addition reactions. Compare unsatu-
rated. 2.(of a solution) Containing
the maximum equilibrium amount
of solute at a given temperature. In a
saturated solution the dissolved sub-
stance is in equilibrium with undis-
solved substance; i.e. the rate at
which solute particles leave the solu-
tion is exactly balanced by the rate at
which they dissolve. A solution con-
taining less than the equilibrium
amount is said to be unsaturated.
One containing more than the equi-
librium amount is supersaturated.
Supersaturated solutions can be
made by slowly cooling a saturated
solution. Such solutions are meta-
stable; if a small crystal seed is added
the excess solute crystallizes out of
solution. 3.(of a vapour) See vapour
pressure.
saturationSee supersaturation.
saturation spectroscopy A spec-
troscopic technique using lasers to lo-
cate absorption maxima with great
precision. In saturation spectroscopy
the laser beam is split into an intense
saturating beam and a less intense
beam that pass through the cavity
containing the sample in almost op-
posite directions. The saturating
beam sometimes excites molecules,
which are shifted to its frequency by
the Doppler effect. The other beam
gives a modulated signal at the detec-
tor if it is interacting with the same
Doppler-shifted molecules. These
molecules are not moving parallel to
the beams and have an extremely
small Doppler shift, thus providing
very high resolution. See also lamb-dip
spectroscopy.
sawhorse projection A type
of *projection in which a three-
dimensional view is drawn. See con-
formation.
s-block elementsThe elements of
theÜrst two groups of the *periodic
table; i.e. groups 1 (Li, Na, K, Rb, Cs,
Fr) and 2 (Be, Mg, Ca, Sr, Ba, Ra). The
outer electronic conÜgurations of
these elements all have inert-gas
structures plus outer ns^1 (group 1) or
ns^2 (group 2) electrons. The term thus
excludes elements with incomplete
inner d-levels (transition metals) or
with incomplete inner f-levels (lan-
thanoids and actinoids) even though
these often have outer ns^2 or occa-
sionally ns^1 conÜgurations. Typically,
the s-block elements are reactive
metals forming stable ionic com-
pounds containing M+or M2+ions.
See alkali metals; alkaline-earth
metals.
scSynclinal. See torsion angle.scandiumSymbol Sc. A rare soft
silvery metallic element belonging to
group 3 (formerly IIIA) of the peri-
odic table; a.n. 21; r.a.m. 44.956; r.d.
2.989 (alpha form), 3.19 (beta form);
m.p. 1541°C; b.p. 2831°C. Scandium
often occurs in *lanthanoid ores,
from which it can be separated on ac-
count of the greater solubility of its
thiocyanate in ether. The only nat-
ural isotope, which is not radioactive,
is scandium–45, and there are nine
radioactive isotopes, all with rela-
tively short half-lives. Because of the
metal’s high reactivity and high cost
no substantial uses have been found
for either the metal or its com-
pounds. Predicted in 1869 by Dmitri
*Mendeleev, and then called eka-
boron, the oxide (called scandia) was
isolated by Lars Nilson (1840–99) in
1879.
A- Information from the WebElements site
scanning electron microscope
See electron microscope.scanning tunnelling microscope
(STM)A type of electron microscope
that uses the quantum-mechanical
*tunnel effect to study atomic struc-473 scanning tunnelling microscope
s