility and strength. A characteristic of metals of toxicological importance is that they may often react in biological systems by losing one or more electrons to form cations (Vouk, 1986). In the periodic table, within a group there is often a gradual transition from nonmetallic to metallic properties going from lighter to heavier atoms (eg, Group IVa transitions from carbon to lead). Metals often exhibit variable oxidation states. Various names are applied to subsets of metallic elements including alkali metals (eg, lithium and sodium), the alkaline earth metals (eg, beryllium and magnesium), the transition (or “heavy”) metals (eg, cadmium), and the metalloids (eg, arsenic and antimony), the latter of which show characteristics of metals and nonmetals.
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Over 75% of the elements in the periodic table are regarded as metals and several are considered metalloids. This chapter discusses metals, and certain metal complexes or molecules, that have been reported to produce significant toxicity in humans. The discussion includes major toxic metals (eg, lead, cadmium), essential metals (eg, zinc, copper), medicinal metals (eg, platinum, bismuth), and minor toxic metals including metals of technological significance (eg, indium, uranium). This chapter will also discuss toxic metalloids (eg, arsenic, antimony) and certain nonmetallic elemental toxicants (eg, selenium, fluoride). An overview of toxic effects of metals is shown in Fig. 23-1.
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