of destructive interference, the pho-
toelectron has a lower probability of
appearing. The technique therefore
provides information about the sur-
roundings of an atom. Analysis of re-
sults from SEXAFS has given a great
deal of information about the struc-
ture of surfaces and how they re-
spond to adsorption.
sexivalent(hexavalent)Having a
valency of six.shaleA form of *clay that occurs in
thin layers. Shales are very common
sedimentary rocks. See also oil shale.
shellSee atom.shellacA hard resin produced as a
secretion by a plant parasite, the
south-east Asian lac insect Lacifer
lacca. It is used in sealing wax, var-
nish (French polish), and electrical in-
sulators.
sherardizing The process of coat-
ing iron or steel with a zinc corro-
sion-resistant layer by heating it in
contact with zinc dust to a tempera-
ture slightly below the melting point
of zinc. At a temperature of about
371 °C the two metals amalgamate to
form internal layers of zinc–iron al-
loys and an external layer of pure
zinc. The process was invented by
Sherard Cowper-Coles (d. 1935).shock waveA very narrow region
of high pressure and temperature
formed in aÛuid when theÛuid
Ûows supersonically over a stationary
object or a projectileÛying superson-
ically passes through a stationary
Ûuid. A shock wave may also be gen-
erated by violent disturbances in a
Ûuid, such as a lightning stroke or a
bomb blast. Shock waves are gener-
ated experimentally to excite mol-
ecules for spectroscopic
investigations.short periodSee periodic table.sialThe rocks that form the earth’s
continental crust. These are granite
rock types rich in silica (SiO 2 ) and
aluminium (Al), hence the name.
Compare sima.side chainSee chain.
side reaction A chemical reaction
that occurs at the same time as a
main reaction but to a lesser extent,
thus leading to other products mixed
with the main products.sideriteA brown or grey-green
mineral form of iron(II) carbonate,
FeCO 3 , often with magnesium and
manganese substituting for the iron.
It occurs in sedimentary deposits or
in hydrothermal veins and is an im-
portant iron ore. It is found in Eng-
land, Greenland, Spain, N Africa, and
the USA.siemensSymbol S. The SI unit of
electrical conductance equal to the
conductance of a circuit or element
that has a resistance of 1 ohm. 1 S =
10 –1Ω. The unit was formerly called
the mho or reciprocal ohm. It is
named after Ernst Werner von
Siemens (1816–92).sievertThe SI unit of dose equiva-
lent (see radiation units). It is
named after the Swedish physicist
Rolf Sievert (1896–1966).sigma bondSee orbital.sigma electron An electron in a
sigma orbital. See orbital.
sigmatropic reaction A type of
rearrangement in which a sigma
bond is formed between two non-
linked atoms at the same time as an
existing sigma bond is broken. Sig-
matropic rearrangements are a type
of *pericyclic reaction. An example is
the *Cope rearrangement.
silane(silicane)1.A colourless gas,
SiH 4 , which is insoluble in water; d.
1.44 g dm–3; r.d. 0.68 (liquid); m.p.sexivalent 480s