<P> There is no widely agreed criterion - based definition of a heavy metal . Different meanings may be attached to the term, depending on the context . In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, and a chemist would likely be more concerned with chemical behaviour . </P> <P> Density criteria range from above 3.5 g / cm to above 7 g / cm . Atomic weight definitions can range from greater than sodium (atomic weight 22.98); greater than 40 (excluding s - and f - block metals, hence starting with scandium); or more than 200, i.e. from mercury onwards . Atomic numbers of heavy metals are generally given as greater than 20 (calcium); sometimes this is capped at 92 (uranium). Definitions based on atomic number have been criticised for including metals with low densities . For example, rubidium in group (column) 1 of the periodic table has an atomic number of 37 but a density of only 1.532 g / cm, which is below the threshold figure used by other authors . The same problem may occur with atomic weight based definitions . </P> <P> Criteria based on chemical behaviour or periodic table position have been used or suggested . The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides ." In 1997, Stephen Hawkes, a chemistry professor writing in the context of fifty years' experience with the term, said it applied to "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water and whose complexes are usually colored". On the basis of the metals he had seen referred to as heavy metals, he suggested it would useful to define them as (in general) all the metals in periodic table columns 3 to 16 that are in row 4 or greater, in other words, the transition metals and post-transition metals . The lanthanides satisfy Hawkes' three - part description; the status of the actinides is not completely settled . </P> <P> In biochemistry, heavy metals are sometimes defined--on the basis of the Lewis acid (electronic pair acceptor) behaviour of their ions in aqueous solution--as class B and borderline metals . In this scheme, class A metal ions prefer oxygen donors; class B ions prefer nitrogen or sulfur donors; and borderline or ambivalent ions show either class A or B characteristics, depending on the circumstances . Class A metals, which tend to have low electronegativity and form bonds with large ionic character, are the alkali and alkaline earths, aluminium, the group 3 metals, and the lanthanides and actinides . Class B metals, which tend to have higher electronegativity and form bonds with considerable covalent character, are mainly the heavier transition and post-transition metals . Borderline metals largely comprise the lighter transition and post-transition metals (plus arsenic and antimony). The distinction between the class A metals and the other two categories is sharp . A frequently cited proposal to use these classification categories instead of the more evocative name heavy metal has not been widely adopted . </P>

Where are the heavy metals located on the periodic table
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