Heavy metals are generally defined as metals
with relatively high densities, atomic weights,
or atomic numbers. The criteria used, and
whether metalloids are included, vary depending
on the author and context. In metallurgy,
for example, a heavy metal may be defined
on the basis of density, whereas in physics
the distinguishing criterion might be atomic
number, while a chemist would likely be more
concerned with chemical behaviour. More specific
definitions have been published, but none
of these have been widely accepted. The definitions
surveyed in this article encompass up to 96
out of the 118 known chemical elements; only
mercury, lead and bismuth meet all of them.
Despite this lack of agreement, the term (plural
or singular) is widely used in science. A
density of more than 5 g/cm3 is sometimes
quoted as a commonly used criterion and is
used in the body of this article.
The earliest known metals—common metals
such as iron, copper, and tin, and precious
metals such as silver, gold, and platinum—are
heavy metals. From 1809 onwards, light metals,
such as magnesium, aluminium, and titanium,
were discovered, as well as less well-known
heavy metals including gallium, thallium,
and hafnium.
Some heavy metals are either essential nutrients
(typically iron, cobalt, and zinc), or relatively
harmless (such as ruthenium, silver, and indium),
but can be toxic in larger amounts or certain
forms. Other heavy metals, such as cadmium,
mercury, and lead, are highly poisonous. Potential
sources of heavy metal poisoning include mining,
tailings, industrial wastes, agricultural
runoff, occupational exposure, paints and
treated timber.
Physical and chemical characterisations of
heavy metals need to be treated with caution,
as the metals involved are not always consistently
defined. As well as being relatively dense,
heavy metals tend to be less reactive than
lighter metals and have much less soluble
sulfides and hydroxides. While it is relatively
easy to distinguish a heavy metal such as
tungsten from a lighter metal such as sodium,
a few heavy metals, such as zinc, mercury,
and lead, have some of the characteristics
of lighter metals, and, lighter metals such
as beryllium, scandium, and titanium, have
some of the characteristics of heavier metals.
Heavy metals are relatively scarce in the
Earth's crust but are present in many aspects
of modern life. They are used in, for example,
golf clubs, cars, antiseptics, self-cleaning
ovens, plastics, solar panels, mobile phones,
and particle accelerators.
== Definitions ==
There is no widely agreed criterion-based
definition of a heavy metal. Different meanings
may be attached to the term, depending on
the context. In metallurgy, for example, a
heavy metal may be defined on the basis of
density, whereas in physics the distinguishing
criterion might be atomic number, and a chemist
would likely be more concerned with chemical
behaviour.Density criteria range from above
3.5 g/cm3 to above 7 g/cm3. Atomic weight
definitions can range from greater than sodium
(atomic weight 22.98); greater than 40 (excluding
s- and f-block metals, hence starting with
scandium); or more than 200, i.e. from mercury
onwards. Atomic numbers of heavy metals are
generally given as greater than 20 (calcium);
sometimes this is capped at 92 (uranium).
Definitions based on atomic number have been
criticised for including metals with low densities.
For example, rubidium in group (column) 1
of the periodic table has an atomic number
of 37 but a density of only 1.532 g/cm3, which
is below the threshold figure used by other
authors. The same problem may occur with atomic
weight based definitions.The United States
Pharmacopeia includes a test for heavy metals
that involves precipitating metallic impurities
as their coloured sulfides." In 1997, Stephen
Hawkes, a chemistry professor writing in the
context of fifty years' experience with the
term, said it applied to "metals with insoluble
sulfides and hydroxides, whose salts produce
colored solutions in water and whose complexes
are usually colored". On the basis of the
metals he had seen referred to as heavy metals,
he suggested it would useful to define them
as (in general) all the metals in periodic
table columns 3 to 16 that are in row 4 or
greater, in other words, the transition metals
and post-transition metals. The lanthanides
satisfy Hawkes' three-part description; the
status of the actinides is not completely
settled.In biochemistry, heavy metals are
sometimes defined—on the basis of the Lewis
acid (electronic pair acceptor) behaviour
of their ions in aqueous solution—as class
B and borderline metals. In this scheme, class
A metal ions prefer oxygen donors; class B
ions prefer nitrogen or sulfur donors; and
borderline or ambivalent ions show either
class A or B characteristics, depending on
the circumstances. 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.
A frequently cited proposal to use these classification
categories instead of the more evocative name
heavy metal has not been widely adopted.
=== List of heavy metals based on density
===
A density of more than 5 g/cm3 is sometimes
mentioned as a common heavy metal defining
factor and, in the absence of a unanimous
definition, is used to populate this list
and (unless otherwise stated) guide the remainder
of the article. Metalloids meeting the applicable
criteria–arsenic and antimony for example—are
sometimes counted as heavy metals, particularly
in environmental chemistry, as is the case
here. Selenium (density 4.8 g/cm3) is also
included in the list. It falls marginally
short of the density criterion and is less
commonly recognised as a metalloid but has
a waterborne chemistry similar in some respects
to that of arsenic and antimony. Other metals
sometimes classified or treated as "heavy"
metals, such as beryllium (density 1.8 g/cm3),
aluminium (2.7 g/cm3), calcium (1.55 g/cm3),
and barium (3.6 g/cm3) are here treated as
light metals and, in general, are not further
considered.
== Origins and use of the term ==
The heaviness of naturally occurring metals
such as gold, copper, and iron may have been
noticed in prehistory and, in light of their
malleability, led to the first attempts to
craft metal ornaments, tools, and weapons.
All metals discovered from then until 1809
had relatively high densities; their heaviness
was regarded as a singularly distinguishing
criterion.From 1809 onwards, light metals
such as sodium, potassium, and strontium were
isolated. Their low densities challenged conventional
wisdom and it was proposed to refer to them
as metalloids (meaning "resembling metals
in form or appearance"). This suggestion was
ignored; the new elements came to be recognised
as metals, and the term metalloid was then
used to refer to nonmetallic elements and,
later, elements that were hard to describe
as either metals or nonmetals.An early use
of the term "heavy metal" dates from 1817,
when the German chemist Leopold Gmelin divided
the elements into nonmetals, light metals,
and heavy metals. Light metals had densities
of 0.860–5.0 g/cm3; heavy metals 5.308–22.000.
The term later became associated with elements
of high atomic weight or high atomic number.
It is sometimes used interchangeably with
the term heavy element. For example, in discussing
the history of nuclear chemistry, Magee notes
that the actinides were once thought to represent
a new heavy element transition group whereas
Seaborg and co-workers "favoured ... a heavy
metal rare-earth like series ...". In astronomy,
however, a heavy element is any element heavier
than hydrogen and helium.
=== Criticism ===
In 2002, Scottish toxicologist John Duffus
reviewed the definitions used over the previous
60 years and concluded they were so diverse
as to effectively render the term meaningless.
Along with this finding, the heavy metal status
of some metals is occasionally challenged
on the grounds that they are too light, or
are involved in biological processes, or rarely
constitute environmental hazards. Examples
include scandium (too light); vanadium to
zinc (biological processes); and rhodium,
indium, and osmium (too rare).
=== Popularity ===
Despite its questionable meaning, the term
heavy metal appears regularly in scientific
literature. A 2010 study found that it had
been increasingly used and seemed to have
become part of the language of science. It
is said to be an acceptable term, given its
convenience and familiarity, as long as it
is accompanied by a strict definition. The
counterparts to the heavy metals, the light
metals, are alluded to by The Minerals, Metals
and Materials Society as including "aluminium,
magnesium, beryllium, titanium, lithium, and
other reactive metals." The named metals have
densities of 0.534 to 4.54 g/cm3.
== Biological role ==
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);
vanadium and manganese (enzyme regulation
or functioning); chromium (glucose utilisation);
nickel (cell growth); arsenic (metabolic growth
in some animals and possibly in humans) and
selenium (antioxidant functioning and hormone
production). Periods 5 and 6 contain fewer
essential heavy metals, consistent with the
general pattern that heavier elements tend
to be less abundant and that scarcer elements
are less likely to be nutritionally essential.
In period 5, molybdenum is required for the
catalysis of redox reactions; cadmium is used
by some marine diatoms for the same purpose;
and tin may be required for growth in a few
species. In period 6, tungsten is required
by some archaea and bacteria for metabolic
processes. A deficiency of any of these period
4–6 essential heavy metals may increase
susceptibility to heavy metal poisoning (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).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 ==
The focus of this section is mainly on the
more serious toxic effects of heavy metals,
including cancer, brain damage or death, rather
than the harm they may cause to one more of
the skin, lungs, stomach, kidneys, liver,
or heart. For more specific information see
Metal toxicity, Toxic heavy metal, or the
articles on individual elements or compounds.Heavy
metals are often assumed to be highly toxic
or damaging to the environment. Some are,
while certain others are toxic only if taken
in excess or encountered in certain forms.
=== Environmental heavy metals ===
Chromium, arsenic, cadmium, mercury, and lead
have the greatest potential to cause harm
on account of their extensive use, the toxicity
of some of their combined or elemental forms,
and their widespread distribution in the environment.
Hexavalent chromium, for example, is highly
toxic as are mercury vapour and many mercury
compounds. These five elements have a strong
affinity for sulfur; in the human body they
usually bind, via thiol groups (–SH), to
enzymes responsible for controlling the speed
of metabolic reactions. The resulting sulfur-metal
bonds inhibit the proper functioning of the
enzymes involved; human health deteriorates,
sometimes fatally. Chromium (in its hexavalent
form) and arsenic are carcinogens; cadmium
causes a degenerative bone disease; and mercury
and lead damage the central nervous system.
Lead is the most prevalent heavy metal contaminant.
Levels in the aquatic environments of industrialised
societies have been estimated to be two to
three times those of pre-industrial levels.
As a component of tetraethyl lead, (CH3CH2)4Pb,
it was used extensively in gasoline during
the 1930s–1970s. Although the use of leaded
gasoline was largely phased out in North America
by 1996, soils next to roads built before
this time retain high lead concentrations.
Later research demonstrated a statistically
significant correlation between the usage
rate of leaded gasoline and violent crime
in the United States; taking into account
a 22-year time lag (for the average age of
violent criminals), the violent crime curve
virtually tracked the lead exposure curve.Other
heavy metals noted for their potentially hazardous
nature, usually as toxic environmental pollutants,
include manganese (central nervous system
damage); cobalt and nickel (carcinogens);
copper, zinc, selenium and silver (endocrine
disruption, congenital disorders, or general
toxic effects in fish, plants, birds, or other
aquatic organisms); tin, as organotin (central
nervous system damage); antimony (a suspected
carcinogen); and thallium (central nervous
system damage).
=== Nutritionally essential heavy metals ===
Heavy metals essential for life can be toxic
if taken in excess; some have notably toxic
forms. Vanadium pentoxide (V2O5) is carcinogenic
in animals and, when inhaled, causes DNA damage.
The purple permanganate ion MnO–4 is a liver
and kidney poison. Ingesting more than 0.5
grams of iron can induce cardiac collapse;
such overdoses most commonly occur in children
and may result in death within 24 hours. Nickel
carbonyl (Ni2(CO)4), at 30 parts per million,
can cause respiratory failure, brain damage
and death. Imbibing a gram or more of copper
sulfate (CuSO4) can be fatal; survivors may
be left with major organ damage. More than
five milligrams of selenium is highly toxic;
this is roughly ten times the 0.45 milligram
recommended maximum daily intake; long-term
poisoning can have paralytic effects.
=== Other heavy metals ===
A few other non-essential heavy metals have
one or more toxic forms. Kidney failure and
fatalities have been recorded arising from
the ingestion of germanium dietary supplements
(~15 to 300 g in total consumed over a period
of two months to three years). Exposure to
osmium tetroxide (OsO4) may cause permanent
eye damage and can lead to respiratory failure
and death. Indium salts are toxic if more
than few milligrams are ingested and will
affect the kidneys, liver, and heart. Cisplatin
(PtCl2(NH3)2), which is an important drug
used to kill cancer cells, is also a kidney
and nerve poison. Bismuth compounds can cause
liver damage if taken in excess; insoluble
uranium compounds, as well as the dangerous
radiation they emit, can cause permanent kidney
damage.
=== Exposure sources ===
Heavy metals can degrade air, water, and soil
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 and industrial
wastes; vehicle emissions; lead-acid batteries;
fertilisers; paints; and treated timber; aging
water supply infrastructure; and microplastics
floating in the world's oceans. Recent examples
of heavy metal contamination and health risks
include the occurrence of Minamata disease,
in Japan (1932–1968; lawsuits ongoing as
of 2016); the Bento Rodrigues dam disaster
in Brazil, and high levels of lead in drinking
water supplied to the residents of Flint,
Michigan, in the north-east of the United
States.
== Formation, abundance, occurrence, and extraction
==
Heavy metals up to the vicinity of iron (in
the periodic table) are largely made via stellar
nucleosynthesis. In this process, lighter
elements from hydrogen to silicon undergo
successive fusion reactions inside stars,
releasing light and heat and forming heavier
elements with higher atomic numbers.Heavier
heavy metals are not usually formed this way
since fusion reactions involving such nuclei
would consume rather than release energy.
Rather, they are largely synthesised (from
elements with a lower atomic number) by neutron
capture, with the two main modes of this repetitive
capture being the s-process and the r-process.
In the s-process ("s" stands for "slow"),
singular captures are separated by years or
decades, allowing the less stable nuclei to
beta decay, while in the r-process ("rapid"),
captures happen faster than nuclei can decay.
Therefore, the s-process takes a more or less
clear path: for example, stable cadmium-110
nuclei are successively bombarded by free
neutrons inside a star until they form cadmium-115
nuclei which are unstable and decay to form
indium-115 (which is nearly stable, with a
half-life 30000 times the age of the universe).
These nuclei capture neutrons and form indium-116,
which is unstable, and decays to form tin-116,
and so on. In contrast, there is no such path
in the r-process. The s-process stops at bismuth
due to the short half-lives of the next two
elements, polonium and astatine, which decay
to bismuth or lead. The r-process is so fast
it can skip this zone of instability and go
on to create heavier elements such as thorium
and uranium.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, thereby increasing the abundance
of elements heavier than helium in the interstellar
medium. When gravitational attraction causes
this matter to coalesce and collapse new stars
and planets are formed.The Earth's crust is
made of approximately 5% of heavy metals by
weight, with iron comprising 95% of this quantity.
Light metals (~20%) and nonmetals (~75%) make
up the other 95% of the crust. Despite their
overall scarcity, heavy metals can become
concentrated in economically extractable quantities
as a result of mountain building, erosion,
or other geological processes.Heavy metals
are primarily found as lithophiles (rock-loving)
or chalcophiles (ore-loving). Lithophile heavy
metals are mainly f-block elements and the
more reactive of the d-block elements. They
have a strong affinity for oxygen and mostly
exist as relatively low density silicate minerals.
Chalcophile heavy metals are mainly the less
reactive d-block elements, and period 4–6
p-block metals and metalloids. They are usually
found in (insoluble) sulfide minerals. Being
denser than the lithophiles, hence sinking
lower into the crust at the time of its solidification,
the chalcophiles tend to be less abundant
than the lithophiles.On the other hand, gold
is a siderophile, or iron-loving element.
It does not readily form compounds with either
oxygen or sulfur. At the time of the Earth's
formation, and as the most noble (inert) of
metals, gold sank into the core due to its
tendency to form high-density metallic alloys.
Consequently, it is a relatively rare metal.
Some other (less) noble heavy metals—molybdenum,
rhenium, the platinum group metals (ruthenium,
rhodium, palladium, osmium, iridium, and platinum),
germanium, and tin—can be counted as siderophiles
but only in terms of their primary occurrence
in the Earth (core, mantle and crust), rather
the crust. These metals otherwise occur in
the crust, in small quantities, chiefly as
chalcophiles (less so in their native form).Concentrations
of heavy metals below the crust are generally
higher, with most being found in the largely
iron-silicon-nickel core. Platinum, for example,
comprises approximately 1 part per billion
of the crust whereas its concentration in
the core is thought to be nearly 6,000 times
higher. Recent speculation suggests that uranium
(and thorium) in the core may generate a substantial
amount of the heat that drives plate tectonics
and (ultimately) sustains the Earth's magnetic
field.The winning of heavy metals from their
ores is a complex function of ore type, the
chemical properties of the metals involved,
and the economics of various extraction methods.
Different countries and refineries may use
different processes, including those that
differ from the brief outlines listed here.
Broadly speaking, and with some exceptions,
lithophile heavy metals can be extracted from
their ores by electrical or chemical treatments,
while chalcophile heavy metals are obtained
by roasting their sulphide ores to yield the
corresponding oxides, and then heating these
to obtain the raw metals. Radium occurs in
quantities too small to be economically mined
and is instead obtained from spent nuclear
fuels. The chalcophile platinum group metals
(PGM) mainly occur in small (mixed) quantities
with other chalcophile ores. The ores involved
need to be smelted, roasted, and then leached
with sulfuric acid to produce a residue of
PGM. This is chemically refined to obtain
the individual metals in their pure forms.
Compared to other metals, PGM are expensive
due to their scarcity and high production
costs.Gold, a siderophile, is most commonly
recovered by dissolving the ores in which
it is found in a cyanide solution. The gold
forms a dicyanoaurate(I), for example: 2 Au
+ H2O +½ O2 + KCN → 2 K[Au(CN)2] + 2 KOH.
Zinc is added to the mix and, being more reactive
than gold, displaces the gold: 2[Au(CN)2]
+ Zn → K2[Zn(CN)4] + 2 Au. The gold precipitates
out of solution as a sludge, and is filtered
off and melted.
== Properties compared with light metals ==
Some general physical and chemical properties
of light and heavy metals are summarised in
the table. The comparison should be treated
with caution since the terms light metal and
heavy metal are not always consistently defined.
Also the physical properties of hardness and
tensile strength can vary widely depending
on purity, grain size and pre-treatment.
These properties make it relatively easy to
distinguish a light metal like sodium from
a heavy metal like tungsten, but the differences
become less clear at the boundaries. Light
structural metals like beryllium, scandium,
and titanium have some of the characteristics
of heavy metals, such as higher melting points;
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 be found in, or used to produce,
20% of all consumer goods.Some common uses
of heavy metals depend on the general characteristics
of metals such as electrical conductivity
and reflectivity or the general characteristics
of heavy metals such as density, strength,
and durability. Other uses depend on the characteristics
of the specific element, such as their biological
role as nutrients or poisons or some other
specific atomic properties. Examples of such
atomic properties include: partly filled d-
or f- orbitals (in many of the transition,
lanthanide, and actinide heavy metals) that
enable the formation of coloured compounds;
the capacity of most heavy metal ions (such
as platinum, cerium or bismuth) to exist in
different oxidation states and therefore act
as catalysts; poorly overlapping 3d or 4f
orbitals (in iron, cobalt, and nickel, or
the lanthanide heavy metals from europium
through thulium) that give rise to magnetic
effects; and high atomic numbers and electron
densities that underpin their nuclear science
applications. Typical uses of heavy metals
can be broadly grouped into the following
six categories.
=== Weight- or density-based ===
Some uses of heavy metals, including in sport,
mechanical engineering, military ordnance,
and nuclear science, take advantage of their
relatively high densities. In underwater diving,
lead is used as a ballast; in handicap horse
racing each horse must carry a specified lead
weight, based on factors including past performance,
so as to equalize the chances of the various
competitors. In golf, tungsten, brass, or
copper inserts in fairway clubs and irons
lower the centre of gravity of the club making
it easier to get the ball into the air; and
golf balls with tungsten cores are claimed
to have better flight characteristics. In
fly fishing, sinking fly lines have a PVC
coating embedded with tungsten powder, so
that they sink at the required rate. In track
and field sport, steel balls used in the hammer
throw and shot put events are filled with
lead in order to attain the minimum weight
required under international rules. Tungsten
was used in hammer throw balls at least up
to 1980; the minimum size of the ball was
increased in 1981 to eliminate the need for
what was, at that time, an expensive metal
(triple the cost of other hammers) not generally
available in all countries. Tungsten hammers
were so dense that they penetrated too deeply
into the turf.In mechanical engineering, heavy
metals are used for ballast in boats, aeroplanes,
and motor vehicles; or in balance weights
on wheels and crankshafts, gyroscopes, and
propellers, and centrifugal clutches, in situations
requiring maximum weight in minimum space
(for example in watch movements).
In military ordnance, tungsten or uranium
is used in armour plating and armour piercing
projectiles, as well as in nuclear weapons
to increase efficiency (by reflecting neutrons
and momentarily delaying the expansion of
reacting materials). 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. 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. Doubts
have been raised about the safety (or green
credentials) of tungsten.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.
=== Strength- or durability-based ===
The strength or durability of heavy metals
such as chromium, iron, nickel, copper, zinc,
molybdenum, tin, tungsten, and lead, as well
as their alloys, makes them useful for the
manufacture of artefacts such as tools, machinery,
appliances, utensils, pipes, railroad tracks,
buildings and bridges, automobiles, locks,
furniture, ships, planes, coinage and jewellery.
They are also used as alloying additives for
enhancing the properties of other metals.
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 jewellery
as are (for example) nickel, copper, indium,
and cobalt in coloured gold. Low-cost jewellery
and children's toys may be made, to a significant
degree, of heavy metals such as chromium,
nickel, cadmium, or lead.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, zinc carbonate, tin oxide, or a mixture
of lead oxide, carbonate, and sulfate, confer
valuable protective properties. Copper and
lead are therefore used, for example, as roofing
materials; zinc acts as an anti-corrosion
agent in galvanised steel; and tin serves
a similar purpose on steel cans.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 and steel alloys to improve their machinability;
and to lead to make it harder and more acid-resistant.
=== 
Biological and chemical ===
The biocidal effects of some heavy metals
have been known since antiquity. Platinum,
osmium, copper, ruthenium, and other heavy
metals, including arsenic, are used in anti-cancer
treatments, or have shown potential. Antimony
(anti-protozoal), bismuth (anti-ulcer), gold
(anti-arthritic), and iron (anti-malarial)
are also important in medicine. Copper, zinc,
silver, gold, or mercury are used in antiseptic
formulations; small amounts of some heavy
metals are used to control algal growth in,
for example, cooling towers. Depending on
their intended use as fertilisers or biocides,
agrochemicals may contain heavy metals such
as chromium, cobalt, nickel, copper, zinc,
arsenic, cadmium, mercury, or lead.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). 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).
=== Colouring and optics ===
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.
Tattoo inks may contain heavy metals, such
as chromium, cobalt, nickel, and copper. 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.
=== Electronics, magnets, and lighting ===
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. Ruthenium(IV) oxide coated
titanium anodes are used for the industrial
production of chlorine. 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 contact switches,
as a result of their high electrical conductivity
and capacity to resist or minimise the formation
of impurities on their surfaces. 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. Heavy
metals have been used in batteries for over
200 years, at least since Volta invented his
copper and silver voltaic pile in 1800. Promethium,
lanthanum, and mercury are further examples
found in, respectively, atomic, nickel-metal
hydride, and button cell batteries.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.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;
and copper, iridium, and platinum are used
in LEDs (the latter three in organic LEDs).
=== Nuclear ===
Niche uses of heavy metals with high atomic
numbers occur in diagnostic imaging, electron
microscopy, and nuclear science. In diagnostic
imaging, heavy metals such as cobalt or tungsten
make up the anode materials found in x-ray
tubes. In electron microscopy, heavy metals
such as lead, gold, palladium, platinum, or
uranium are used to make conductive coatings
and to introduce electron density into biological
specimens by staining, negative staining,
or vacuum deposition. In nuclear science,
nuclei of heavy metals such as chromium, iron,
or zinc are sometimes fired at other heavy
metal targets to produce superheavy elements;
heavy metals are also employed as spallation
targets for the production of neutrons or
radioisotopes such as astatine (using lead,
bismuth, thorium, or uranium in the latter
case).
== Notes ==
== 
Sources ==
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Citations
