Everything about Allotropy totally explained
Allotropy is the property of some
chemical elements to be able to take two or more different forms, where the
atoms are arranged differently by
chemical bonds. The forms are known as
allotropes of that element. The phenomenon of allotropy is sometimes also called allotropism. For example,
carbon has two common allotropes:
diamond, where the carbon atoms are bonded together in a
tetrahedral lattice arrangement, and
graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice.
The word
allotropy comes from the
Greek allos, meaning "other", and
tropos, "manner".
Allotropy refers only to different forms of an element within the same phase or
state of matter (for example different
solid,
liquid or
gas forms). Changes of state (between solid, liquid and gas) are not considered allotropy. Some elements have allotropes that persist in different phases - for example, the two allotropes of
oxygen (
dioxygen, O
2, and
ozone, O
3), can both exist in the solid, liquid and gaseous states. Other elements maintain distinct allotropes only in some phases - for example
phosphorus has many solid allotropes, which all revert to the same P
4 form when melted to the liquid state.
History
The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron
Jons Jakob Berzelius (1779-1848) who offered no explanation. After the acceptance of
Avogadro's hypothesis in 1860 it was understood that elements could exist as polyatomic molecules, and the two allotropes of oxygen were recognized as O
2 and O
3. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure.
By 1912,
Ostwald noted that the allotropy of elements is just a special case of the phenomenon of
polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism. Although many other chemists have repeated this advice,
IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.
Differences in properties of an element's allotropes
Allotropes are different structural forms of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is triggered by the same forces that affect other structures, for example
pressure,
light, and
temperature. Therefore the stability of the particular allotropes depends on particular conditions. For instance,
iron changes from a
body-centered cubic structure
(ferrite) to a
face-centered cubic structure (
austenite) above 906 °C, and
tin undergoes a transformation known as
tin pest from a
metallic phase to a
semiconductor phase below 13.2 °C.
List of allotropes
Typically, elements capable of variable
coordination number and/or
oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to
catenate. Allotropes are typically more noticeable in
non-metals and
metalloids. Nevertheless,
metals tend to have many allotropes.
Examples of allotropes include:
Non-metals
Carbon:
- diamond - an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.
- graphite - a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices, which are then layered in sheets.
- amorphous carbon
- fullerenes, including "buckyballs", such as C60, and carbon nanotubes
Phosphorus:
White phosphorus - crystalline solid
Red phosphorus - polymeric solid
Scarlet phosphorus
Violet phosphorus
Black phosphorus - semiconductor, analogous to graphite
Diphosphorus
Oxygen:
dioxygen, O2 - colorless
ozone, O3 - blue
tetraoxygen, O4 - metastable
octaoxygen, - red
Nitrogen:
dinitrogen
tetranitrogen
trinitrogen
two solid forms: one hexagonal close-packed and the other alpha cubic
Sulfur:
Plastic (amorphous) sulfur - polymeric solid
Rhombic sulfur - large crystals composed of S8 molecules
Monoclinic sulfur - fine needle-like crystals
Other ring molecules such as S7 and S12
Selenium:
"Red selenium," cyclo-Se8
Gray selenium, polymeric Se
Black selenium
Metalloids
Boron
amorphous boron - brown powder
crystalline boron - black, hard (9.3 on Mohs' scale), and a weak conductor at room temperature.
Silicon
amorphous silicon - brown powder
nanocrystalline silicon - similar to the amorphous silicon
crystalline silicon - has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown with a process known as the Czochralski process
Arsenic:
Yellow arsenic - molecular non-metallic As4
Gray arsenic, polymeric As (metalloid)
Black arsenic (metalloid) and several similar other ones.
Antimony:
blue-white antimony - the stable form (metalloid)
yellow antimony (non-metallic)
black antimony (non-metallic)
(a fourth one too)
Polonium has two metallic allotropes.
Metals
Tin
grey tin (alpha-tin)
white tin (beta tin)
rhombic tin (gamma)
Iron
ferrite (alpha iron) - forms below 770°C (the Curie point, Tc ); the iron becomes magnetic in its alpha form; BCC
beta - forms below 912°C (BCC)
gamma - forms below 1401°C; face centred cubic (FCC) crystal structure
delta - forms from cooling down molten iron below 1535°C; has a body-centred cubic (BCC) crystal structure
Titanium has two allotropes
Strontium has three allotropes
Lantanides and actinides
Plutonium has six distinct solid allotropes under "normal" pressures. Their densities vary within a ratio of some 4:3, which vastly complicates all kinds of work with the metal (particularly casting, machining, and storage). A seventh plutonium allotrope exists at very high pressures, which adds further difficulties in exotic applications.
Ytterbium has three allotropes
Terbium has two crystalline allotropes
Promethium has two allotropic forms
Curium has 3 allotropes (also Americium, Berkelium, Californium do) Further Information
Get more info on 'Allotropy'.
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