Heavy metals are generally defined as metals with relatively high densities,atomic weights, or atomic numbers.

These metals include transition metalssuch as lanthanides and actinides as well as semi-metals (metalloids) such asArsenic and Antimony. The term heavy metal can also be referred to elements ofthe periodic table of elements with atomic number 21 or higher (eg. Zinc).

Thisterm heavy metal mainly came aboutwith dialogues about pollutants that were released to the environment in theform of air, water or soil contaminants. Some heavy metals are chiefly knownfor their toxicity to man and other organisms. On the other hand, several of them(eg.

Zinc, Iron, Copper, Chromium, Cobalt) are vital for metabolic function inmany organisms in their right quantities (Arielet al, 1973). Despitetheir significance (some of the heavy metals) as micronutrients for animals,plants and many microbes, all heavy metals prove toxic to living organismsthrough metabolic interference and genetic mutations. This usually is based onthe route, and dose of these heavy metals that enters the organism. This toxicimpacts span from fitness reduction, through to reproductive interference,carcinoma and in extreme exposure, being lethal. Variousways for classifying heavy metals have been adopted and usually been based onatomic number, toxicity levels among other factors; meanwhile, the use of thisterm has centered around the treatment of certain metallic elements that haveeffect on animals when released in high concentrations into the environment.

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Based on the extent of relevance, the leading heavy metals of consideration inenvironmental science are Silver, Nickel, Zinc, Copper, Manganese, Mercury,Cadmium, Chromium. In some cases, lanthanides and actinides are excluded fromheavy metals while mostly transition metals (elements with incomplete d-shell)are considered (Baird and Cann, 2012). Heavy metals mostly show good electrical and heatconductivities in their natural pure state. Some literature refers to”heavy” metal to a metal on the periodic table with specific gravity rangingfrom 3.5-6 g/cm3, rather than the atomic number as shown above. Thetransition metals which are mostly considered as heavy metals show varyingchemical species or oxidation states and can form variety of complexes withsome chemical species or ligands. All of the transition metals have two or moreoxidation states (Brady and Holum, 1995).