cous it becomes. This property is
used in nondrip paints (which are
more viscous on the brush than on
the wall) and in lubricating oils
(which become thinner when the
parts they are lubricating start to
move). Another example is the nonNewtonian Ûow of macromolecules
in solution or in polymer melts. In
this case the shearing force F is not
parallel to the shear planes and the
linear relationship does not apply.
In general, the many types of nonNewtonian Ûuid are somewhat complicated and no theory has been
developed to accommodate them
fully.
NHOMO Next-to-highest occupied
molecular orbital. See subjacent orbitals.
niacin See nicotinic acid.
Nichrome Tradename for a group
of nickel–chromium alloys used for
wire in heating elements as they possess good resistance to oxidation and
have a high resistivity. Typical is
Nichrome V containing 80% nickel
and 19.5% chromium, the balance
consisting of manganese, silicon, and
carbon.
nickel Symbol Ni. A malleable ductile silvery metallic *transition element; a.n. 28; r.a.m. 58.70; r.d. 8.9;
m.p. 1450°C; b.p. 2732°C. It is found
in the minerals pentlandite (NiS),
pyrrhoite ((Fe,Ni)S), and garnierite
((Ni,Mg) 6 (OH) 6 Si 4 O 11 .H 2 O). Nickel is
also present in certain iron meteorites (up to 20%). The metal is
extracted by roasting the ore to give
the oxide, followed by reduction
with carbon monoxide and puriÜcation by the *Mond process. Alternatively electolysis is used. Nickel metal
is used in special steels, in Invar, and,
being ferromagnetic, in magnetic alloys, such as *Mumetal. It is also an
effective catalyst, particularly for hydrogenation reactions (see also raney
nickel). The main compounds are
formed with nickel in the +2 oxidation state; the +3 state also exists (e.g.
the black oxide, Ni 2 O 3 ). Nickel was
discovered by Axel Cronstedt
(1722–65) in 1751.
A
• Information from the WebElements site
nickel arsenide structure A type
of ionic crystal structure in which
the anions have a distorted hexagonal close packed arrangement with
the cations occupying the octahedral
holes. Each type of ion has a coordination number of 6. Examples of
compounds with this structure are
NiAs, NiS, FeS, and CoS.
A
• An interactive version of the structure
nickel–cadmium cell See
nickel–iron accumulator.
nickel carbonyl A colourless
volatile liquid, Ni(CO) 4 ; m.p. –25°C;
b.p. 43°C. It is formed by direct combination of nickel metal with carbon
monoxide at 50–60°C. The reaction is
reversed at higher temperatures, and
the reactions are the basis of the
*Mond process for purifying nickel.
The nickel in the compound has an
oxidation state of zero, and the compound is a typical example of a complex with pi-bonding *ligands, in
which Ülled d-orbitals on the nickel
overlap with empty p-orbitals on the
carbon.
nickelic compounds Compounds
of nickel in its +3 oxidation state; e.g.
nickelic oxide is nickel(III) oxide
(Ni 2 O 3 ).
nickel–iron accumulator (Edison
cell; NIFE cell) A *secondary cell devised by Thomas Edison (1847–1931)
having a positive plate of nickel
oxide and a negative plate of iron
both immersed in an electrolyte of
NHOMO
370
n
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used in nondrip paints (which are
more viscous on the brush than on
the wall) and in lubricating oils
(which become thinner when the
parts they are lubricating start to
move). Another example is the nonNewtonian Ûow of macromolecules
in solution or in polymer melts. In
this case the shearing force F is not
parallel to the shear planes and the
linear relationship does not apply.
In general, the many types of nonNewtonian Ûuid are somewhat complicated and no theory has been
developed to accommodate them
fully.
NHOMO Next-to-highest occupied
molecular orbital. See subjacent orbitals.
niacin See nicotinic acid.
Nichrome Tradename for a group
of nickel–chromium alloys used for
wire in heating elements as they possess good resistance to oxidation and
have a high resistivity. Typical is
Nichrome V containing 80% nickel
and 19.5% chromium, the balance
consisting of manganese, silicon, and
carbon.
nickel Symbol Ni. A malleable ductile silvery metallic *transition element; a.n. 28; r.a.m. 58.70; r.d. 8.9;
m.p. 1450°C; b.p. 2732°C. It is found
in the minerals pentlandite (NiS),
pyrrhoite ((Fe,Ni)S), and garnierite
((Ni,Mg) 6 (OH) 6 Si 4 O 11 .H 2 O). Nickel is
also present in certain iron meteorites (up to 20%). The metal is
extracted by roasting the ore to give
the oxide, followed by reduction
with carbon monoxide and puriÜcation by the *Mond process. Alternatively electolysis is used. Nickel metal
is used in special steels, in Invar, and,
being ferromagnetic, in magnetic alloys, such as *Mumetal. It is also an
effective catalyst, particularly for hydrogenation reactions (see also raney
nickel). The main compounds are
formed with nickel in the +2 oxidation state; the +3 state also exists (e.g.
the black oxide, Ni 2 O 3 ). Nickel was
discovered by Axel Cronstedt
(1722–65) in 1751.
A
• Information from the WebElements site
nickel arsenide structure A type
of ionic crystal structure in which
the anions have a distorted hexagonal close packed arrangement with
the cations occupying the octahedral
holes. Each type of ion has a coordination number of 6. Examples of
compounds with this structure are
NiAs, NiS, FeS, and CoS.
A
• An interactive version of the structure
nickel–cadmium cell See
nickel–iron accumulator.
nickel carbonyl A colourless
volatile liquid, Ni(CO) 4 ; m.p. –25°C;
b.p. 43°C. It is formed by direct combination of nickel metal with carbon
monoxide at 50–60°C. The reaction is
reversed at higher temperatures, and
the reactions are the basis of the
*Mond process for purifying nickel.
The nickel in the compound has an
oxidation state of zero, and the compound is a typical example of a complex with pi-bonding *ligands, in
which Ülled d-orbitals on the nickel
overlap with empty p-orbitals on the
carbon.
nickelic compounds Compounds
of nickel in its +3 oxidation state; e.g.
nickelic oxide is nickel(III) oxide
(Ni 2 O 3 ).
nickel–iron accumulator (Edison
cell; NIFE cell) A *secondary cell devised by Thomas Edison (1847–1931)
having a positive plate of nickel
oxide and a negative plate of iron
both immersed in an electrolyte of
NHOMO
370
n
www.AzShimi.ir www.AzShimi.com