ciated with anti-Stokes radiation are
called anti-Stokes lines. The quantum
theory of the Raman effect provides
an explanation for the intensity of
the anti-Stokes line being much less
than the intensity of the Stokes line.
ap Antiperiplanar. See torsion
angle.
apatite A complex mineral form of
*calcium phosphate, Ca 5 (PO 4 ) 3 -
(OH,F,Cl); the commonest of the
phosphate minerals. It has a hexagonal structure and occurs widely as an
accessory mineral in igneous rocks
(e.g. pegmatite) and often in regional
and contact metamorphic rocks, especially limestone. Large deposits
occur in the Kola Peninsula, Russia. It
is used in the production of fertilizers and is a major source of phosphorus. The enamel of teeth is composed
chieÛy of apatite.
apical Designating certain bonds
and positions in a bipyramidal or
pyramidal structure. For example, in
a trigonal bipyramid the two bonds
aligned through the central atom are
the apical bonds and the groups attached to these bonds are in apical
positions. The three other bonds and
positions are equatorial. Similarly, in
a square pyramidal structure the
bond perpendicular to the base of
the pyramid is the apical bond. The
four other coplanar bonds (and positions) are said to be basal.
aprotic See solvent.
aqua acid A type of acid in which
the acidic hydrogen is on a water
molecule coordinated to a metal ion.
For example,
Al(OH 2 ) 6
3+ + H 2 O →
Al(OH 2 ) 5 (OH)
2+ + H 3 O
+
aqua regia A mixture of concentrated nitric acid and concentrated
hydrochloric acid in the ratio 1:3 respectively. It is a very powerful oxidizing mixture and will dissolve all
metals (except silver, which forms an
insoluble chloride) including such
noble metals as gold and platinum,
hence its name (‘royal water’). Nitrosyl chloride (NOCl) is believed to be
one of the active constituents.
aquation The process in which
water molecules solvate or form coordination complexes with ions.
aqueous Describing a solution in
water.
aquo ion A hydrated positive ion
present in a crystal or in solution.
arachidonic acid An unsaturated
fatty acid, CH 3 (CH 2 ) 3 (CH 2 CH:CH) 4 -
(CH 2 ) 3 COOH, that is essential for
growth in mammals. It can be synthesized from *linoleic acid. Arachidonic acid acts as a precursor to
several biologically active compounds, including prostaglandins,
and plays an important role in membrane production and fat metabolism. The release of arachidonic acid
from membrane phospholipids is
triggered by certain hormones.
arachno structure See borane.
aragonite A rock-forming anhydrous mineral form of calcium carbonate, CaCO 3 . It is much less stable
than *calcite, the commoner form of
calcium carbonate, from which it
may be distinguished by its greater
hardness and speciÜc gravity. Over
time aragonite undergoes recrystallization to calcite. Aragonite occurs
in cavities in limestone, as a deposit
in limestone caverns, as a precipitate
around hot springs and geysers, and
in high-pressure low-temperature
metamorphic rocks; it is also found
in the shells of a number of molluscs
and corals and is the main constituent of pearls. It is white or
colourless when pure but the presap
42
a
www.AzShimi.ir www.AzShimi.com
called anti-Stokes lines. The quantum
theory of the Raman effect provides
an explanation for the intensity of
the anti-Stokes line being much less
than the intensity of the Stokes line.
ap Antiperiplanar. See torsion
angle.
apatite A complex mineral form of
*calcium phosphate, Ca 5 (PO 4 ) 3 -
(OH,F,Cl); the commonest of the
phosphate minerals. It has a hexagonal structure and occurs widely as an
accessory mineral in igneous rocks
(e.g. pegmatite) and often in regional
and contact metamorphic rocks, especially limestone. Large deposits
occur in the Kola Peninsula, Russia. It
is used in the production of fertilizers and is a major source of phosphorus. The enamel of teeth is composed
chieÛy of apatite.
apical Designating certain bonds
and positions in a bipyramidal or
pyramidal structure. For example, in
a trigonal bipyramid the two bonds
aligned through the central atom are
the apical bonds and the groups attached to these bonds are in apical
positions. The three other bonds and
positions are equatorial. Similarly, in
a square pyramidal structure the
bond perpendicular to the base of
the pyramid is the apical bond. The
four other coplanar bonds (and positions) are said to be basal.
aprotic See solvent.
aqua acid A type of acid in which
the acidic hydrogen is on a water
molecule coordinated to a metal ion.
For example,
Al(OH 2 ) 6
3+ + H 2 O →
Al(OH 2 ) 5 (OH)
2+ + H 3 O
+
aqua regia A mixture of concentrated nitric acid and concentrated
hydrochloric acid in the ratio 1:3 respectively. It is a very powerful oxidizing mixture and will dissolve all
metals (except silver, which forms an
insoluble chloride) including such
noble metals as gold and platinum,
hence its name (‘royal water’). Nitrosyl chloride (NOCl) is believed to be
one of the active constituents.
aquation The process in which
water molecules solvate or form coordination complexes with ions.
aqueous Describing a solution in
water.
aquo ion A hydrated positive ion
present in a crystal or in solution.
arachidonic acid An unsaturated
fatty acid, CH 3 (CH 2 ) 3 (CH 2 CH:CH) 4 -
(CH 2 ) 3 COOH, that is essential for
growth in mammals. It can be synthesized from *linoleic acid. Arachidonic acid acts as a precursor to
several biologically active compounds, including prostaglandins,
and plays an important role in membrane production and fat metabolism. The release of arachidonic acid
from membrane phospholipids is
triggered by certain hormones.
arachno structure See borane.
aragonite A rock-forming anhydrous mineral form of calcium carbonate, CaCO 3 . It is much less stable
than *calcite, the commoner form of
calcium carbonate, from which it
may be distinguished by its greater
hardness and speciÜc gravity. Over
time aragonite undergoes recrystallization to calcite. Aragonite occurs
in cavities in limestone, as a deposit
in limestone caverns, as a precipitate
around hot springs and geysers, and
in high-pressure low-temperature
metamorphic rocks; it is also found
in the shells of a number of molluscs
and corals and is the main constituent of pearls. It is white or
colourless when pure but the presap
42
a
www.AzShimi.ir www.AzShimi.com