acid–base *indicators that changes
colour (e.g. red-yellow-orange-greenblue) over a range of pH.
unnil- See transactinide elements.
unsaturated 1. (of a compound)
Having double or triple bonds in its
molecules. Unsaturated compounds
can undergo addition reactions as well
as substitution. Compare saturated.
2. (of a solution) See saturated.
unstable equilibrium See equilibrium.
UPS Ultraviolet photoelectron spectroscopy. See photoelectron spectroscopy.
UPVC Unplasticized PVC: a tough
hardwearing form of PVC used for
window frames and similar applications.
uracil A *pyrimidine derivative and
one of the major component bases of
*nucleotides and the nucleic acid
*RNA.
unnil542
u
N
H
NH
O
O
Uracil
uraninite A mineral form of uranium(IV) oxide, containing minute
amounts of radium, thorium, polonium, lead, and helium. When uraninite occurs in a massive form with a
pitchy lustre it is known as pitchblende, the chief ore of uranium.
Uraninite occurs in Saxony (Germany), Romania, Norway, the UK
(Cornwall), E Africa (Democratic Republic of Congo), USA, and Canada
(Great Bear Lake).
uranium Symbol U. A white radioactive metallic element belonging to
the *actinoids; a.n. 92; r.a.m. 238.03;
r.d. 19.05 (20°C); m.p. 1132±1°C; b.p.
3818°C. It occurs as *uraninite, from
which the metal is extracted by an
ion-exchange process. Three isotopes
are found in nature: uranium–238
(99.28%), uranium–235 (0.71%), and
uranium–234 (0.006%). As uranium–235 undergoes nuclear Üssion
with slow neutrons it is the fuel used
in nuclear reactors and nuclear
weapons; uranium has therefore assumed enormous technical and political importance since their invention.
It was discovered by Martin Klaproth
(1747–1817) in 1789.
A
• Information from the WebElements site
uranium(VI) Ûuoride (uranium
hexaÛuoride) A volatile white solid,
UF 6 ; r.d. 4.68; m.p. 64.5°C. It is used
in the separation of uranium isotopes
by gas diffusion.
uranium hexaÛuoride See uranium(vi) fluoride.
uranium–lead dating A group of
methods of dating certain rocks that
depends on the decay of the radioisotope uranium–238 to lead–206 (halflife 4.5 × 10
9 years) or the decay of
uranium–235 to lead–207 (half-life
7.1 × 10
8 years). One form of uranium–lead dating depends on measuring the ratio of the amount of
helium trapped in the rock to the
amount of uranium present (since
the decay
238 U →
206 Pb releases eight
alpha-particles). Another method of
calculating the age of the rocks is to
measure the ratio of radiogenic lead
(
206 Pb,
207 Pb, and
208 Pb) present to
nonradiogenic lead (
204 Pb). These
methods give reliable results for ages
of the order 10
7 –10
9 years.
uranium(IV) oxide A black solid,
UO 2 ; r.d. 10.9; m.p. 3000°C. It occurs
naturally as *uraninite and is used in
nuclear reactors.
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colour (e.g. red-yellow-orange-greenblue) over a range of pH.
unnil- See transactinide elements.
unsaturated 1. (of a compound)
Having double or triple bonds in its
molecules. Unsaturated compounds
can undergo addition reactions as well
as substitution. Compare saturated.
2. (of a solution) See saturated.
unstable equilibrium See equilibrium.
UPS Ultraviolet photoelectron spectroscopy. See photoelectron spectroscopy.
UPVC Unplasticized PVC: a tough
hardwearing form of PVC used for
window frames and similar applications.
uracil A *pyrimidine derivative and
one of the major component bases of
*nucleotides and the nucleic acid
*RNA.
unnil542
u
N
H
NH
O
O
Uracil
uraninite A mineral form of uranium(IV) oxide, containing minute
amounts of radium, thorium, polonium, lead, and helium. When uraninite occurs in a massive form with a
pitchy lustre it is known as pitchblende, the chief ore of uranium.
Uraninite occurs in Saxony (Germany), Romania, Norway, the UK
(Cornwall), E Africa (Democratic Republic of Congo), USA, and Canada
(Great Bear Lake).
uranium Symbol U. A white radioactive metallic element belonging to
the *actinoids; a.n. 92; r.a.m. 238.03;
r.d. 19.05 (20°C); m.p. 1132±1°C; b.p.
3818°C. It occurs as *uraninite, from
which the metal is extracted by an
ion-exchange process. Three isotopes
are found in nature: uranium–238
(99.28%), uranium–235 (0.71%), and
uranium–234 (0.006%). As uranium–235 undergoes nuclear Üssion
with slow neutrons it is the fuel used
in nuclear reactors and nuclear
weapons; uranium has therefore assumed enormous technical and political importance since their invention.
It was discovered by Martin Klaproth
(1747–1817) in 1789.
A
• Information from the WebElements site
uranium(VI) Ûuoride (uranium
hexaÛuoride) A volatile white solid,
UF 6 ; r.d. 4.68; m.p. 64.5°C. It is used
in the separation of uranium isotopes
by gas diffusion.
uranium hexaÛuoride See uranium(vi) fluoride.
uranium–lead dating A group of
methods of dating certain rocks that
depends on the decay of the radioisotope uranium–238 to lead–206 (halflife 4.5 × 10
9 years) or the decay of
uranium–235 to lead–207 (half-life
7.1 × 10
8 years). One form of uranium–lead dating depends on measuring the ratio of the amount of
helium trapped in the rock to the
amount of uranium present (since
the decay
238 U →
206 Pb releases eight
alpha-particles). Another method of
calculating the age of the rocks is to
measure the ratio of radiogenic lead
(
206 Pb,
207 Pb, and
208 Pb) present to
nonradiogenic lead (
204 Pb). These
methods give reliable results for ages
of the order 10
7 –10
9 years.
uranium(IV) oxide A black solid,
UO 2 ; r.d. 10.9; m.p. 3000°C. It occurs
naturally as *uraninite and is used in
nuclear reactors.
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