Heavy metals

Heat map of heavy metals 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		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	Nh	Fl	Mc	Lv	Ts	Og
				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
	Number of criteria met: Number of elements:    10 3    9 5    8 14    6–7 56    4–5 14    1–3 4    0 3    nonmetals 19
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. 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).[16][n 2] 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.

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,[17] whereas in physics the distinguishing criterion might be atomic number,[18] and a chemist or biologist would likely be more concerned with chemical behaviour.[10]

Density criteria range from above 3.5 g/cm³ to above 7 g/cm³.[3] Atomic weight definitions can range from greater than sodium (atomic weight 22.98);[3] greater than 40 (excluding s- and f-block metals, hence starting with scandium);[4] or more than 200, i.e. from mercury onwards.[5] Atomic numbers of heavy metals are generally given as greater than 20 (calcium);[3] sometimes this is capped at 92 (uranium).[6] 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.[19] The same problem may occur with definitions which are based on atomic weight.[20]

The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides."[7][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 be 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.[10][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.[41] 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.[45] A frequently cited proposal[46] to use these classification categories instead of the more evocative[11] name heavy metal has not been widely adopted.[47]

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.[64] All metals discovered from then until 1809 had relatively high densities; their heaviness was regarded as a singularly distinguishing criterion.[65]

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").[66] 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.[67]

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.[68] Light metals had densities of 0.860–5.0 g/cm³; heavy metals 5.308–22.000.[69][n 9] The term later became associated with elements of high atomic weight or high atomic number.[19] It is sometimes used interchangeably with the term heavy element. For example, in discussing the history of nuclear chemistry, Magee[70] 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.[71]

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); zinc (hydroxylation);[83] 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).[84] 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.[85] 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.[86] In period 6, tungsten is required by some archaea and bacteria for metabolic processes.[87] A deficiency of any of these period 4–6 essential heavy metals may increase susceptibility to heavy metal poisoning[88] (conversely, an excess may also have adverse biological effects). An average 70 kg human body is about 0.01% heavy metals (~7 g, equivalent to the weight of two dried peas, with iron at 4 g, zinc at 2.5 g, and lead at 0.12 g comprising the three main constituents), 2% light metals (~1.4 kg, the weight of a bottle of wine) and nearly 98% nonmetals (mostly water).[89][n 15]

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[90] (though it is not always considered a heavy metal).

Heavy metals are often assumed to be highly toxic or damaging to the environment.[91] Some are, while certain others are toxic only if taken in excess or encountered in certain forms. Inhalation of certain metals, either as fine dust or most commonly as fumes, can also result in a condition called metal fume fever.

Heavy metals in the Earth's crust:
abundance and main occurrence or source[n 19]
	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		Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
6	Cs	Ba		Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi
7		Ra
				La	Ce	Pr	Nd		Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb
					Th	Pa	U
	Most abundant (56,300 ppm by weight)										Rare (0.01–0.99 ppm)
	Abundant (100–999 ppm)										Very rare (0.0001–0.0099 ppm)
	Uncommon (1–99 ppm)										Least abundant (~0.000001 ppm)
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.[134]

Heavier heavy metals are not usually formed this way since fusion reactions involving such nuclei would consume rather than release energy.[135] 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,[136] 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 30,000 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.[134][137][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.[139]

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,[140][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.[142]

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.[131] Despite their overall scarcity, heavy metals can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes.[143]

Heavy metals are found primarily 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.[144] 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.[145]

In contrast, gold is a siderophile, or iron-loving element. It does not readily form compounds with either oxygen or sulfur.[146] 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.[147] 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).[148][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.[149][150] 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.[151][n 23]

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.[153][n 24] Radium occurs in quantities too small to be economically mined and is instead obtained from spent nuclear fuels.[156] 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.[157] Compared to other metals, PGM are expensive due to their scarcity[158] and high production costs.[159]

Gold, a siderophile, is most commonly recovered by dissolving the ores in which it is found in a cyanide solution.[160] 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.[161]

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. Moreover, the physical properties of hardness and tensile strength can vary widely depending on purity, grain size and pre-treatment.[162]

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.[21][23][24]

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.[186]

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;[187] the capacity of most heavy metal ions (such as platinum,[188] cerium[189] or bismuth[190]) to exist in different oxidation states and therefore act as catalysts;[191] 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;[192] and high atomic numbers and electron densities that underpin their nuclear science applications.[193] Typical uses of heavy metals can be broadly grouped into the following six categories.[194][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.[195]

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;[196] 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.[197] 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;[198] and golf balls with tungsten cores are claimed to have better flight characteristics.[199] In fly fishing, sinking fly lines have a PVC coating embedded with tungsten powder, so that they sink at the required rate.[200] 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.[201] 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.[202] Tungsten hammers were so dense that they penetrated too deeply into the turf.[203]

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 mechanical engineering, heavy metals are used for ballast in boats,[204] aeroplanes,[205] and motor vehicles;[206] or in balance weights on wheels and crankshafts,[207] gyroscopes, and propellers,[208] and centrifugal clutches,[209] in situations requiring maximum weight in minimum space (for example in watch movements).[205]

In military ordnance, tungsten or uranium is used in armour plating[210] and armour piercing projectiles,[211] as well as in nuclear weapons to increase efficiency (by reflecting neutrons and momentarily delaying the expansion of reacting materials).[212] 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.[213] Less-toxic heavy metals, such as copper, tin, tungsten, and bismuth, and probably manganese (as well as boron, a metalloid), have replaced lead and antimony in the green bullets used by some armies and in some recreational shooting munitions.[214] Doubts have been raised about the safety (or green credentials) of tungsten.[215]

Because denser materials absorb more radioactive emissions than lighter ones, heavy metals are useful for radiation shielding and to focus radiation beams in linear accelerators and radiotherapy applications.[216]

Strength- or durability-based

The Statue of Liberty. A stainless steel alloy[217] armature provides structural strength; a copper skin confers corrosion resistance.[n 30]

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,[219] appliances,[220] utensils,[221] pipes,[220] railroad tracks,[222] buildings[223] and bridges,[224] automobiles,[220] locks,[225] furniture,[226] ships,[204] planes,[227] coinage[228] and jewellery.[229] 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.[231][n 32] Gold, silver, and platinum are used in jewellery[n 33] as are, for example, nickel, copper, indium, and cobalt in coloured gold.[234] Low-cost jewellery and children's toys may be made, to a significant degree, of heavy metals such as chromium, nickel, cadmium, or lead.[235]

Copper, zinc, tin, and lead are mechanically weaker metals but have useful corrosion prevention properties. While each of them will react with air, the resulting patinas of either various copper salts,[236] zinc carbonate, tin oxide, or a mixture of lead oxide, carbonate, and sulfate, confer valuable protective properties.[237] Copper and lead are therefore used, for example, as roofing materials;[238][n 34] zinc acts as an anti-corrosion agent in galvanised steel;[239] and tin serves a similar purpose on steel cans.[240]

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.[241]

Biological and chemical

Cerium(IV) oxide is used as a catalyst in self-cleaning ovens.[242]

The biocidal effects of some heavy metals have been known since antiquity.[243] Platinum, osmium, copper, ruthenium, and other heavy metals, including arsenic, are used in anti-cancer treatments, or have shown potential.[244] Antimony (anti-protozoal), bismuth (anti-ulcer), gold (anti-arthritic), and iron (anti-malarial) are also important in medicine.[245] Copper, zinc, silver, gold, or mercury are used in antiseptic formulations;[246] small amounts of some heavy metals are used to control algal growth in, for example, cooling towers.[247] 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.[248]

Selected heavy metals are used as catalysts in fuel processing (rhenium, for example), synthetic rubber and fibre production (bismuth), emission control devices (palladium), and in self-cleaning ovens (where cerium(IV) oxide in the walls of such ovens helps oxidise carbon-based cooking residues).[249] 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).[250]

Colouring and optics

Neodymium sulfate (Nd₂(SO₄)₃), used to colour glassware[251]

The colours of glass, ceramic glazes, paints, pigments, and plastics are commonly produced by the inclusion of heavy metals (or their compounds) such as chromium, manganese, cobalt, copper, zinc, selenium, zirconium, molybdenum, silver, tin, praseodymium, neodymium, erbium, tungsten, iridium, gold, lead, or uranium.[252] Tattoo inks may contain heavy metals, such as chromium, cobalt, nickel, and copper.[253] The high reflectivity of some heavy metals is important in the construction of mirrors, including precision astronomical instruments. Headlight reflectors rely on the excellent reflectivity of a thin film of rhodium.[254]

Electronics, magnets, and lighting

The Topaz Solar Farm, in southern California, features nine million cadmium-tellurium photovoltaic modules covering an area of 25.6 square kilometres (9.9 square miles).

Heavy metals or their compounds can be found in electronic components, electrodes, and wiring and solar panels where they may be used as either conductors, semiconductors, or insulators. Molybdenum powder is used in circuit board inks.[255] Ruthenium(IV) oxide coated titanium anodes are used for the industrial production of chlorine.[256] Home electrical systems, for the most part, are wired with copper wire for its good conducting properties.[257] Silver and gold are used in electrical and electronic devices, particularly in contact switches, as a result of their high electrical conductivity and capacity to resist or minimise the formation of impurities on their surfaces.[258] The semiconductors cadmium telluride and gallium arsenide are used to make solar panels. Hafnium oxide, an insulator, is used as a voltage controller in microchips; tantalum oxide, another insulator, is used in capacitors in mobile phones.[259] Heavy metals have been used in batteries for over 200 years, at least since Volta invented his copper and silver voltaic pile in 1800.[260] Promethium, lanthanum, and mercury are further examples found in, respectively, atomic, nickel-metal hydride, and button cell batteries.[261]

Magnets are made of heavy metals such as manganese, iron, cobalt, nickel, niobium, bismuth, praseodymium, neodymium, gadolinium, and dysprosium. Neodymium magnets are the strongest type of permanent magnet commercially available. They are key components of, for example, car door locks, starter motors, fuel pumps, and power windows.[262]

Heavy metals are used in lighting, lasers, and light-emitting diodes (LEDs). Flat panel displays incorporate a thin film of electrically conducting indium tin oxide. Fluorescent lighting relies on mercury vapour for its operation. Ruby lasers generate deep red beams by exciting chromium atoms; the lanthanides are also extensively employed in lasers. Gallium, indium, and arsenic;[263] and copper, iridium, and platinum are used in LEDs (the latter three in organic LEDs).[264]

Nuclear

An X-ray tube with a rotating anode, typically a tungsten-rhenium alloy on a molybdenum core, backed with graphite[265][n 35]

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.[268] 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.[269] 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;[270] heavy metals are also employed as spallation targets for the production of neutrons[271] or radioisotopes such as astatine (using lead, bismuth, thorium, or uranium in the latter case).[272]

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".
• Duffus J. H. 2002, "'Heavy metals'—A meaningless term?", Pure and Applied Chemistry, vol. 74, no. 5, pp. 793–807, doi:10.1351/pac200274050793. Includes a survey of the term's various meanings.
• Hawkes S. J. 1997, "What is a 'heavy metal'?", Journal of Chemical Education, vol. 74, no. 11, p. 1374, doi:10.1021/ed074p1374. A chemist's perspective.
• 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, ISBN 1-4292-7704-1. 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.
• Association between Heavy Metal Exposure and Parkinson’s Disease: A Review of the Mechanisms Related to Oxidative Stress.

Formation

• Hadhazy A. 2016, "Galactic 'gold mine' explains the origin of nature's heaviest elements Archived 2016-05-24 at the Wayback Machine", Science Spotlights, 10 May, accessed 11 July 2016

Uses

• Koehler C. S. W. 2001, "Heavy metal medicine", Chemistry Chronicles, American Chemical Society, accessed 11 July 2016
• Morowitz N. 2006, "The heavy metals", Modern Marvels, season 12, episode 14, HistoryChannel.com
• Öhrström L. 2014, "Tantalum oxide", Chemistry World, 24 September, accessed 4 October 2016. The author explains how tantalum(V) oxide banished brick-sized mobile phones. Also available as a podcast.

• Media related to Heavy metals at Wikimedia Commons