Heavy metal (chemical element)

Samples of tungsten, a heavy metal. Shown are a 1 cm3 cube; and some rods with evaporated crystals; the foremost rod is prominently tarnished due to partial oxidation. The word tungsten comes from the Swedish tung sten, "heavy stone". With a density of 19.52 gm/cm3, tungsten is about 70% heavier than lead (density 11.34).

A heavy metal, broadly speaking, is any relatively dense metal.[1] More specific definitions have been proposed (sometimes including metalloids such as arsenic and antimony), but no definition has obtained widespread acceptance. Because of their heaviness and many specialized properties, heavy metals are useful in nearly all aspects of modern economic activity. Some are toxic, but a number are essential nutrients in trace amounts.

Definitions

Densities of metals and metalloids in the periodic table
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1  H He
2  Li Be B C N O F Ne
3  Na Mg Al Si P S Cl Ar
4  K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
5  Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te  I  Xe
6  Cs Ba 1 asterisk Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
7  Fr Ra † Lr Rf Db Sg Bh Hs Mt Ds Rg Cn 113 Fl 115 Lv 117 118
 
1 asterisk La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
† Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No
 
   < 3.5 g/cm3
  
   7 to 9.99
   3.5 to 4.99
  
   > 10
   5 to 6.99
  

There is no widely agreed criteria-based definition of a heavy metal. Duffus concluded in 2002 that so many different definitions had been used in the previous 60 years that it was an effectively meaningless term.[2]

Quantitative criteria used to define heavy metals have included density, atomic weight and atomic number.[2] Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. Atomic weight definitions start at greater than sodium (22.98) to greater than 40,[n 1] or 200 or more.[4] Atomic numbers of heavy metals are generally given as greater than 20; sometimes this is capped at 92 (uranium). Density is the most commonly used of these criteria. Metalloids meeting the applicable criteria are often counted as heavy metals, particularly in environmental chemistry.[2]

Criteria based on chemical behaviour or periodic table position are known or have been used. The United States Pharmacopeia describes heavy metals as "metallic impurities that are colored by sulfide ion."[5] Hawkes, writing in 1997, and in the context of fifty years of experience with the term, said it referred to, "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water, and whose complexes are usually colored." He suggested referring to heavy metals as all of the metals in groups 3 to 16 that are in period 4 or greater, in other words, the transition metals and post-transition metals.[6][n 2][n 3]

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.[20] 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.[21][22] A frequently cited proposal[n 4] to use these classification categories instead of the more evocative[21] name heavy metal has not been widely adopted.[23]

Despite its questionable meaning, references to the term "heavy metal" appear 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.[23] It is said to be an acceptable term, given its convenience and familiarity, as long as it is accompanied by a strict definition.[20]

Elements of atomic number 104 (rutherfordium) onwards are sometimes referred to as superheavy metals.[24] It remains to be seen if element 118, which is the heaviest element in the noble gas group, is in fact a metal. Predicted densities of elements 104–118 range from around 5 gm/cm3 for element 118, to 41 gm/cm3 for hassium (element 108);[25] the latter figure is 3.6 times that of lead (at 11.35 gm/cm3).

Etymology

The high densities of native metals such as copper, iron and gold may have been noticed in prehistory.[26] All other metals discovered from then until 1800 had fairly high densities. 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 in form or appearance").[27] This suggestion was not taken up and the new elements came to be recognised as metals.[28]

An early use of the term dates from 1817, when the German chemist Leopold Gmelin divided the elements into nonmetals, light metals and heavy metals.[29] Light metals had densities of 0.860–5.0 gm/cm3; heavy metals 5.308–22.000.[30]

Uses

Several lead bricks each the size of small pillows, stacked up like lego bricks to form a fort-like structure
A lead castle built to shield a radioactive sample in a lab

Heavy metals pervade nearly all aspects of modern economic activity.[31] They find general uses in, for example, metalworking and plating, electronics, batteries, catalysis, paint and pigments, jewellery, fertilisers, pesticides and herbicides.[32]

Some uses of heavy metals, including in sport and mechanical engineering, 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 competitors. In golf, tungsten, brass or copper inserts in fairway clubs and irons lower the centre of gravity of the club making it easier to get the ball into the air;[33] and golf balls with tungsten cores are claimed to have better flight characteristics.[34] In fly fishing, sinking fly lines have a PVC coating embedded with tungsten powder so as to achieve the required sink rate.[35] 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.[36] 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.[37] Tungsten hammers were so dense that they penetrated too deeply into the turf.[38] In mechanical engineering, heavy metals are used for balance weights on wheels and crankshafts,[39] gyroscopes and propellers,[40] and centrifugal clutches;[41] or as ballast in boats,[42] aeroplanes,[43] and motor vehicles,[44] in situations requiring maximum weight in minimum space (for example in watch movements).[43]

Niche uses occur in electron microscopy and nuclear science. 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 shadowing.[45] In nuclear science, heavy metals are used for radiation shielding and to focus radiation beams in linear accelerators and radiotherapy applications.[46]

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)

In military ordnance, tungsten or uranium is used in armour plating[47] and armour piercing projectiles; and in nuclear weapons to increase efficiency (by reflecting neutrons and momentarily delaying the expansion of reacting materials).[48] In the 1970s, tantalum was found to be more effective than copper in shaped charge and explosively formed anti-armour weapons on account of its higher density, allowing greater force concentration, and better deformability.[49] Less toxic heavy metals, such as copper, tin, tungsten and bismuth, and probably manganese (and boron, a metalloid), have replaced lead and antimony in the bullets used by some armies and in some recreational shooting munitions.[50] Doubts have been raised about the green credentials of tungsten.[51][52]

A white-gloved technician examines a large slab of rectangular transparent red glass, about the same width and length of a long surfboard, and a bit thicker, which is lying on a roller conveyer
A slab of neodymium-doped laser glass

Other references or instances of heavy metals occur in soap chemistry; glass making, ceramics and pyrotechnics; medicine; numismatics; and with respect to children's toys and low-cost jewellery. In soap chemistry, heavy metals form insoluble soaps that are used in lubricating greases, paint dryers, and fungicides (apart from lithium, the alkali metals and the ammonium ion form soluble soaps).[53] The colours of glass, ceramic glazes and pyrotechnics are produced by the inclusion of heavy metals (or their compounds) such as iron, nickel, copper, chromium, manganese, cobalt, gold, silver or neodymium. The biocidal effects of some heavy metals have been known since antiquity.[54] Platinum, osmium, copper, ruthenium and other heavy metals, including arsenic, are used in anti-cancer treatments, or have shown potential.[55] Antimony (anti-protozoal), bismuth (anti-ulcer), gold (anti-arthritic), and iron (anti-malarial) are also important in medicine.[56] Copper, zinc, silver, gold or mercury are used in antiseptic formulations;[57] small amounts of selected heavy metals are used to control algal growth in, for example, cooling towers.[58] In numismatics, of two dozen elements that have been used in the world's monetised coinage only two, carbon and aluminium, are not heavy metals.[59][n 5] Children's toys and low-cost jewellery may be made, to a significant degree, of heavy metals such as chromium, nickel, cadmium or lead.[61]

Toxicity

Some heavy metals, especially chromium, arsenic, cadmium, mercury, lead and thallium are potentially hazardous due to their toxicity in combined or elemental forms. Hexavalent chromium, for example, is highly toxic as is mercury vapour and many mercury compounds.[62] These six elements have a strong affinity for sulfur; in the human body they usually bind, via sulfhydryl 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.[63] Other heavy metals noted for their potential toxicity, particularly in an environmental science context, include nickel, copper, zinc, silver, tin, and antimony.[64][n 6] Health risks associated with some heavy metals have resulted in efforts to limit their indiscriminate use or to replace them with less toxic alternatives. For example, tetraethyl lead, (CH
3CH
2)
4Pb was added to petrol as an anti-knock agent, particularly during the 1930‒1970s,[66] but this practice has largely been phased out.[67]

Nutrition

Trace amounts of some heavy metals are required for certain biological processes. These include vanadium and manganese (enzyme regulation or activation), iron and copper (oxygen and electron transport); cobalt (complex syntheses and cell metabolism), zinc (hydroxylation) and molybdenum (catalysis of redox reactions).[22][68] A deficiency of these essential elements may increase susceptibility to heavy metal poisoning; high doses can have adverse effects.[69]

Notes

  1. ↑ Excluding s- and f-block metals, hence starting with scandium[3]
  2. ↑ In this case aluminium is not regarded as a post-transition metal
  3. ↑ Lanthanide sulfides are highly coloured[7] and insoluble;[8] the hydroxides are insoluble.[9] Simple ions of lanthanum, europium, gadolinium, ytterbium and lutetium are colourless in water;[10] lanthanide complexes have much the same colour as their aqua ions (the majority of which are coloured).[11] Actinide sulfides from actinium[12] to at least americium[13] are coloured;[14] the literature is incomplete or contradictory on the solubility of actinide sulfides.[15][16][17] The hydroxides, on the other hand, are insoluble.[16] Simple ions of actinium, thorium, and proactinium are colourless in water[18] however many actinide complexes have "deep and vivid" colours.[19]
  4. ↑ Google recorded 945 citations for the paper in question,[21] as at 19 April 2016.
  5. ↑ Weller[60] classifies coinage metals as precious metals (e.g. silver, gold, platinum); heavy metals of very high durability (nickel); heavy metals of low durability (copper, iron, zinc, tin and lead); and light metals (aluminium).
  6. ↑ Tungsten may be another such toxic heavy metal.[65]

Sources

Citations

  1. ↑ Qian 2009, p. 496
  2. 1 2 3 Duffus 2002
  3. ↑ Rand, Wells & McCarty 1995, p. 23
  4. ↑ Baldwin & Marshall 1999, p. 267: "The term 'heavy metal'…should probably be reserved for those elements with an atomic mass of 200 or greater" i.e. mercury onwards.
  5. ↑ The United States Pharmacopeia 1985, p. 1189
  6. ↑ Hawkes 1997
  7. ↑ Remy 1956, p. 496
  8. ↑ Topp 1965, p. 106
  9. ↑ Schweitzer & Pesterfield 2010, p. 284
  10. ↑ Schweitzer & Pesterfield 2010, pp. 267–283
  11. ↑ Cotton 2006, pp. 66
  12. ↑ CHB 1881, p. 1104
  13. ↑ Silva 1995, p. 126
  14. ↑ Samsonov 1961, pp. 31–33
  15. ↑ Albutt & Dell 1963, p. 1796; Wiberg 2001, p. 1722-1724
  16. 1 2 Edelstein et al. 2010, p. 1796
  17. ↑ Deschlag 2011, p. 226; Haynes 2015, p. 4-95
  18. ↑ Schweitzer & Pesterfield 2010, pp. 408–410
  19. ↑ Ahrland, Liljenzin & Rydberg 1973, p. 478
  20. 1 2 Rainbow 1991, p. 416
  21. 1 2 3 Nieboer & Richardson 1980
  22. 1 2 Nieboer & Richardson 1978
  23. 1 2 Hübner, Astin & Herbert 2010
  24. ↑ Loveland 2014
  25. ↑ Hoffman, Lee & Pershina 2011, pp. 1691,1723; Bonchev & Kamenska 1981, p. 1182
  26. ↑ Raymond 1984, p. 9
  27. ↑ Oxford English Dictionary 1989; Gordh, Gordh & Headrick 2003, p. 753
  28. ↑ Goldsmith 1982, p. 526
  29. ↑ Habashi 2009, p. 31
  30. ↑ Gmelin 1849, p. 2
  31. ↑ Provenzano 1975, p. 339
  32. ↑ Landis, Sofield & Yu 2011, p. 269
  33. ↑ Jackson & Summitt 2006, pp. 10, 13
  34. ↑ Shedd 2002, p. 80.5; Kantra 2001, p. 10
  35. ↑ Spolek 2007, p. 239
  36. ↑ White 2010, p. 139
  37. ↑ Dapena & Teves 1982, p. 78
  38. ↑ Burkett 2010, p. 80
  39. ↑ VanGelder 2014, pp. 354, 801
  40. ↑ National Materials Advisory Board 1971, pp. 35–37
  41. ↑ Frick 2000, p. 342
  42. ↑ Moore & Ramamoorthy 1984, p. 102
  43. 1 2 National Materials Advisory Board 1973, p. 58
  44. ↑ Livesey 2012, p. 57
  45. ↑ Chandler & Roberson 2009, p. 47; Ismail, Khulbe & Matsuura 2015, pp. 302, 367, 373
  46. ↑ Scoullos et al. 2001, p. 315; Ariel, Barta & Brandon 1973, p. 126
  47. ↑ Rockhoff 2012, p. 314
  48. ↑ Morstein 2005, p. 129
  49. ↑ Russell & Lee 2005, pp. 218–219
  50. ↑ Lach et al. 2015; DiMaio 2016, p. 154
  51. ↑ Preschel 2005
  52. ↑ Guandalini et al. 2011
  53. ↑ Elliot 1946, p. 11; Warth 1956, p. 571
  54. ↑ Weber & Rutula 2001, p. 415
  55. ↑ Dunn 2009; Bonetti et al. 2009
  56. ↑ Desoize 2004
  57. ↑ Atlas 1986, p. 359; Lima et al. 2013
  58. ↑ Volesky 1990, p. 174
  59. ↑ Roe & Roe 1992
  60. ↑ Weller 1976, p. 4
  61. ↑ Guney & Zagury 2012; Cui et al. 2015
  62. ↑ Kozin & Hansen 2013, p. 80
  63. ↑ Baird & Cann 2012, pp. 519–520; 567; Rusyniak et al. 2010, p. 387
  64. ↑ Baird & Cann 2012, pp. 558; United States Government 2014
  65. ↑ United States Environmental Protection Agency 2014
  66. ↑ Lovei 1998, p. 15
  67. ↑ Mason 2015, p. 130
  68. ↑ Bánfalvi 2011, p. 12; Emsley 2011, pp. 138, 310, 331, 604
  69. ↑ Venugopal & Luckey 1978, p. 307

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