Heavy metals

Heavy metals is a controversial and ambiguous term for metallic elements with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context, and arguably, the term "heavy metal" should be avoided. A heavy metal may be defined on the basis of density, atomic number, or chemical behaviour. More specific definitions have been published, none of which has been widely accepted. The definitions surveyed in this article encompass up to 96 of the 118 known chemical elements; only mercury, lead, and bismuth meet all of them. Despite this lack of agreement, the term (plural or singular) is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article. The earliest known metals—common metals such as iron, copper, and tin, and precious metals such as silver, gold, and platinum—are heavy metals. From 1809 onward, light metals, such as magnesium, aluminium, and titanium, were discovered, as well as less well-known heavy metals, including gallium, thallium, and hafnium. Some heavy metals are either essential nutrients (typically iron, cobalt, copper, and zinc), or relatively harmless (such as ruthenium, silver, and indium), but can be toxic in larger amounts or certain forms. Other heavy metals, such as arsenic, cadmium, mercury, and lead, are highly poisonous. Potential sources of heavy-metal poisoning include mining, tailings, smelting, industrial waste, agricultural runoff, occupational exposure, paints, and treated timber. Physical and chemical characterisations of heavy metals need to be treated with caution, as the metals involved are not always consistently defined. Heavy metals, as well as being relatively dense, tend to be less reactive than lighter metals, and have far fewer soluble sulfides and hydroxides. While distinguishing a heavy metal such as tungsten from a lighter metal such as sodium is relatively easy, a few heavy metals, such as zinc, mercury, and lead, have some of the characteristics of lighter metals, and lighter metals, such as beryllium, scandium, and titanium, have some of the characteristics of heavier metals. Heavy metals are relatively rare in the Earth's crust, but are present in many aspects of modern life. They are used in, for example, golf clubs, cars, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones, and particle accelerators.

Definitions

Controversial terminology The International Union of Pure and Applied Chemistry (IUPAC), which standardizes nomenclature, says "the term 'heavy metals' is both meaningless and misleading". The IUPAC report focuses on the legal and toxicological implications of describing "heavy metals" as toxins when no scientific evidence supports a connection. The density implied by the adjective "heavy" has almost no biological consequences, and pure metals are rarely the biologically active form. This characterization has been echoed by numerous reviews. The most widely used toxicology textbook, Casarett and Doull's Toxicology uses "toxic metal", not "heavy metal". Nevertheless, there are scientific and science related articles which continue to use "heavy metal" as a term for toxic substances. To be an acceptable term in scientific papers, a strict definition has been encouraged.

Use outside toxicology

Even in applications other than toxicity, no widely agreed criterion-based definition of a heavy metal exists. Reviews have recommended that it not be used. Different meanings may be attached to the term, depending on the context. For example, a heavy metal may be defined on the basis of density, and the distinguishing criterion might be atomic number or chemical behaviour. Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. 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 are sometimes 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/cm3, which is below the threshold figure used by other authors. The same problem may occur with definitions which are based on atomic weight. The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides. On the basis of this type of chemical test, the group would include the transition metals and post-transition metals. A different chemistry-based approach advocates replacing the term "heavy metal" with two groups of metals and a gray area. 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. The distinction between the class A metals and the other two categories is sharp. The class A and class B terminology is analogous to the "hard acid" and "soft base" terminology sometimes used to refer to the behaviour of metal ions in inorganic systems. The system groups the elements by X m 2 r {\displaystyle X_{m}^{2}r} where X m {\displaystyle X_{m}} is the metal ion electronegativity and r {\displaystyle r} is its ionic radius. This index gauges the importance of covalent interactions vs ionic interactions for a given metal ion. This scheme has been applied to analyze biologically active metals in sea water for example, but it has not been widely adopted.

… excerpt ends here. Continue reading the full article.