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 Heat map of heavy metals in the periodic table123456789101112131415161718 1 HHe 2 LiBeBCNOFNe 3 NaMgAlSiPSClAr 4 KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr 5 RbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXe 6 CsBawidth="5%" LuHfTaWReOsIrPtAuHgTlPbBiPoAtRn 7 FrRa LrRfDbSgBhHsMtDsRgCnNhFlMcLvTsOg LaCePrNdPmSmEuGdTbDyHoErTmYbAcThPaUNpPuAmCmBkCfEsFmMdNo Number of elements: This table shows the number of heavy metal criteria met by each metal, out of the ten criteria listed in this section i.e. two based on density, three on atomic weight, two on atomic number, and three on chemical behaviour. It illustrates the lack of agreement surrounding the concept, with the possible exception of mercury, lead, and bismuth. Six elements near the end of periods (rows) 4 to 7 sometimes considered metalloids are treated here as metals: they are germanium (Ge), arsenic (As), selenium (Se), antimony (Sb), tellurium (Te), and astatine (At). Oganesson (Og) is treated as a nonmetal. Metals enclosed by a dashed line have (or, for At and Fm–Ts, are predicted to have) densities of more than 5 g/cm3.
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 where is the metal ion electronegativity and 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.
Origins and use of the term The heaviness of naturally occurring metals such as gold, copper, and iron may have been noticed in prehistory and, in light of their malleability, led to the first attempts to craft metal ornaments, tools, and weapons.
In 1817, German chemist Leopold Gmelin divided the elements into nonmetals, light metals, and heavy metals. Light metals had densities of 0.860–5.0 g/cm3; heavy metals 5.308–22.000. The term heavy metal is sometimes used interchangeably with the term "heavy element". For example, in discussing the history of nuclear chemistry, Magee noted that the actinides were once thought to represent a new heavy-element transition group, whereas Seaborg and co-workers "favoured ... a heavy metal rare-earth like series ...".
The counterparts to the heavy metals, the light metals, are defined by the Minerals, Metals and Materials Society as including "the traditional (aluminium, magnesium, beryllium, titanium, lithium, and other reactive metals) and emerging light metals (composites, laminates, etc.)"
Biological role + Amount of heavy metals inan average 70 kg human body Element Milligrams Iron Zinc Lead Copper Tin Vanadium Cadmium Nickel Selenium Manganese Other Total 7000
Trace amounts of some heavy metals, mostly in period 4, are required for certain biological processes. These are iron and copper (oxygen and electron transport); cobalt (complex syntheses and cell metabolism); vanadium and manganese (enzyme regulation or functioning); chromium (glucose utilisation); nickel (cell growth); arsenic (metabolic growth in some animals and possibly in humans) and selenium (antioxidant functioning and hormone production). Periods 5 and 6 contain fewer essential heavy metals, consistent with the general pattern that heavier elements tend to be less abundant and that scarcer elements are less likely to be nutritionally essential. In period 5, molybdenum is required for the catalysis of redox reactions; cadmium is used by some marine diatoms for the same purpose; and tin may be required for growth in a few species. In period 6, tungsten is required by some archaea and bacteria for metabolic processes. A deficiency of some essential metals may increase susceptibility to metal poisoning (conversely, an excess may also be toxic).
An average human body is about 0.01% heavy metals (~, equivalent to the weight of two dried peas, with iron at , zinc at , and lead at comprising the three main constituents), 2% light metals (~, the weight of a bottle of wine) and nearly 98% nonmetals (mostly water).
A few non-essential heavy metals have been observed to have biological effects. Gallium, germanium (a metalloid), indium, and most lanthanides can stimulate metabolism, and titanium promotes growth in plants (though it is not always considered a heavy metal).
Formation, abundance, occurrence, and extraction
Heavy metals in the Earth's crust:abundance and main occurrence or source123456789101112131415161718 1 HHe 2 LiBeBCNOFNe 3 NaMgAlSiPSClAr 4 KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr 5 RbSrYZrNbMoRuRhPdAgCdInSnSbTe I Xe 6 CsBa width="5%" LuHfTaWReOsIrPtAuHgTlPbBi 7 style="background:#f8f8f8; border-left:1px solid #f8f8f8; border-top:1px solid #f8f8f8; border-right:1px #f8f8f8; border-bottom:1px #f8f8f8" LaCePrNdSmEuGdTbDyHoErTmYbThU Heavy metals left of the dividing line occur (or are sourced) mainly as lithophiles; those to the right, as chalcophiles except gold (a siderophile) and tin (a lithophile).
Heavy metals up to the vicinity of iron (in the periodic table) are largely made via stellar nucleosynthesis. In this process, lighter elements from hydrogen to silicon undergo successive fusion reactions inside stars, releasing light and heat and forming heavier elements with higher atomic numbers.
Heavier heavy metals are not usually formed this way since fusion reactions involving such nuclei would consume rather than release energy. Rather, they are largely synthesised (from elements with a lower atomic number) by neutron capture, with the two main modes of this repetitive capture being the s-process and the r-process. In the s-process ("s" stands for "slow"), singular captures are separated by years or decades, allowing the less stable nuclei to beta decay, while in the r-process ("rapid"), captures happen faster than nuclei can decay. Therefore, the s-process takes a more or less clear path: for example, stable cadmium-110 nuclei are successively bombarded by free neutrons inside a star until they form cadmium-115 nuclei which are unstable and decay to form indium-115 (which is nearly stable, with a half-life times the age of the universe). These nuclei capture neutrons and form indium-116, which is unstable, and decays to form tin-116, and so on. In contrast, there is no such path in the r-process. The s-process stops at bismuth due to the short half-lives of the next two elements, polonium and astatine, which decay to bismuth or lead. The r-process is so fast it can skip this zone of instability and go on to create heavier elements such as thorium and uranium.