Member of a loosely defined subset of elements that exhibit metallic properties
Top 10 Heavy metals related articles
- 1 Definitions
- 2 Origins and use of the term
- 3 Biological role
- 4 Toxicity
- 5 Formation, abundance, occurrence, and extraction
- 6 Properties compared with light metals
- 7 Uses
- 8 Notes
- 9 Sources
- 10 Further reading
- 11 External links
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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 onward, 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 waste, 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 far fewer 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.
Heavy metals Intro articles: 42
|Heat map of heavy metals in the periodic table|
|This table shows the number of heavy metal criteria met by each metal, out of the ten criteria listed in this section i.e. two based on density, three on atomic weight, two on atomic number, and three on chemical behaviour.[n 1] It illustrates the lack of agreement surrounding the concept, with the possible exception of mercury, lead and bismuth.|
Six elements near the end of periods (rows) 4 to 7 sometimes considered metalloids are treated here as metals: they are germanium (Ge), arsenic (As), selenium (Se), antimony (Sb), tellurium (Te), and astatine (At).[n 2] Oganesson (Og) is treated as a nonmetal.
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 atomic weight based definitions.
The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides."[n 3] In 1997, Stephen Hawkes, a chemistry professor writing in the context of fifty years' experience with the term, said it applied to "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water and whose complexes are usually colored". On the basis of the metals he had seen referred to as heavy metals, he suggested it would 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.[n 4] The lanthanides satisfy Hawkes' three-part description; the status of the actinides is not completely settled.[n 5][n 6]
In biochemistry, heavy metals are sometimes defined—on the basis of the Lewis acid (electronic pair acceptor) behaviour of their ions in aqueous solution—as class B and borderline metals. In this scheme, class A metal ions prefer oxygen donors; class B ions prefer nitrogen or sulfur donors; and borderline or ambivalent ions show either class A or B characteristics, depending on the circumstances.[n 7] Class A metals, which tend to have low electronegativity and form bonds with large ionic character, are the alkali and alkaline earths, aluminium, the group 3 metals, and the lanthanides and actinides.[n 8] Class B metals, which tend to have higher electronegativity and form bonds with considerable covalent character, are mainly the heavier transition and post-transition metals. Borderline metals largely comprise the lighter transition and post-transition metals (plus arsenic and antimony). The distinction between the class A metals and the other two categories is sharp. A frequently cited proposal[n 9] 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.
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|Produced mainly by artificial transmutation (informally classified by stability)|