Heavy metal (elements)

This article is about dense metallic elements. For toxic metals, seeToxic heavy metal.For other uses, seeHeavy metal (disambiguation).

Crystals ofosmium,a heavy metal nearly twice as dense aslead

Heavy metalsaremetallicelements with relatively highdensities,atomic weights,oratomic numbers.The criteria used, and whethermetalloidsare included, vary depending on the author and context and has been argued should not be used.A heavy metal may be defined on the basis of density, atomic number orchemical behaviour.More specific definitions have been published, none of which have been widely accepted. The definitions surveyed in this article encompass up to 96 out of the 118 knownchemical elements;onlymercury,leadandbismuthmeet 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/cm³is sometimes quoted as a commonly used criterion and is used in the body of this article.

The earliest known metals—common metals such asiron,copper,andtin,and precious metals such assilver,gold,andplatinum—are heavy metals. From 1809 onward,light metals,such asmagnesium,aluminium,andtitanium,were discovered, as well as less well-known heavy metals includinggallium,thallium,andhafnium.

Some heavy metals are either essential nutrients (typically iron,cobalt,copper andzinc), or relatively harmless (such asruthenium,silver andindium), but can be toxic in larger amounts or certain forms. Other heavy metals, such asarsenic,cadmium,mercury, and lead, are highly poisonous. Potential sources of heavy metal poisoning includemining,tailings,smelting,industrial waste,agricultural runoff,occupational exposure,paintsandtreated timber.

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 lessreactivethan lighter metals and have far fewersolublesulfidesandhydroxides.While it is relatively easy to distinguish a heavy metal such astungstenfrom a lighter metal such assodium,a few heavy metals, such as zinc, mercury, and lead, have some of the characteristics of lighter metals; and lighter metals such asberyllium,scandium,and titanium, have some of the characteristics of heavier metals.

Heavy metals are relatively scarce in theEarth's crustbut 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,andparticle accelerators.

Definitions

Controversial terminology

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

Use outside toxicology

Even in applications other than toxicity, there no widely agreed criterion-based definition of a heavy metal. 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 ofdensity,the distinguishing criterion might beatomic number,or the chemical behaviour.

Density criteria range from above 3.5 g/cm³to above 7 g/cm³.Atomic weight definitions can range from greater thansodium(atomic weight 22.98);greater than 40 (excludings-andf-blockmetals, hence starting withscandium);or more than 200, i.e. frommercuryonwards.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,rubidiumingroup (column) 1of theperiodic tablehas 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.

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 ondensity,three onatomic weight,two onatomic number,and three on chemical behaviour.It illustrates the lack of agreement surrounding the concept, with the possible exception ofmercury,leadandbismuth. Six elements near the end ofperiods(rows) 4 to 7 sometimes considered metalloids are treated here as metals: they aregermanium(Ge),arsenic(As),selenium(Se),antimony(Sb),tellurium(Te), andastatine(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.

TheUnited States Pharmacopeiaincludes a test for heavy metals that involves precipitating metallic impurities as their colouredsulfides.On the basis of this type of chemical test, the group would include thetransition metalsandpost-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 preferoxygendonors; class B ions prefernitrogenorsulfurdonors; and borderline or ambivalent ions show either class A or B characteristics, depending on the circumstances.The distinction between the class A metals and the other two categories is sharp. The class A and class B terminology is analogous to the"hard acid" and "soft base"terminology sometimes used to refer to the behaviour of metal ions in inorganic systems.The system groups the elements by$X_{m}^{2}r$where$X_{m}$is the metal ionelectronegativityand$r$is itsionic radius.This index gauges the importance ofcovalentinteractions vsionicinteractions 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.

List of heavy metals based on density

A density of more than 5 g/cm³is sometimes mentioned as a common heavy metal defining factorand, 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 inenvironmental chemistry,as is the case here. Other metals sometimes classified or treated as "heavy" metals, such asberyllium(density 1.8 g/cm³),aluminium(2.7 g/cm³),calcium(1.55 g/cm³),and barium(3.6 g/cm³)are here treated aslight metalsand, in general, are not further considered.

Produced mainly by commercial mining(informally classified by economic significance)
Strategic (30) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Considered vital to multiple nations' strategic interests These 30 include 22 listed here and 8 below (6 precious & 2 commodity).	Antimony† Cerium Dysprosium Erbium Europium Gadolinium Gallium Germanium† Holmium Indium Lanthanum Lutetium Neodymium Niobium Praseodymium Samarium Tantalum Terbium Thulium Tungsten Uranium☢ Ytterbium
Precious (8) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Rare and costly	Strategic: Iridium Osmium Palladium Platinum Rhodium Ruthenium Non-strategic: Gold Silver
Commodity (9) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Traded by thetonneon theLME	Strategic: Chromium Cobalt Non-strategic: Copper Iron Lead Molybdenum Nickel Tin Zinc
Minor (14) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Neither strategic, precious, nor commodity	Arsenic† Bismuth Cadmium Hafnium Manganese Mercury Rhenium Tellurium† Thallium Thorium☢ Vanadium Zirconium
Produced mainly byartificial transmutation(informally classified by stability)
Long-lived (14) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Half-lifegreater than 1 day	Actinium☢¶ Americium☢ Berkelium☢ Californium☢ Curium☢ Einsteinium☢ Fermium☢ Mendelevium☢ Neptunium☢¶ Plutonium☢¶ Polonium☢¶ Protactinium☢¶ Promethium☢¶ Radium☢¶ Technetium☢¶
Ephemeral (17) Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Half-life less than 1 day	Astatine☢‡ ¶ Bohrium☢ Copernicium☢ Darmstadtium☢ Dubnium☢ Flerovium☢ Hassium☢ Lawrencium☢ Livermorium☢ Meitnerium☢ Moscovium☢ Nihonium☢ Nobelium☢ Roentgenium☢ Rutherfordium☢ Seaborgium☢ Tennessine☢
† Antimony, arsenic, germanium and tellurium are commonly recognised asmetalloids ‡ Astatine is predicted to be a metal. ☢ Allisotopesof 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 19billionbillion years is over a billion times the 13.8-billion-year estimatedage of the universe. ¶ These nine elements do occur naturally but in amounts too small for economically viable extraction.

Origins and use of the term

The heaviness ofnaturally occurring metalssuch asgold,copper,andironmay have been noticed inprehistoryand, in light of theirmalleability,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, andstrontiumwere isolated. Their low densities challenged conventional wisdom and it was proposed to refer to them asmetalloids(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.

In 1817 the German chemistLeopold Gmelindivided 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 later became associated with elements of high atomic weight or high atomic number.It is sometimes used interchangeably with the termheavy element.For example, in discussing the history ofnuclear chemistry,Mageenotes that the actinides were once thought to represent a new heavy element transition group whereasSeaborgand co-workers "favoured... a heavy metalrare-earthlike series... ".

The counterparts to the heavy metals, thelight metals,are defined byThe Minerals, Metals and Materials Societyas 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 in
an average 70 kg human body

Element	Milligrams
Iron	4000	4000
Zinc	2500	2500
Lead	120	120
Copper	70	70
Tin	30	30
Vanadium	20	20
Cadmium	20	20
Nickel	15	15
Selenium	14	14
Manganese	12	12
Other	200	200
Total	7000

See also:Essential trace element

Trace amounts of some heavy metals, mostly in period 4, are required for certain biological processes. These areironandcopper(oxygenandelectron transport);cobalt(complex syntheses and cell metabolism);zinc(hydroxylation);vanadiumandmanganese(enzyme regulationor functioning);chromium(glucoseutilisation);nickel(cell growth);arsenic(metabolic growth in some animals and possibly in humans) andselenium(antioxidantfunctioning andhormoneproduction).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.Inperiod 5,molybdenumis required for thecatalysisofredoxreactions;cadmiumis used by some marinediatomsfor the same purpose; andtinmay be required for growth in a few species.Inperiod 6,tungstenis required by somearchaeaand bacteria formetabolic processes.A deficiency of any of these period 4–6 essential heavy metals may increase susceptibility toheavy metal poisoning(conversely, an excess may also haveadverse biological effects). An average 70 kghuman bodyis 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 (mostlywater).

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).

Toxicity

Main articles:Toxic heavy metalandMetal toxicity

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. Inhalation of certain metals, either as fine dust or most commonly as fumes, can also result in a condition calledmetal fume fever.

Environmental heavy metals

Chromium, arsenic, cadmium, mercury, and lead have the greatest potential to cause harm on account of their extensive use, thetoxicityof some of their combined or elemental forms, and their widespread distribution in the environment.Hexavalent chromium,for example, is highly toxicas are mercury vapour and many mercury compounds.These five elements have a strong affinity for sulfur; in the human body they usually bind, viathiolgroups (–SH), toenzymesresponsible 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 arecarcinogens;cadmium causes adegenerative bone disease;and mercury and lead damage thecentral nervous system.

• 
  Chromiumcrystals
  and 1 cm³cube
• 
  Arsenic,sealed in a
  container to stop tarnishing
• 
  Cadmiumbar
  and 1 cm³cube
• 
  Mercurybeing
  poured into apetri dish
• 
  Oxidisedlead
  nodules and 1 cm³cube

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 oftetraethyl lead,(CH
₃CH
₂)
₄Pb,it was used extensively ingasolinefrom the 1930s until the 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.

Other heavy metals noted for their potentially hazardous nature, usually as toxic environmental pollutants, include manganese (central nervous system damage);cobalt andnickel(carcinogens);copper,zinc,seleniumandsilver(endocrinedisruption,congenital disorders,or general toxic effects in fish, plants, birds, or other aquatic organisms); tin, asorganotin(central nervous system damage);antimony (a suspected carcinogen);andthallium(central nervous system damage).

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 toosmium tetroxide(OsO₄) may cause permanent eye damage and can lead to respiratory failureand death.Indium salts are toxic if more than few milligrams are ingested and will affect the kidneys, liver, and heart.Cisplatin(PtCl₂(NH₃)₂), an important drug used tokill cancer cells,is also a kidney and nerve poison.Bismuthcompounds can cause liver damage if taken in excess; insoluble uranium compounds, as well as the dangerousradiationthey emit, can cause permanent kidney damage.

Exposure sources

See also:Cement § Heavy metal emissions in the air

Heavy metals can degrade air, water, andsoil quality,and subsequently cause health issues in plants, animals, and people, when they become concentrated as a result of industrial activities.Common sources of heavy metals in this context include mining, smelting and industrial wastes; vehicle emissions;motor oil;fuels used by ships and heavy machineries; construction works; fertilisers;pesticides;paints;dyes and pigments; renovation; illegal depositing of construction and demolition waste; open-top roll-off dumpster; welding, brazing and soldering; glassworking;concrete works; roadworks; use of recycled materials; DIY Metal Projects; incinerators;burning ofjoss paper;open burning of wastein rural areas; contaminated ventilation system; food contaminated by the environment or by the packaging; armaments;lead–acid batteries;electronic wasterecycling yard; andtreated timber;aging water supply infrastructure;andmicroplasticsfloating in the world's oceans.Recent examples of heavy metal contamination and health risks include the occurrence ofMinamata disease,in Japan (1932–1968; lawsuits ongoing as of 2016);theBento Rodrigues dam disasterin Brazil,high levels of lead in drinking water supplied to the residents ofFlint,Michigan, in the north-east of the United Statesand2015 Hong Kong heavy metal in drinking water incidents.

Formation, abundance, occurrence, and extraction

See also:NucleosynthesisandAbundance of the chemical elements

Heavy metals in the Earth's crust:
abundance and main occurrence or source
	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
				La	Ce	Pr	Nd		Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb
					Th		U
	Most abundant (56,300ppm by weight)										Rare (0.01–0.99 ppm)
	Abundant (100–999ppm)										Very rare (0.0001–0.0099 ppm)
	Uncommon (1–99 ppm)
Heavy metals left of the dividing line occur (or are sourced) mainly aslithophiles;those to the right, aschalcophilesexcept gold (asiderophile) and tin (a lithophile).

Heavy metals up to thevicinity of iron(in the periodic table) are largely made viastellar nucleosynthesis.In this process, lighter elements from hydrogen tosiliconundergo successivefusionreactions 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) byneutron capture,with the two main modes of this repetitive capture being thes-processand ther-process.In the s-process ( "s" stands for "slow" ), singular captures are separated by years or decades, allowing the less stable nuclei tobeta 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-life30,000times 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 asthoriumand uranium.

Heavy metals condense in planets as a result of stellar evolution and destruction processes. Stars lose much of their mass when it isejectedlate in their lifetimes, and sometimes thereafter as a result of aneutron starmerger,thereby increasing the abundance of elements heavier than helium in theinterstellar 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 ofmountain building,erosion,or othergeological processes.

Heavy metals are found primarily aslithophiles(rock-loving) orchalcophiles(ore-loving). Lithophile heavy metals are mainly f-block elements and the more reactive of thed-blockelements. They have a strong affinity for oxygen and mostly exist as relatively low densitysilicate minerals.Chalcophile heavy metals are mainly the less reactive d-block elements, and period 4–6p-blockmetals 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.

In contrast, gold is asiderophile,or iron-loving element. It does not readily form compounds with either oxygen or sulfur.At the time of theEarth's formation,and as the mostnoble(inert) of metals, gold sank into thecoredue to its tendency to form high-density metallic alloys. Consequently, it is a relatively rare metal.Some other (less) noble heavy metals—molybdenum,rhenium,theplatinum 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,mantleand crust), rather the crust. These metals otherwise occur in the crust, in small quantities, chiefly as chalcophiles (less so in theirnative form).

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 drivesplate tectonicsand (ultimately) sustains theEarth's magnetic field.

Broadly speaking, and with some exceptions, lithophile heavy metals can be extracted from their ores byelectricalorchemical treatments,while chalcophile heavy metals are obtained byroastingtheir sulphide ores to yield the corresponding oxides, and then heating these to obtain the raw metals.Radium occurs in quantities too small to be economically mined and is instead obtained from spentnuclear fuels.The chalcophile platinum group metals (PGM) mainly occur in small (mixed) quantities with other chalcophile ores. The ores involved need to besmelted,roasted, and thenleachedwithsulfuric acidto 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 scarcityand high production costs.

Gold, a siderophile, is most commonly recovered by dissolving the ores in which it is found in acyanide solution.The gold forms a dicyanoaurate(I), for example: 2 Au +H₂O+½ O₂+ 4KCN→ 2 K[Au(CN)₂] + 2KOH.Zinc is added to the mix and, being morereactivethan 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 termslight metalandheavy metalare not always consistently defined. Moreover, the physical properties of hardness and tensile strength can vary widely depending on purity,grain sizeand pre-treatment.

Properties of light and heavy metals

Physical properties	Light metals	Heavy metals
Density	Usually lower	Usually higher
Hardness	Tend to be soft, easily cut or bent	Most are quite hard
Thermal expansivity	Mostly higher	Mostly lower
Melting point	Mostly low	Low to very high
Tensile strength	Mostly lower	Mostly higher
Chemical properties	Light metals	Heavy metals
Periodic tablelocation	Most found in groups1and2	Nearly all found in groups3through16
Abundance in Earth's crust	More abundant	Less abundant
Main occurrence (or source)	Lithophiles	Lithophiles orchalcophiles(Auis asiderophile)
Reactivity	More reactive	Less reactive
Sulfides	Soluble to insoluble	Extremely insoluble
Hydroxides	Soluble to insoluble	Generally insoluble
Salts	Mostly form colourless solutions in water	Mostly form coloured solutions in water
Complexes	Mostly colourless	Mostly coloured
Biological role	Includemacronutrients(Na,Mg,K,Ca)	Includemicronutrients(V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Mo)

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;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,and forming mainly colourless complexes.

Uses

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 befound in, or used to produce, 20% of all consumer goods.

Some common uses of heavy metals depend on the general characteristics of metals such aselectrical conductivityandreflectivityor 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 filledd-orf- 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,ceriumor bismuth) to exist in differentoxidation statesand are used in catalysts;strongexchange interactionsin 3d or 4f orbitals (in iron, cobalt, and nickel, or the lanthanide heavy metals) that give rise to magnetic effects;and high atomic numbers andelectron densitiesthat underpin their nuclear science applications.Typical uses of heavy metals can be broadly grouped into the following six categories.

Weight- or density-based

In acello(example shown above) or aviolathe C-stringsometimes incorporatestungsten;its high density permits a smaller diameter string and improves responsiveness.

Some uses of heavy metals, including in sport,mechanical engineering,military ordnance,andnuclear science,take advantage of their relatively high densities. Inunderwater diving,lead is used as aballast;inhandicap horse racingeach horse must carry a specified lead weight, based on factors including past performance, so as to equalize the chances of the various competitors.Ingolf,tungsten,brass,or copper inserts infairwayclubs and ironslower 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.Infly fishing,sinking fly lines have aPVCcoating embedded with tungsten powder, so that they sink at the required rate.Intrack and fieldsport,steelballs used in thehammer throwandshot putevents 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,andRe... are expensive...Uoffers 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)

Heavy metals are used for ballast in boats,aeroplanes,and motor vehicles;or inbalance weights on wheelsandcrankshafts,gyroscopes,andpropellers,andcentrifugal clutches,in situations requiring maximum weight in minimum space (for example inwatch movements).

In military ordnance, tungsten or uranium is used inarmour platingandarmour piercing projectiles,as well as innuclear weaponsto increase efficiency (byreflecting neutronsand momentarily delaying the expansion of reacting materials).In the 1970s,tantalumwas found to be more effective than copper inshaped chargeandexplosively formed anti-armour weaponson account of its higher density, allowing greater force concentration, and better deformability.Less-toxic heavy metals,such as copper, tin, tungsten, and bismuth, and probably manganese (as well asboron,a metalloid), have replaced lead and antimony in thegreen bulletsused by some armies and in some recreational shooting munitions.Doubts have been raised about the safety (orgreen credentials) of tungsten.

Because denser materials absorb more of certain types of radioactive emissions such asgamma raysthan lighter ones, heavy metals are useful forradiation shieldingand tofocus radiation beamsinlinear acceleratorsandradiotherapyapplications.

Strength- or durability-based

TheStatue of Liberty.Astainless steelalloyarmatureprovides structural strength; acopperskin confers corrosion resistance.

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 tools, machinery,appliances,utensils,pipes,railroad tracks,buildingsand bridges,automobiles,locks,furniture,ships,planes,coinageand jewellery.They are also used as alloying additives for enhancing the properties of other metals.Of the two dozen elements that have been used in the world's monetised coinage only two, carbon and aluminium, are not heavy metals.Gold, silver, and platinum are used in jewelleryas are, for example, nickel, copper, indium, and cobalt incoloured gold.Low-cost jewelleryandchildren's toysmay be made, to a significant degree, of heavy metals such as chromium, nickel, cadmium, or lead.

Copper, zinc, tin, and lead are mechanically weaker metals but have usefulcorrosionprevention properties. While each of them will react with air, the resultingpatinasof either various copper salts,zinc carbonate,tin oxide,or a mixture oflead oxide,carbonate,andsulfate,confer valuableprotective properties.Copper and lead are therefore used, for example, asroofing materials;zinc acts as ananti-corrosionagent ingalvanised steel;and tin serves a similar purpose onsteel cans.

The workability and corrosion resistance of iron and chromium are increased by addinggadolinium;thecreep resistanceof nickel is improved with the addition of thorium.Tellurium is added to copper (tellurium copper) and stainless steel to improve their machinability; and to lead to make it harder and more acid-resistant.

Biological and chemical

Cerium(IV) oxideis used as acatalystinself-cleaning ovens.

Thebiocidaleffects ofsome heavy metalshave 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 inantisepticformulations;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,agrochemicalsmay contain heavy metals such as chromium, cobalt, nickel, copper, zinc, arsenic, cadmium, mercury, or lead.

Selected heavy metals are used as catalysts in fuel processing (rhenium, for example),synthetic rubberand fibre production (bismuth),emission control devices(palladium and platinum), and inself-cleaning ovens(wherecerium(IV) oxidein the walls of such ovens helpsoxidisecarbon-based cooking residues).In soap chemistry, heavy metals form insoluble soaps that are used inlubricating greases,paint dryers, andfungicides(apart from lithium, the alkali metals and theammoniumion form soluble soaps).

Colouring and optics

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

The colours ofglass,ceramic glazes,paints,pigments,andplasticsare commonly produced by the inclusion of heavy metals (or their compounds) such as chromium, manganese, cobalt, copper, zinc,zirconium,molybdenum, silver, tin,praseodymium,neodymium,erbium,tungsten, iridium, gold, lead, or uranium.Tattooinks may contain heavy metals, such as chromium, cobalt, nickel, and copper.The high reflectivity of some heavy metals is important in the construction ofmirrors,including precisionastronomical instruments.Headlight reflectors rely on the excellent reflectivity of a thin film of rhodium.

Electronics, magnets, and lighting

TheTopaz Solar Farm,in southern California, features nine millioncadmium-tellurium photovoltaicmodules covering an area of 25.6 square kilometres (9.9 square miles).

Heavy metals or their compounds can be found inelectronic components,electrodes,andwiringandsolar panelswhere they may be used as either conductors, semiconductors, or insulators. Molybdenum powder is used incircuit boardinks.Ruthenium(IV) oxidecoated titaniumanodesare used for the industrial production ofchlorine.Home electrical systems, for the most part, are wired with copper wire for its good conducting properties.Silver and gold are used in electrical and electronic devices, particularly in contactswitches,as a result of their high electrical conductivity and capacity to resist or minimise the formation of impurities on their surfaces.Hafnium oxide,an insulator, is used as avoltage controllerinmicrochips;tantalum oxide,another insulator, is used incapacitorsinmobile phones.Heavy metals have been used in batteries for over 200 years, at least sinceVoltainvented his copper and silvervoltaic pilein 1800.

Magnetsare often made of heavy metals such as manganese, iron, cobalt, nickel, niobium, bismuth, praseodymium, neodymium, gadolinium, anddysprosium.Neodymium magnets are the strongest type ofpermanent magnetcommercially available. They are key components of, for example, car door locks,starter motors,fuel pumps,andpower windows.

Heavy metals are used inlighting,lasers,andlight-emitting diodes(LEDs).Flat panel displaysincorporate a thin film of electrically conductingindium tin oxide.Fluorescent lightingrelies on mercury vapour for its operation.Ruby lasersgenerate deep red beams by exciting chromium atoms inaluminum oxide;the lanthanides are also extensively employed in lasers. Gallium, indium, and arsenic;and copper, iridium, and platinum are used in LEDs (the latter three inorganic LEDs).

Nuclear

AnX-ray tubewith a rotating anode, typically atungsten-rheniumalloy on amolybdenumcore, backed withgraphite

Niche uses of heavy metals with high atomic numbers occur indiagnostic imaging,electron microscopy,and nuclear science. In diagnostic imaging, heavy metals such as cobalt or tungsten make up the anode materials found inx-ray tubes.In electron microscopy, heavy metals such as lead, gold, palladium, platinum, or uranium have been used in the past to make conductive coatings and to introduce electron density into biological specimens bystaining,negative staining,orvacuum deposition.In nuclear science, nuclei of heavy metals such as chromium, iron, or zinc are sometimes fired at other heavy metal targets to producesuperheavy elements;heavy metals are also employed asspallationtargets for the production ofneutronsor isotopes of non-primordial elements such as astatine (using lead, bismuth, thorium, or uranium in the latter case).

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• Yang D. J., Jolly W. L. & O'Keefe A. 1977, "Conversion of hydrous germanium(II) oxide to germynyl sesquioxide, (HGe)₂O₃",'Inorganic Chemistry,vol. 16, no. 11, pp. 2980–2982,doi:10.1021/ic50177a070.
• Yousif N. 2007,Geochemistry of stream sediment from the state of Colorado using NURE data,ETD Collection for the University of Texas, El Paso,paper AAI3273991.

Further reading

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,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.
• 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 elementsArchived2016-05-24 at theWayback 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 apodcast.

External links

• Media related toHeavy metalsat Wikimedia Commons