of destructive interference, the photoelectron has a lower probability of
appearing. The technique therefore
provides information about the surroundings of an atom. Analysis of results from SEXAFS has given a great
deal of information about the structure of surfaces and how they respond to adsorption.
sexivalent (hexavalent) Having a
valency of six.
shale A form of *clay that occurs in
thin layers. Shales are very common
sedimentary rocks. See also oil shale.
shell See atom.
shellac A hard resin produced as a
secretion by a plant parasite, the
south-east Asian lac insect Lacifer
lacca. It is used in sealing wax, varnish (French polish), and electrical insulators.
sherardizing The process of coating iron or steel with a zinc corrosion-resistant layer by heating it in
contact with zinc dust to a temperature 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 alloys and an external layer of pure
zinc. The process was invented by
Sherard Cowper-Coles (d. 1935).
shock wave A very narrow region
of high pressure and temperature
formed in a Ûuid when the Ûuid
Ûows supersonically over a stationary
object or a projectile Ûying supersonically passes through a stationary
Ûuid. A shock wave may also be generated by violent disturbances in a
Ûuid, such as a lightning stroke or a
bomb blast. Shock waves are generated experimentally to excite molecules for spectroscopic
investigations.
short period See periodic table.
sial The 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 chain See 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.
siderite A 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 important iron ore. It is found in England, Greenland, Spain, N Africa, and
the USA.
siemens Symbol 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
–1
Ω. The unit was formerly called
the mho or reciprocal ohm. It is
named after Ernst Werner von
Siemens (1816–92).
sievert The SI unit of dose equivalent (see radiation units). It is
named after the Swedish physicist
Rolf Sievert (1896–1966).
sigma bond See 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 nonlinked atoms at the same time as an
existing sigma bond is broken. Sigmatropic 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
480
s
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appearing. The technique therefore
provides information about the surroundings of an atom. Analysis of results from SEXAFS has given a great
deal of information about the structure of surfaces and how they respond to adsorption.
sexivalent (hexavalent) Having a
valency of six.
shale A form of *clay that occurs in
thin layers. Shales are very common
sedimentary rocks. See also oil shale.
shell See atom.
shellac A hard resin produced as a
secretion by a plant parasite, the
south-east Asian lac insect Lacifer
lacca. It is used in sealing wax, varnish (French polish), and electrical insulators.
sherardizing The process of coating iron or steel with a zinc corrosion-resistant layer by heating it in
contact with zinc dust to a temperature 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 alloys and an external layer of pure
zinc. The process was invented by
Sherard Cowper-Coles (d. 1935).
shock wave A very narrow region
of high pressure and temperature
formed in a Ûuid when the Ûuid
Ûows supersonically over a stationary
object or a projectile Ûying supersonically passes through a stationary
Ûuid. A shock wave may also be generated by violent disturbances in a
Ûuid, such as a lightning stroke or a
bomb blast. Shock waves are generated experimentally to excite molecules for spectroscopic
investigations.
short period See periodic table.
sial The 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 chain See 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.
siderite A 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 important iron ore. It is found in England, Greenland, Spain, N Africa, and
the USA.
siemens Symbol 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
–1
Ω. The unit was formerly called
the mho or reciprocal ohm. It is
named after Ernst Werner von
Siemens (1816–92).
sievert The SI unit of dose equivalent (see radiation units). It is
named after the Swedish physicist
Rolf Sievert (1896–1966).
sigma bond See 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 nonlinked atoms at the same time as an
existing sigma bond is broken. Sigmatropic 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
480
s
www.AzShimi.ir www.AzShimi.com