CaWO 4 . The ore is heated with concentrated sodium hydroxide solution
to form a soluble tungstate. The
oxide WO 3 is precipitated from this
by adding acid, and is reduced to the
metal using hydrogen. It is used in
various alloys, especially high-speed
steels (for cutting tools) and in lamp
Ülaments. Tungsten forms a protective oxide in air and can be oxidized
at high temperature. It does not dissolve in dilute acids. It forms compounds in which the oxidation state
ranges from +2 to +6. The metal was
Ürst isolated by Juan d’Elhuyer and
Fausto d’Elhuyer (1755–1833) in
1783.
A
• Information from the WebElements site
tungsten carbide A black powder,
WC, made by heating powdered
tungsten metal with lamp black at
1600°C. It is extremely hard (9.5 on
Mohs’ scale) and is used in dies and
cutting tools. A ditungsten carbide,
W 2 C, also exists.
tunnel effect An effect in which
electrons are able to tunnel through
a narrow potential barrier that would
constitute a forbidden region if the
electrons were treated as classical
particles. That there is a Ünite probability of an electron tunnelling from
one classically allowed region to another arises as a consequence of
*quantum mechanics. The effect is
made use of in the tunnel diode.
turpentine An oily liquid extracted
from pine resin. It contains pinene,
C 10 H 16 , and other terpenes and is
mainly used as a solvent.
turquoise A mineral consisting of a
hydrated phosphate of aluminium
and copper, CuAl 6 (PO 4 ) 4 (OH) 8 .4H 2 O),
that is prized as a semiprecious
stone. It crystallizes in the triclinic
system and is generally blue in colour, the ‘robin’s egg’ blue variety
being the most sought after. It usually occurs in veinlets and as masses
and is formed by the action of surface waters on aluminium-rich rocks.
The Ünest specimens are obtained
from Iran.
twinning The growth of crystals in
such a way that two distinct crystals
form sharing a common plane of
atoms.
twist See ring conformations.
Tyndall effect The scattering of
light as it passes through a medium
containing small particles. If a polychromatic beam of light is passed
through a medium containing particles with diameters less than about
one-twentieth of the wavlength of
light, the scattered light appears
blue. This accounts for the blue appearance of tobacco smoke. At
higher particle diameters, the scattered light remains polychromatic.
The effect is seen in suspensions and
certain colloids. It is named after
John Tyndall (1820–93).
type A and B metals A classiÜcation of metal ions according to the
stability of their complexes for a
given ligand. Type A metals cations
include the ions of group 1 (Li
+ to
Cs
+ ), the ions of group 2 (Be
2+ to
Ba
2+ ), and ions of lighter transition
metals in high oxidation states (e.g.
Co
3+
, Ti
4+ , Fe
3+ ). The type B metal
cations are those of heavier transition metals in lower oxidation states
(e.g. Ag
+ , Cu
+ , Ni
2+ , Pd
2+ , Pt
2+
). Certain
ligands tend to form more stable
complexes with type A metals; others
form more stable complexes with
type B. For example, the tendency of
halide anions to complex with type A
metals is in the sequence
F
–
> Cl
–
> Br
–
> I
– .
Their tendency to complex with type
B metals is the opposite sequence.
This led to a classiÜcation of ligands
tungsten carbide
538
t
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to form a soluble tungstate. The
oxide WO 3 is precipitated from this
by adding acid, and is reduced to the
metal using hydrogen. It is used in
various alloys, especially high-speed
steels (for cutting tools) and in lamp
Ülaments. Tungsten forms a protective oxide in air and can be oxidized
at high temperature. It does not dissolve in dilute acids. It forms compounds in which the oxidation state
ranges from +2 to +6. The metal was
Ürst isolated by Juan d’Elhuyer and
Fausto d’Elhuyer (1755–1833) in
1783.
A
• Information from the WebElements site
tungsten carbide A black powder,
WC, made by heating powdered
tungsten metal with lamp black at
1600°C. It is extremely hard (9.5 on
Mohs’ scale) and is used in dies and
cutting tools. A ditungsten carbide,
W 2 C, also exists.
tunnel effect An effect in which
electrons are able to tunnel through
a narrow potential barrier that would
constitute a forbidden region if the
electrons were treated as classical
particles. That there is a Ünite probability of an electron tunnelling from
one classically allowed region to another arises as a consequence of
*quantum mechanics. The effect is
made use of in the tunnel diode.
turpentine An oily liquid extracted
from pine resin. It contains pinene,
C 10 H 16 , and other terpenes and is
mainly used as a solvent.
turquoise A mineral consisting of a
hydrated phosphate of aluminium
and copper, CuAl 6 (PO 4 ) 4 (OH) 8 .4H 2 O),
that is prized as a semiprecious
stone. It crystallizes in the triclinic
system and is generally blue in colour, the ‘robin’s egg’ blue variety
being the most sought after. It usually occurs in veinlets and as masses
and is formed by the action of surface waters on aluminium-rich rocks.
The Ünest specimens are obtained
from Iran.
twinning The growth of crystals in
such a way that two distinct crystals
form sharing a common plane of
atoms.
twist See ring conformations.
Tyndall effect The scattering of
light as it passes through a medium
containing small particles. If a polychromatic beam of light is passed
through a medium containing particles with diameters less than about
one-twentieth of the wavlength of
light, the scattered light appears
blue. This accounts for the blue appearance of tobacco smoke. At
higher particle diameters, the scattered light remains polychromatic.
The effect is seen in suspensions and
certain colloids. It is named after
John Tyndall (1820–93).
type A and B metals A classiÜcation of metal ions according to the
stability of their complexes for a
given ligand. Type A metals cations
include the ions of group 1 (Li
+ to
Cs
+ ), the ions of group 2 (Be
2+ to
Ba
2+ ), and ions of lighter transition
metals in high oxidation states (e.g.
Co
3+
, Ti
4+ , Fe
3+ ). The type B metal
cations are those of heavier transition metals in lower oxidation states
(e.g. Ag
+ , Cu
+ , Ni
2+ , Pd
2+ , Pt
2+
). Certain
ligands tend to form more stable
complexes with type A metals; others
form more stable complexes with
type B. For example, the tendency of
halide anions to complex with type A
metals is in the sequence
F
–
> Cl
–
> Br
–
> I
– .
Their tendency to complex with type
B metals is the opposite sequence.
This led to a classiÜcation of ligands
tungsten carbide
538
t
www.AzShimi.ir www.AzShimi.com