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Heavy Metal Chemistry
Crystals of osmium, a heavy metal nearly twice as dense as lead.
Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and 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, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published, but none of these have been widely accepted. The definitions surveyed in this article encompass up to 96 out 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.
Physical and chemical characterisations of heavy metals need to be treated with caution, as the metals involved are not always consistently defined. As well as being relatively dense, heavy metals tend to be less reactive than lighter metals and have far fewer solublesulfides and hydroxides. While it is relatively easy to distinguish a heavy metal such as tungsten from a lighter metal such as sodium, 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.
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.[n 1] It illustrates the lack of agreement surrounding the concept, with the possible exception of mercury, lead and bismuth.
Metals enclosed by a dashed line have (or, for At and Fm-Ts, are predicted to have) densities of more than 5 g/cm3.
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 or biologist would likely be more concerned with 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 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/cm3, which is below the threshold figure used by other authors. The same problem may occur with atomic weight based definitions.
The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides."[n 3] 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.[n 4] The lanthanides satisfy Hawkes' three-part description; the status of the actinides is not completely settled.[n 5][n 6]
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.[n 7] 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.[n 8] 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[n 9] to use these classification categories instead of the more evocative name heavy metal has not been widely adopted.
List of heavy metals based on density
A density of more than 5 g/cm3 is sometimes mentioned as a common heavy metal defining factor and, in the absence of a unanimous definition, is used to populate this list and (unless otherwise stated) guide the remainder of the article. Metalloids meeting the applicable criteria-arsenic and antimony for example--are sometimes counted as heavy metals, particularly in environmental chemistry, as is the case here. Selenium (density 4.8 g/cm3) is also included in the list. It falls marginally short of the density criterion and is less commonly recognised as a metalloid but has a waterborne chemistry similar in some respects to that of arsenic and antimony. Other metals sometimes classified or treated as "heavy" metals, such as beryllium (density 1.8 g/cm3), aluminium (2.7 g/cm3), calcium (1.55 g/cm3), and barium (3.6 g/cm3) are here treated as light metals and, in general, are not further considered.
Produced mainly by commercial mining (informally classified by economic significance)
All isotopes of these 34 elements are unstable and hence radioactive. While this is also true of bismuth, it is not so marked since its half-life of 19 billion billion years is over a billion times the 13.8 billion year estimated age of the universe.
These eight elements do occur naturally but in amounts too small for economically viable extraction.
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. All metals discovered from then until 1809 had relatively high densities; their heaviness was regarded as a singularly distinguishing criterion.
From 1809 onwards, light metals such as sodium, potassium, and strontium were isolated. Their low densities challenged conventional wisdom and it was proposed to refer to them as metalloids (meaning "resembling metals in form or appearance"). This suggestion was ignored; the new elements came to be recognised as metals, and the term metalloid was then used to refer to nonmetallic elements and, later, elements that were hard to describe as either metals or nonmetals.
An early use of the term "heavy metal" dates from 1817, when the 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.[n 10] The term later became associated with elements of high atomic weight or high atomic number. It is sometimes used interchangeably with the term heavy element. For example, in discussing the history of nuclear chemistry, Magee notes 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 ...". In astronomy, however, a heavy element is any element heavier than hydrogen and helium.
In 2002, Scottish toxicologist John Duffus reviewed the definitions used over the previous 60 years and concluded they were so diverse as to effectively render the term meaningless. Along with this finding, the heavy metal status of some metals is occasionally challenged on the grounds that they are too light, or are involved in biological processes, or rarely constitute environmental hazards. Examples include scandium (too light);vanadium to zinc (biological processes); and rhodium, indium, and osmium (too rare).
Despite its questionable meaning, the term heavy metal appears regularly in scientific literature. A 2010 study found that it had been increasingly used and seemed to have become part of the language of science. It is said to be an acceptable term, given its convenience and familiarity, as long as it is accompanied by a strict definition. The counterparts to the heavy metals, the light metals, are alluded to by The Minerals, Metals and Materials Society as including "aluminium, magnesium, beryllium, titanium, lithium, and other reactive metals." The named metals have densities of 0.534 to 4.54 g/cm3.
Amount of heavy metals in an average 70 kg human body
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).
The focus of this section is mainly on the more serious toxic effects of heavy metals, including cancer, brain damage or death, rather than the harm they may cause to one more of the skin, lungs, stomach, kidneys, liver, or heart. For more specific information see Metal toxicity, Toxic heavy metal, or the articles on individual elements or compounds.
Heavy metals are often assumed to be highly toxic or damaging to the environment. Some are, while certain others are toxic only if taken in excess or encountered in certain forms.
Environmental heavy metals
Chromium, arsenic, cadmium, mercury, and lead have the greatest potential to cause harm on account of their extensive use, the toxicity of some of their combined or elemental forms, and their widespread distribution in the environment.Hexavalent chromium, for example, is highly toxic as are mercury vapour and many mercury compounds. These five elements have a strong affinity for sulfur; in the human body they usually bind, via thiol groups (-SH), to enzymes responsible for controlling the speed of metabolic reactions. The resulting sulfur-metal bonds inhibit the proper functioning of the enzymes involved; human health deteriorates, sometimes fatally. Chromium (in its hexavalent form) and arsenic are carcinogens; cadmium causes a degenerative bone disease; and mercury and lead damage the central nervous system.
Lead is the most prevalent heavy metal contaminant. Levels in the aquatic environments of industrialised societies have been estimated to be two to three times those of pre-industrial levels. As a component of tetraethyl lead, , it was used extensively in gasoline during the 1930s-1970s. Although the use of leaded gasoline was largely phased out in North America by 1996, soils next to roads built before this time retain high lead concentrations. Later research demonstrated a statistically significant correlation between the usage rate of leaded gasoline and violent crime in the United States; taking into account a 22-year time lag (for the average age of violent criminals), the violent crime curve virtually tracked the lead exposure curve.
Heavy metals essential for life can be toxic if taken in excess; some have notably toxic forms. Vanadium pentoxide (V2O5) is carcinogenic in animals and, when inhaled, causes DNA damage. The purple permanganate ion MnO- 4 is a liver and kidney poison. Ingesting more than 0.5 grams of iron can induce cardiac collapse; such overdoses most commonly occur in children and may result in death within 24 hours.Nickel carbonyl (Ni(CO)4), at 30 parts per million, can cause respiratory failure, brain damage and death. Imbibing a gram or more of copper sulfate (CuSO4) can be fatal; survivors may be left with major organ damage. More than five milligrams of selenium is highly toxic; this is roughly ten times the 0.45 milligram recommended maximum daily intake; long-term poisoning can have paralytic effects.[n 18]
Other heavy metals
A few other non-essential heavy metals have one or more toxic forms. Kidney failure and fatalities have been recorded arising from the ingestion of germanium dietary supplements (~15 to 300 g in total consumed over a period of two months to three years). Exposure to osmium tetroxide (OsO4) may cause permanent eye damage and can lead to respiratory failure and death. Indium salts are toxic if more than few milligrams are ingested and will affect the kidneys, liver, and heart.Cisplatin (PtCl2(NH3)2), which is an important drug used to kill cancer cells, is also a kidney and nerve poison.Bismuth compounds can cause liver damage if taken in excess; insoluble uranium compounds, as well as the dangerous radiation they emit, can cause permanent kidney damage.
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.[n 20] 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.
Heavy metals condense in planets as a result of stellar evolution and destruction processes. Stars lose much of their mass when it is ejected late in their lifetimes, and sometimes thereafter as a result of a neutron star merger,[n 21] thereby increasing the abundance of elements heavier than helium in the interstellar medium. When gravitational attraction causes this matter to coalesce and collapse, new stars and planets are formed.
The Earth's crust is made of approximately 5% of heavy metals by weight, with iron comprising 95% of this quantity. Light metals (~20%) and nonmetals (~75%) make up the other 95% of the crust. Despite their overall scarcity, heavy metals can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes.
Heavy metals are primarily found as lithophiles (rock-loving) or chalcophiles (ore-loving). Lithophile heavy metals are mainly f-block elements and the more reactive of the d-block elements. They have a strong affinity for oxygen and mostly exist as relatively low density silicate minerals. Chalcophile heavy metals are mainly the less reactive d-block elements, and period 4-6 p-block metals and metalloids. They are usually found in (insoluble) sulfide minerals. Being denser than the lithophiles, hence sinking lower into the crust at the time of its solidification, the chalcophiles tend to be less abundant than the lithophiles.
On the other hand, gold is a siderophile, or iron-loving element. It does not readily form compounds with either oxygen or sulfur. At the time of the Earth's formation, and as the most noble (inert) of metals, gold sank into the core due to its tendency to form high-density metallic alloys. Consequently, it is a relatively rare metal. Some other (less) noble heavy metals--molybdenum, rhenium, the platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum), germanium, and tin--can be counted as siderophiles but only in terms of their primary occurrence in the Earth (core, mantle and crust), rather the crust. These metals otherwise occur in the crust, in small quantities, chiefly as chalcophiles (less so in their native form).[n 22]
Concentrations of heavy metals below the crust are generally higher, with most being found in the largely iron-silicon-nickel core. Platinum, for example, comprises approximately 1 part per billion of the crust whereas its concentration in the core is thought to be nearly 6,000 times higher. Recent speculation suggests that uranium (and thorium) in the core may generate a substantial amount of the heat that drives plate tectonics and (ultimately) sustains the Earth's magnetic field.[n 23]
The winning of heavy metals from their ores is a complex function of ore type, the chemical properties of the metals involved, and the economics of various extraction methods. Different countries and refineries may use different processes, including those that differ from the brief outlines listed here.
Broadly speaking, and with some exceptions, lithophile heavy metals can be extracted from their ores by electrical or chemical treatments, while chalcophile heavy metals are obtained by roasting their sulphide ores to yield the corresponding oxides, and then heating these to obtain the raw metals.[n 24] Radium occurs in quantities too small to be economically mined and is instead obtained from spent nuclear fuels. The chalcophile platinum group metals (PGM) mainly occur in small (mixed) quantities with other chalcophile ores. The ores involved need to be smelted, roasted, and then leached with sulfuric acid to produce a residue of PGM. This is chemically refined to obtain the individual metals in their pure forms. Compared to other metals, PGM are expensive due to their scarcity and high production costs.
Gold, a siderophile, is most commonly recovered by dissolving the ores in which it is found in a cyanide solution. The gold forms a dicyanoaurate(I), for example: 2 Au + H2O +½ O2 + 4 KCN -> 2 K[Au(CN)2] + 2 KOH. Zinc is added to the mix and, being more reactive than gold, displaces the gold: 2 K[Au(CN)2] + Zn -> K2[Zn(CN)4] + 2 Au. The gold precipitates out of solution as a sludge, and is filtered off and melted.
Properties compared with light metals
Some general physical and chemical properties of light and heavy metals are summarised in the table. The comparison should be treated with caution since the terms light metal and heavy metal are not always consistently defined. Also the physical properties of hardness and tensile strength can vary widely depending on purity, grain size and pre-treatment.
These properties make it relatively easy to distinguish a light metal like sodium from a heavy metal like tungsten, but the differences become less clear at the boundaries. Light structural metals like beryllium, scandium, and titanium have some of the characteristics of heavy metals, such as higher melting points;[n 27] post-transition heavy metals like zinc, cadmium, and lead have some of the characteristics of light metals, such as being relatively soft, having lower melting points,[n 28] and forming mainly colourless complexes.
Heavy metals are present in nearly all aspects of modern life. Iron may be the most common as it accounts for 90% of all refined metals. Platinum may be the most ubiquitous given it is said to be found in, or used to produce, 20% of all consumer goods.
Some common uses of heavy metals depend on the general characteristics of metals such as electrical conductivity and reflectivity or the general characteristics of heavy metals such as density, strength, and durability. Other uses depend on the characteristics of the specific element, such as their biological role as nutrients or poisons or some other specific atomic properties. Examples of such atomic properties include: partly filled d- or f- orbitals (in many of the transition, lanthanide, and actinide heavy metals) that enable the formation of coloured compounds; the capacity of most heavy metal ions (such as platinum, cerium or bismuth) to exist in different oxidation states and therefore act as catalysts; poorly overlapping 3d or 4f orbitals (in iron, cobalt, and nickel, or the lanthanide heavy metals from europium through thulium) that give rise to magnetic effects; and high atomic numbers and electron densities that underpin their nuclear science applications. Typical uses of heavy metals can be broadly grouped into the following six categories.[n 29]
Weight- or density-based
In a cello (example shown above) or a viola the C-string sometimes incorporates tungsten; its high density permits a smaller diameter string and improves responsiveness.
Some uses of heavy metals, including in sport, mechanical engineering, military ordnance, and nuclear science, take advantage of their relatively high densities. In underwater diving, lead is used as a ballast; in handicap horse racing each horse must carry a specified lead weight, based on factors including past performance, so as to equalize the chances of the various competitors. In golf, tungsten, brass, or copper inserts in fairwayclubs and irons lower the centre of gravity of the club making it easier to get the ball into the air; and golf balls with tungsten cores are claimed to have better flight characteristics. In fly fishing, sinking fly lines have a PVC coating embedded with tungsten powder, so that they sink at the required rate. In track and field sport, steel balls used in the hammer throw and shot put events are filled with lead in order to attain the minimum weight required under international rules. Tungsten was used in hammer throw balls at least up to 1980; the minimum size of the ball was increased in 1981 to eliminate the need for what was, at that time, an expensive metal (triple the cost of other hammers) not generally available in all countries. Tungsten hammers were so dense that they penetrated too deeply into the turf.
The higher the projectile density, the more effectively it can penetrate heavy armor plate ... Os, Ir, Pt, and Re ... are expensive ... U offers an appealing combination of high density, reasonable cost and high fracture toughness.
AM Russell and KL Lee Structure-property relations in nonferrous metals (2005, p. 16)
The strength or durability of heavy metals such as chromium, iron, nickel, copper, zinc, molybdenum, tin, tungsten, and lead, as well as their alloys, makes them useful for the manufacture of artefacts such as tools, machinery,appliances, utensils, pipes,railroad tracks, buildings and bridges, automobiles, locks, furniture, ships, planes, coinage and jewellery. They are also used as alloying additives for enhancing the properties of other metals.[n 31] Of the two dozen elements that have been used in the world's monetised coinage only two, carbon and aluminium, are not heavy metals.[n 32] Gold, silver, and platinum are used in jewellery[n 33] as are (for example) nickel, copper, indium, and cobalt in coloured gold.Low-cost jewellery and children's toys may be made, to a significant degree, of heavy metals such as chromium, nickel, cadmium, or lead.
The workability and corrosion resistance of iron and chromium are increased by adding gadolinium; the creep resistance of nickel is improved with the addition of thorium. Tellurium is added to copper (Tellurium Copper) and steel alloys to improve their machinability; and to lead to make it harder and more acid-resistant.
The biocidal effects of some heavy metals have been known since antiquity. Platinum, osmium, copper, ruthenium, and other heavy metals, including arsenic, are used in anti-cancer treatments, or have shown potential. Antimony (anti-protozoal), bismuth (anti-ulcer), gold (anti-arthritic), and iron (anti-malarial) are also important in medicine. Copper, zinc, silver, gold, or mercury are used in antiseptic formulations; small amounts of some heavy metals are used to control algal growth in, for example, cooling towers. Depending on their intended use as fertilisers or biocides, agrochemicals may contain heavy metals such as chromium, cobalt, nickel, copper, zinc, arsenic, cadmium, mercury, or lead.
Niche uses of heavy metals with high atomic numbers occur in diagnostic imaging, electron microscopy, and nuclear science. In diagnostic imaging, heavy metals such as cobalt or tungsten make up the anode materials found in x-ray tubes. In electron microscopy, heavy metals such as lead, gold, palladium, platinum, or uranium are used to make conductive coatings and to introduce electron density into biological specimens by staining, negative staining, or vacuum deposition. In nuclear science, nuclei of heavy metals such as chromium, iron, or zinc are sometimes fired at other heavy metal targets to produce superheavy elements; heavy metals are also employed as spallation targets for the production of neutrons or radioisotopes such as astatine (using lead, bismuth, thorium, or uranium in the latter case).
^ Criteria used were density: (1) above 3.5 g/cm3; (2) above 7 g/cm3; atomic weight: (3) > 22.98; (4) > 40 (excluding s- and f-block metals); (5) > 200;atomic number: (6) > 20; (7) 21-92;chemical behaviour: (8) United States Pharmacopeia; (9) Hawkes' periodic table-based definition (excluding the lanthanides and actinides); and (10) Nieboer and Richardson's biochemical classifications. Densities of the elements are mainly from Emsley. Predicted densities have been used for At, Fr and Fm-Ts. Indicative densities were derived for Fm, Md, No and Lr based on their atomic weights, estimated metallic radii, and predicted close-packed crystalline structures. Atomic weights are from Emsley, inside back cover
^Metalloids were, however, excluded from Hawkes' periodic table-based definition given he noted it was "not necessary to decide whether semimetals [i.e. metalloids] should be included as heavy metals."
^Lanthanide (Ln) sulfides and hydroxides are insoluble; the latter can be obtained from aqueous solutions of Ln salts as coloured gelatinous precipitates; and Ln complexes have much the same colour as their aqua ions (the majority of which are coloured). Actinide (An) sulfides may or may not be insoluble, depending on the author. Divalent uranium monosulfide is not attacked by boiling water. Trivalent actinide ions behave similarly to the trivalent lanthanide ions hence the sulfides in question may be insoluble but this is not explicitly stated. Tervalent An sulfides decompose but Edelstein et al. say they are soluble whereas Haynes says thorium(IV) sulfide is insoluble. Early in the history of nuclear fission it had been noted that precipitation with hydrogen sulfide was a "remarkably" effective way of isolating and detecting transuranium elements in solution. In a similar vein, Deschlag writes that the elements after uranium were expected to have insoluble sulfides by analogy with third row transition metals. But he goes on to note that the elements after actinium were found to have properties different from those of the transition metals and claims they do not form insoluble sulfides. The An hydroxides are, however, insoluble and can be precipitated from aqueous solutions of their salts. Finally, many An complexes have "deep and vivid" colours.
^The heavier elements commonly to less commonly recognised as metalloids--Ge; As, Sb; Se, Te, Po; At--satisfy some of the three parts of Hawkes' definition. All of them have insoluble sulfides but only Ge, Te, and Po apparently have effectively insoluble hydroxides. All bar At can be obtained as coloured (sulfide) precipitates from aqueous solutions of their salts; astatine is likewise precipitated from solution by hydrogen sulfide but, since visible quantities of At have never been synthesised, the colour of the precipitate is not known. As p-block elements, their complexes are usually colourless.
^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.
^Be and Al are exceptions to this general trend. They have somewhat higher electronegativity values. Being relatively small their +2 or +3 ions have high charge densities, thereby polarising nearby electron clouds. The net result is that Be and Al compounds have considerable covalent character.
^If Gmelin had been working with the imperial system of weights and measures he may have chosen 300 lb/ft3 as his light/heavy metal cutoff in which case selenium (density 300.27 lb/ft3 ) would have made the grade, whereas 5 g/cm3 = 312.14lb/ft3.
^Lead, which is a cumulative poison, has a relatively high abundance due to its extensive historical use and human-caused discharge into the environment.
^Iyengar records a figure of 5 mg for nickel; Haynes shows an amount of 10 mg
^Encompassing 45 heavy metals occurring in quantities of less than 10 mg each, including As (7 mg), Mo (5), Co (1.5), and Cr (1.4)
^Of the elements commonly recognised as metalloids, B and Si were counted as nonmetals; Ge, As, Sb, and Te as heavy metals.
^Ni, Cu, Zn, Se, Ag and Sb appear in the United States Government's Toxic Pollutant List; Mn, Co, and Sn are listed in the Australian Government's National Pollutant Inventory.
^Tungsten could be another such toxic heavy metal.
^Selenium is the most toxic of the heavy metals that are essential for mammals.
^Trace elements having an abundance equalling or much less than one part per trillion (namely Tc, Pm, Po, At, Ra, Ac, Pa, Np, and Pu) are not shown. Abundances are from Lide and Emsley; occurrence types are from McQueen.
^The ejection of matter when two neutron stars collide is attributed to the interaction of their tidal forces, possible crustal disruption, and shock heating (which is what happens if you floor the accelerator in car when the engine is cold).
^Iron, cobalt, nickel, germanium and tin are also siderophiles from a whole of Earth perspective.
^Heat escaping from the inner solid core is believed to generate motion in the outer core, which is made of liquid iron alloys. The motion of this liquid generates electrical currents which give rise to a magnetic field.
^Heavy metals that occur naturally in quantities too small to be economically mined (Tc, Pm, Po, At, Ac, Np and Pu) are instead produced by artificial transmutation. The latter method is also used to produce heavy metals from americium onwards.
^Sulfides of the Group 1 and 2 metals, and aluminium, are hydrolysed by water; scandium, yttrium and titanium sulfides are insoluble.
^For example, the hydroxides of potassium, rubidium, and caesium have solubilities exceeding 100 grams per 100 grams of water whereas those of aluminium (0.0001) and scandium (<0.000 000 15 grams) are regarded as being insoluble.
^Beryllium has what is described as a "high" melting point of 1560 K; scandium and titanium melt at 1814 and 1941 K.
^Zinc is a soft metal with a Moh's hardness of 2.5; cadmium and lead have lower hardness ratings of 2.0 and 1.5. Zinc has a "low" melting point of 693 K; cadmium and lead melt at 595 and 601 K.
^Some violence and abstraction of detail was applied to the sorting scheme in order to keep the number of categories to a manageable level.
^Emsley estimates a global loss of six tonnes of gold a year due to 18-carat wedding rings slowly wearing away.
^Sheet lead exposed to the rigours of industrial and coastal climates will last for centuries
^Electrons impacting the tungsten anode generate X-rays; rhenium gives tungsten better resistance to thermal shock; molybdenum and graphite act as heat sinks. Molybdenum also has a density nearly half that of tungsten thereby reducing the weight of the anode.
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Definition and usage
Ali H. & Khan E. 2017, "What are heavy metals? long-standing controversy over the scientific use of the term 'heavy metals'--proposal of a comprehensive definition", Toxicological & Environmental Chemistry, pp. 1-25, doi:10.1080/02772248.2017.1413652. Suggests defining heavy metals as "naturally occurring metals having atomic number (Z) greater than 20 and an elemental density greater than 5 g cm-3".
Hübner R., Astin K. B. & Herbert R. J. H. 2010, " 'Heavy metal'--time to move on from semantics to pragmatics?", Journal of Environmental Monitoring, vol. 12, pp. 1511-1514, doi:10.1039/C0EM00056F. Finds that, despite its lack of specificity, the term appears to have become part of the language of science.
Toxicity and biological role
Baird C. & Cann M. 2012, Environmental Chemistry, 5th ed., chapter 12, "Toxic heavy metals", W. H. Freeman and Company, New York, ISBN1-4292-7704-1. Discusses the use, toxicity, and distribution of Hg, Pb, Cd, As, and Cr.
Nieboer E. & Richardson D. H. S. 1980, "The replacement of the nondescript term 'heavy metals' by a biologically and chemically significant classification of metal ions", Environmental Pollution Series B, Chemical and Physical, vol. 1, no. 1, pp. 3-26, doi:10.1016/0143-148X(80)90017-8. A widely cited paper, focusing on the biological role of heavy metals.