white metallic element belonging to
*group 1 (formerly IA) of the periodic
table; a.n. 37; r.a.m. 85.47; r.d. 1.53;
m.p. 38.89°C; b.p. 688°C. It is found
in a number of minerals (e.g. lepido-
lite) and in certain brines. The metal
is obtained by electrolysis of molten
rubidium chloride. The naturally oc-
curring isotope rubidium–87 is radio-
active (see rubidium–strontium
dating). The metal is highly reactive,
with properties similar to those of
other group 1 elements, igniting
spontaneously in air. It was discov-
ered spectroscopically by Robert Bun-
sen and Gustav Kirchhoff in 1861.
A
- Information from the WebElements site
rubidium–strontium dating A
method of dating geological speci-
mens based on the decay of the ra-
dioisotope rubidium–87 into the
stable isotope strontium–87. Natural
rubidium contains 27.85% of rubid-
ium–87, which has a half-life of 4.7 ×
1010 years. The ratio^87 Rb/^87 Sr in a
specimen gives an estimate of its age
(up to several thousand million
years).
ruby The transparent red variety of
the mineral *corundum, the colour
being due to the presence of traces of
chromium. It is a valuable gemstone,
more precious than diamonds. The
Ünest rubies are obtained from
Mogok in Burma, where they occur
in metamorphic limestones; Sri
Lanka and Thailand are the only
other important sources. Rubies have
been produced synthetically by the
VerneuilÛame-fusion process. Indus-
trial rubies are used in lasers,
watches, and other precision instru-
ments.
Russell–Saunders coupling (L–S
coupling)A type of coupling in sys-
tems involving many *fermions.
These systems include electrons in
atoms and nucleons in nuclei, in
which the energies associated with
electrostatic repulsion are much
greater than the energies associated
with *spin–orbit coupling. *Multi-
plets of many-electron atoms with a
low atomic number are characterized
by Russell–Saunders coupling. It is
named after the US physicists Henry
Norris Russell (1877–1957) and Fred-
erick Saunders (1875–1963) who pos-
tulated this type of coupling to
explain the spectra of many-electron
atoms with low atomic number in- The multiplets of heavy atoms
and nuclei are better described by *j-
j coupling or intermediate coupling,
i.e. a coupling in which the energies
of electrostatic repulsion and
spin–orbit coupling are similar in
size.
rustingCorrosion of iron (or steel)
to form a hydrated iron(III) oxide
Fe 2 O 3 .xH 2 O. Rusting occurs only in
the presence of both water and oxy-
gen. It is an electrochemical process
in which different parts of the iron
surface act as electrodes in a cell re-
action. At the anode, iron atoms dis-
solve as Fe2+ions:
Fe(s) →Fe2+(aq) + 2e
At the cathode, hydroxide ions are
formed:
O 2 (aq) + 2H 2 O(l) + 4e →4OH–(aq)
The Fe(OH) 2 in solution is oxidized to
Fe 2 O 3. Rusting is accelerated by im-
purities in the iron and by the pres-
ence of acids or other electrolytes in
the water.
rutheniumSymbol Ru. A hard
white metallic *transition element;
a.n. 44; r.a.m. 101.07; r.d. 12.3; m.p.
2310 °C; b.p. 3900°C. It is found asso-
ciated with platinum and is used as a
catalyst and in certain platinum al-
loys. Chemically, it dissolves in fused
alkalis but is not attacked by acids. It
reacts with oxygen and halogens at467 ruthenium
r