Relative abundance of the chemical elements in the Earth's upper continental crust, on a per-atom basis
In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed.
Primordial nuclides were present in the interstellar medium from which
the solar system was formed, and were formed in, or after, the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation,
and potentially from other processes. They are the stable nuclides plus
the long-lived fraction of radionuclides surviving in the primordial
solar nebula through planet accretion until the present; 286 such nuclides are known.
Because the age of the Earth is 4.58×109 years (4.6 billion years), the half-life of the given nuclides must be greater than about 108 years (100 million years) for practical considerations. For example, for a nuclide with half-life 6×107 years (60 million years), this means 77 half-lives have elapsed, meaning that for each mole (6.02×1023 atoms) of that nuclide being present at the formation of Earth, only 4 atoms remain today.
The four shortest-lived primordial nuclides (i.e., the nuclides
with the shortest half-lives) to have been experimentally verified are 232 Th (1.4×1010 years), 238 U (4.5×109 years), 40 K (1.25×109 years), and 235 U (7.0×108 years).
These are the four nuclides with half-lives comparable to, or somewhat less than, the estimated age of the universe. (232Th
has a half life slightly longer than the age of the universe.) For a
complete list of the 35 known primordial radionuclides, including the
next 30 with half-lives much longer than the age of the universe, see
the complete list below. For practical purposes, nuclides with
half-lives much longer than the age of the universe may be treated as if
they were stable. 232Th and 238U have half-lives long enough that their decay is limited over geological time scales; 40K and 235U have shorter half-lives and are hence severely depleted, but are still long-lived enough to persist significantly in nature.
The longest-lived isotope not proven to be primordial is 146 Sm , which has a half-life of 1.03×108 years, followed by 244 Pu (8.08×107 years) and 92 Nb (3.5×107 years). 244Pu has been reported to exist in nature as a primordial nuclide, although a later study did not detect it. Taking into account that all these nuclides must exist for at least 4.6×109 years, 146Sm must survive 45 half-lives (and hence be reduced by 245 ≈ 4×1013), 244Pu must survive 57 (and be reduced by a factor of 257 ≈ 1×1017), and 92Nb must survive 130 (and be reduced by 2130 ≈ 1×1039). Mathematically, considering the likely initial abundances of these nuclides, primordial 146Sm and 244Pu
should persist somewhere within the Earth today, even if they are not
identifiable in the relatively minor portion of the Earth's crust
available to human assays, while 92Nb and all shorter-lived nuclides should not. Nuclides such as 92Nb that were present in the primordial solar nebula but have long since decayed away completely are termed extinct radionuclides if they have no other means of being regenerated.
Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and three additional primordial elements have only radioactive isotopes (bismuth, thorium, and uranium).
Naturally occurring nuclides that are not primordial
Some unstable isotopes which occur naturally (such as 14 C , 3 H , and 239 Pu ) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as 14 C and 3 H ), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of 237 Np and 239 Pu ). Other examples of common naturally occurring but non-primordial nuclides are isotopes of radon, polonium, and radium, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. The stable argon isotope 40Ar is actually more common as a radiogenic nuclide than as a primordial nuclide, forming almost 1% of the Earth's atmosphere, which is regenerated by the beta decay of the extremely long-lived radioactive primordial isotope 40K,
whose half-life is on the order of a billion years and thus has been
generating argon since early in the Earth's existence. (Primordial argon
was dominated by the alpha process nuclide 36Ar, which is significantly rarer than 40Ar on Earth.)
A similar radiogenic series is derived from the long-lived radioactive primordial nuclide 232Th. These nuclides are described as geogenic, meaning that they are decay or fission products of uranium or other actinides in subsurface rocks.
All such nuclides have shorter half-lives than their parent radioactive
primordial nuclides. Some other geogenic nuclides do not occur in the decay chains of 232Th, 235U, or 238U but can still fleetingly occur naturally as products of the spontaneous fission of one of these three long-lived nuclides, such as 126Sn, which makes up about 10−14 of all natural tin.
A primordial element is a chemical element
with at least one primordial nuclide. There are 251 stable primordial
nuclides and 35 radioactive primordial nuclides, but only 80 primordial
stable elements—hydrogen through lead, atomic numbers 1 to 82, with the exceptions of technetium (43) and promethium (61)—and three radioactive primordial elements—bismuth
(83), thorium (90), and uranium (92). Bismuth's half-life is so long
that it is often classed with the 80 primordial stable elements instead,
since its radioactivity is not a cause for serious concern. The number
of elements is smaller than the number of nuclides, because many of the
primordial elements are represented by multiple isotopes. See chemical element for more information.
Naturally occurring stable nuclides
As noted, these number about 251. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the "stable" 251 nuclides may be in some respect unstable, see list of nuclides and stable nuclide.
These questions do not impact the question of whether a nuclide is
primordial, since all "nearly stable" nuclides, with half-lives longer
than the age of the universe, are also primordial.
Radioactive primordial nuclides
Although it is estimated that about 35 primordial nuclides are radioactive
(list below), it becomes very difficult to determine the exact total
number of radioactive primordials, because the total number of stable
nuclides is uncertain. There exist many extremely long-lived nuclides
whose half-lives are still unknown. For example, it is predicted
theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by alpha emission, but as of 2013 this could only be measured experimentally for 180 W . Similarly, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long (some exceeding 10100
years) that such decays could hardly be observed in the near future.
Nevertheless, the number of nuclides with half-lives so long that they
cannot be measured with present instruments—and are considered from this
viewpoint to be stable nuclides—is limited. Even when a "stable" nuclide is found to be radioactive, it merely moves from the stable to the unstable
list of primordial nuclides, and the total number of primordial
nuclides remains unchanged. For practical purposes, these nuclides may
be considered stable for all purposes outside specialized research.
List of 35 radioactive primordial nuclides and measured half-lives
These
35 primordial nuclides represent radioisotopes of 28 distinct chemical
elements (cadmium, neodymium, osmium, samarium, tellurium, uranium, and
xenon each have two primordial radioisotopes). The radionuclides are
listed in order of stability, with the longest half-life beginning the
list. These radionuclides in many cases are so nearly stable that they
compete for abundance with stable isotopes of their respective elements.
For three chemical elements, indium, tellurium, and rhenium, a very long-lived radioactive primordial nuclide is found in greater abundance than a stable nuclide.
The longest-lived radionuclide, 128Te, has a half-life of 2.2×1024 years, which is 160 trillion times the age of the Universe.
Only four of these 35 nuclides have half-lives shorter than, or equal
to, the age of the universe. Most of the remaining 30 have half-lives
much longer. The shortest-lived primordial isotope, 235U, has a half-life of 703.8 million years, about one sixth of the age of the Earth and the Solar System. Many of these nuclides decay by double beta decay, although some like 209Bi decay by other methods such as alpha decay.
At the end of the list, two more nuclides have been added: 146Sm and 244Pu.
They have not been confirmed as primordial, but their half-lives are
long enough that minute quantities should persist today.
A running positive integer for reference. These
numbers may change slightly in the future since there are 251 nuclides
now classified as stable, but which are theoretically predicted to be
unstable (see Stable nuclide § Still-unobserved decay),
so that future experiments may show that some are in fact unstable. The
number starts at 252, to follow the 251 (observationally) stable
nuclides.
Nuclide
Nuclide identifiers are given by their mass number A and the symbol for the corresponding chemical element (implies a unique proton number).
Energy
Mass of the average nucleon of this nuclide relative to the mass of a neutron (so all nuclides get a positive value) in MeV/c2, formally: mn − mnuclide / A.
An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms).
Because these substitute particles are usually unstable, exotic atoms
typically have very short lifetimes and no exotic atom observed so far
can persist under normal conditions.
Muonic atoms
Muonic helium, made out of 2 protons, 2 neutrons, 1 muon and 1 electron.
In a muonic atom (previously called a mu-mesic atom, now known to be a misnomer as muons are not mesons), an electron is replaced by a muon, which, like the electron, is a lepton. Since leptons are only sensitive to weak, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction.
Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics.
Muonic
hydrogen is like normal hydrogen with the electron replaced by a
negative muon - that is a proton orbited by a muon. It is important in
addressing the proton radius puzzle.
Muonic helium (Hydrogen-4.1)
The symbol 4.1H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (4He-μ), which is like helium-4 in having 2 protons and 2 neutrons. However one of its electrons is replaced by a muon,
which also has charge –1. Because the muon's orbital radius is less
than 1/200th the electron's orbital radius (due to the mass ratio), the
muon can be considered as a part of the nucleus. The atom then has a nucleus
with 2 protons, 2 neutrons and 1 muon, with total nuclear charge +1
(from 2 protons and 1 muon) and only one electron outside, so that it is
effectively an isotope of hydrogen instead of an isotope of helium. A
muon's weight is approximately 0.1 Da
so the isotopic mass is 4.1. Since there is only one electron outside
the nucleus, the hydrogen-4.1 atom can react with other atoms. Its
chemical behavior is that of a hydrogen atom and not a noble helium
atom.
The muon could react with the protons in the nucleus, but in this case
the reaction is not energetically favourable. Therefore, the atom decays
with the muon's half-life, 1.52 microseconds (1.528312(±0.000015)×10−6 seconds).
Other muonic atoms can be formed when negative muons interact with ordinary matter.
Hadronic atoms
A hadronic atom is an atom in which one or more of the orbital electrons are replaced by a negatively charged hadron. Possible hadrons include mesons such as the pion or kaon, yielding a pionic atom or a kaonic atom (see Kaonic hydrogen), collectively called mesonic atoms; antiprotons, yielding an antiprotonic atom; and the Σ− particle, yielding a Σ− or sigmaonic atom.
Unlike leptons, hadrons can interact via the strong force, so the orbitals of hadronic atoms are influenced by nuclear forces between the nucleus
and the hadron. Since the strong force is a short-range interaction,
these effects are strongest if the atomic orbital involved is close to
the nucleus, when the energy levels involved may broaden or disappear
because of the absorption of the hadron by the nucleus. Hadronic atoms, such as pionic hydrogen and kaonic hydrogen, thus provide experimental probes of the theory of strong interactions, quantum chromodynamics.
An onium (plural: onia) is the bound state of a particle and its antiparticle. The classic onium is positronium, which consists of an electron and a positron bound together as a metastable state, with a relatively long lifetime of 142 ns in the triplet state. Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.
Pionium, a bound state of two oppositely-charged pions, is useful for exploring the strong interaction. This should also be true of protonium,
which is a proton–antiproton bound state. Understanding bound states of
pionium and protonium is important in order to clarify notions related
to exotic hadrons such as mesonic molecules and pentaquark states. Kaonium, which is a bound state of two oppositely charged kaons, has not been observed experimentally yet.
The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain mesons, the quarkonium states, which are made of a heavy quark such as the charm or bottom quark and its antiquark. (Top quarks are so heavy that they decay through the weak force before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and lattice QCD are increasingly important tests of quantum chromodynamics.
Muonium, despite its name, is not
an onium containing a muon and an antimuon, because IUPAC assigned that
name to the system of an antimuon bound with an electron. However, the
production of a muon–antimuon bound state, which is an onium (called true muonium), has been theorized.
Crystals of osmium, a heavy metal nearly twice as dense as lead
Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy,
for example, a heavy metal may be defined on the basis of density,
whereas in physics the distinguishing criterion might be atomic number,
while a chemist would likely be more concerned with chemical behaviour.
More specific definitions have been published, none of which have been
widely accepted. The definitions surveyed in this article encompass up
to 96 out of the 118 known chemical elements; only mercury, lead and bismuth
meet all of them. Despite this lack of agreement, the term (plural or
singular) is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
Physical and chemical characterisations of heavy metals need to
be treated with caution, as the metals involved are not always
consistently defined. As well as being relatively dense, heavy metals
tend to be less reactive than lighter metals and have far fewer solublesulfides and hydroxides. While it is relatively easy to distinguish a heavy metal such as tungsten from a lighter metal such as sodium,
a few heavy metals, such as zinc, mercury, and lead, have some of the
characteristics of lighter metals; and lighter metals such as beryllium, scandium, and titanium, have some of the characteristics of heavier metals.
This
table shows the number of heavy metal criteria met by each metal, out
of the ten criteria listed in this section i.e. two based on density, three on atomic weight, two on atomic number, and three on chemical behaviour. It illustrates the lack of agreement surrounding the concept, with the possible exception of mercury, lead and bismuth.
Metals enclosed by a dashed line have (or, for At and Fm–Ts, are predicted to have) densities of more than 5 g/cm3.
There is no widely agreed criterion-based definition of a heavy
metal. Different meanings may be attached to the term, depending on the
context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, and a chemist or biologist would likely be more concerned with chemical behaviour.
Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. Atomic weight definitions can range from greater than sodium (atomic weight 22.98); greater than 40 (excluding s- and f-block metals, hence starting with scandium); or more than 200, i.e. from mercury onwards. Atomic numbers of heavy metals are generally given as greater than 20 (calcium); sometimes this is capped at 92 (uranium). Definitions based on atomic number have been criticised for including metals with low densities. For example, rubidium in group (column) 1 of the periodic table has an atomic number of 37 but a density of only 1.532 g/cm3, which is below the threshold figure used by other authors. The same problem may occur with definitions which are based on atomic weight.
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 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. 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, though it falls marginally short of the
density criterion and is less commonly recognised as a metalloid but has a waterborne chemistry similar in some respects to that of arsenic and antimony. Other metals sometimes classified or treated as "heavy" metals, such as beryllium (density 1.8 g/cm3), aluminium (2.7 g/cm3), calcium (1.55 g/cm3), and barium (3.6 g/cm3) are here treated as light metals and, in general, are not further considered.
Produced mainly by commercial mining (informally classified by economic significance)
Antimony, arsenic, germanium and tellurium are commonly recognised as metalloids; selenium less commonly so.
‡
Astatine is predicted to be a metal.
☢
All isotopes
of these 34 elements are unstable and hence radioactive. While this is
also true of bismuth, it is not so marked since its half-life of 19 billion billion years is over a billion times the 13.8-billion-year estimated age of the universe.
¶
These eight elements do occur naturally but in amounts too small for economically viable extraction.
Origins and use of the term
The heaviness of naturally occurring metals such as gold, copper, and iron may have been noticed in prehistory and, in light of their malleability, led to the first attempts to craft metal ornaments, tools, and weapons.
All metals discovered from then until 1809 had relatively high
densities; their heaviness was regarded as a singularly distinguishing
criterion.
From 1809 onwards, light metals such as sodium, potassium, and strontium were isolated. Their low densities challenged conventional wisdom and it was proposed to refer to them as metalloids (meaning "resembling metals in form or appearance").
This suggestion was ignored; the new elements came to be recognised as
metals, and the term metalloid was then used to refer to nonmetallic
elements and, later, elements that were hard to describe as either
metals or nonmetals.
An early use of the term heavy metal dates from 1817, when the German chemist Leopold Gmelin divided the elements into nonmetals, light metals, and heavy metals. Light metals had densities of 0.860–5.0 g/cm3; heavy metals 5.308–22.000.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."
Biological role
Amount of heavy metals in an average 70 kg human body
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 the more serious toxic effects of heavy
metals, including cancer, brain damage and death, rather than the harm
they may cause to one or more of the skin, lungs, stomach, kidneys,
liver, or heart. For more specific information, see Metal toxicity, Toxic heavy metal, or 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. Inhalation of certain metals, either as
fine dust or most commonly as fumes, can also result in a condition
called metal fume fever.
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, (CH 3CH 2) 4Pb, it was used extensively in gasoline from 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 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 (Ni(CO)4), at 30 parts per million, can cause respiratory failure, brain damage and death. Imbibing a gram or more of copper sulfate (CuSO4) can be fatal; survivors may be left with major organ damage.
More than five milligrams of selenium is highly toxic; this is roughly
ten times the 0.45 milligram recommended maximum daily intake; long-term poisoning can have paralytic effects.
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), 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, 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 of joss paper; open burning of waste in rural areas; contaminated ventilation system; food contaminated by the environment or by the packaging; armaments; lead–acid batteries; electronic waste recycling yard; 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, high levels of lead in drinking water supplied to the residents of Flint, Michigan, in the north-east of the United States and 2015 Hong Kong heavy metal in drinking water incidents.
Heavy metals left of the dividing line occur (or are sourced) mainly as lithophiles; those to the right, as chalcophiles except gold (a siderophile) and tin (a lithophile).
Heavy metals up to the vicinity of iron (in the periodic table) are largely made via stellar nucleosynthesis. In this process, lighter elements from hydrogen to silicon undergo successive fusion reactions inside stars, releasing light and heat and forming heavier elements with higher atomic numbers.
Heavier heavy metals are not usually formed this way since fusion
reactions involving such nuclei would consume rather than release
energy. Rather, they are largely synthesised (from elements with a lower atomic number) by neutron capture, with the two main modes of this repetitive capture being the s-process and the r-process.
In the s-process ("s" stands for "slow"), singular captures are
separated by years or decades, allowing the less stable nuclei to beta decay,
while in the r-process ("rapid"), captures happen faster than nuclei
can decay. Therefore, the s-process takes a more or less clear path: for
example, stable cadmium-110 nuclei are successively bombarded by free
neutrons inside a star until they form cadmium-115 nuclei which are
unstable and decay to form indium-115 (which is nearly stable, with a
half-life 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.
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 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. 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.
In contrast, 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.
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 + 4 KCN → 2 K[Au(CN)2] + 2 KOH. Zinc is added to the mix and, being more reactive than gold, displaces the gold: 2 K[Au(CN)2] + Zn → K2[Zn(CN)4] + 2 Au. The gold precipitates out of solution as a sludge, and is filtered off and melted.
Properties compared with light metals
Some
general physical and chemical properties of light and heavy metals are
summarised in the table. The comparison should be treated with caution
since the terms light metal and heavy metal are not always
consistently defined. Moreover, 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
In a cello (example shown above) or a viola the C-string sometimes incorporates tungsten; its high density permits a smaller diameter string and improves responsiveness.
Some uses of heavy metals, including in sport, mechanical engineering, military ordnance, and nuclear science, take advantage of their relatively high densities. In underwater diving, lead is used as a ballast; in handicap horse racing
each horse must carry a specified lead weight, based on factors
including past performance, so as to equalize the chances of the various
competitors. In golf, tungsten, brass, or copper inserts in fairwayclubs and irons lower the centre of gravity of the club making it easier to get the ball into the air; and golf balls with tungsten cores are claimed to have better flight characteristics. In fly fishing, sinking fly lines have a PVC coating embedded with tungsten powder, so that they sink at the required rate. In track and field sport, steel balls used in the hammer throw and shot put events are filled with lead in order to attain the minimum weight required under international rules.
Tungsten was used in hammer throw balls at least up to 1980; the
minimum size of the ball was increased in 1981 to eliminate the need for
what was, at that time, an expensive metal (triple the cost of other
hammers) not generally available in all countries. Tungsten hammers were so dense that they penetrated too deeply into the turf.
The higher the projectile density, the more effectively it can penetrate heavy armor plate ... Os, Ir, Pt, and Re ... are expensive ... U offers an appealing combination of high density, reasonable cost and high fracture toughness.
AM Russell and KL Lee Structure–property relations in nonferrous metals (2005, p. 16)
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.
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.
The workability and corrosion resistance of iron and chromium are increased by adding gadolinium; the creep resistance of nickel is improved with the addition of thorium. Tellurium is added to copper (tellurium copper) and steel alloys to improve their machinability; and to lead to make it harder and more acid-resistant.
The biocidal effects of some heavy metals have been known since antiquity.
Platinum, osmium, copper, ruthenium, and other heavy metals, including
arsenic, are used in anti-cancer treatments, or have shown potential. Antimony (anti-protozoal), bismuth (anti-ulcer), gold (anti-arthritic), and iron (anti-malarial) are also important in medicine. Copper, zinc, silver, gold, or mercury are used in antiseptic formulations; small amounts of some heavy metals are used to control algal growth in, for example, cooling towers. Depending on their intended use as fertilisers or biocides, agrochemicals may contain heavy metals such as chromium, cobalt, nickel, copper, zinc, arsenic, cadmium, mercury, or lead.
Neodymium sulfate (Nd2(SO4)3), used to colour glassware
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.
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).
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).
_oxide.jpg)
sulfat.JPG)
.jpg)
