tures and observe single atoms. A
Üne-pointed conducting tip near a
surface of a sample causes electrons
to tunnel from the surface to the tip.
Its occurrence depends on the electron density of the surface and the
distance between the tip and the surface itself. The electric current produced is kept constant by moving the
tip up or down as moves across the
surface.
scavenger A reagent that removes
a trace component from a system or
that removes a reactive intermediate
from a reaction.
SCF See self-consistent field.
Scheele, Karl Wilhelm (1742–86)
Swedish chemist, who became an
apothecary and in 1775 set up his
own pharmacy at Köping. He made
many chemical discoveries. In
1772 he prepared oxygen (see also
lavoisier, antoine; *Priestley,
Joseph) and in 1774 he isolated chlorine. He also discovered manganese,
glycerol, hydrocyanic (prussic) acid,
citric acid, and many other substances.
scheelite A mineral form of calcium tungstate, CaWO 4 , used as an
ore of tungsten. It occurs in contact
metamorphosed deposits and vein
deposits as colourless or white tetragonal crystals.
Schiff base An imine with the general formula R
1 R
2 C = NR
3 , where R
1
,
R
2
, and R
3 are alkyl or aryl groups.
Schiff’s base A compound formed
by a condensation reaction between
an aromatic amine and an aldehyde
or ketone, for example
RNH 2 + R′CHO → RN:CHR′ + H 2 O
The compounds are often crystalline
and are used in organic chemistry for
characterizing aromatic amines (by
preparing the Schiff’s base and measuring the melting point). They are
named after the German chemist
Hugo Schiff (1834–1915).
Schiff’s reagent A reagent, devised by Hugo Schiff, used for testing
for aldehydes and ketones; it consists
of a solution of fuchsin dye that has
been decolorized by sulphur dioxide.
Aliphatic aldehydes restore the pink
immediately, whereas aromatic ketones have no effect on the reagent.
Aromatic aldehydes and aliphatic ketones restore the colour slowly.
SchoenÛies system A system for
categorizing symmetries of molecules. C n groups contain only an nfold rotation axis. C nv groups, in
addition to the n-fold rotation axis,
have a mirror plane that contains the
axis of rotation (and mirror planes associated with the existence of the nfold axis). C nh groups, in addition to
the n-fold rotation axis, have a mirror
plane perpendicular to the axis. S n
groups have an n-fold rotation–reÛection axis. D n groups have an n-fold rotation axis and a two-fold axis
perpendicular to the n-fold axis (and
two-fold axes associated with the existence of the n-fold axis). D nh groups
have all the symmetry operations of
D n and also a mirror plane perpendicular to the n-fold axis. D nd groups contain all the symmetry operations of
D n and also mirror planes that contain the n-fold axis and bisect the angles between the two-fold axes. In
the SchoenÛies notation C stands for
‘cyclic’, S stands for ‘spiegel’ (mirror),
and D stands for ‘dihedral’. The
subscripts h, v, and d stand for horizontal, vertical, and diagonal respectively, where these words refer to
the position of the mirror planes
with respect to the n-fold axis (considered to be vertical). In addition to
the noncubic groups referred to so
far, there are cubic groups, which
have several rotation axes with the
same value of n. These are the tetrascavenger
474
s
www.AzShimi.ir www.AzShimi.com
Üne-pointed conducting tip near a
surface of a sample causes electrons
to tunnel from the surface to the tip.
Its occurrence depends on the electron density of the surface and the
distance between the tip and the surface itself. The electric current produced is kept constant by moving the
tip up or down as moves across the
surface.
scavenger A reagent that removes
a trace component from a system or
that removes a reactive intermediate
from a reaction.
SCF See self-consistent field.
Scheele, Karl Wilhelm (1742–86)
Swedish chemist, who became an
apothecary and in 1775 set up his
own pharmacy at Köping. He made
many chemical discoveries. In
1772 he prepared oxygen (see also
lavoisier, antoine; *Priestley,
Joseph) and in 1774 he isolated chlorine. He also discovered manganese,
glycerol, hydrocyanic (prussic) acid,
citric acid, and many other substances.
scheelite A mineral form of calcium tungstate, CaWO 4 , used as an
ore of tungsten. It occurs in contact
metamorphosed deposits and vein
deposits as colourless or white tetragonal crystals.
Schiff base An imine with the general formula R
1 R
2 C = NR
3 , where R
1
,
R
2
, and R
3 are alkyl or aryl groups.
Schiff’s base A compound formed
by a condensation reaction between
an aromatic amine and an aldehyde
or ketone, for example
RNH 2 + R′CHO → RN:CHR′ + H 2 O
The compounds are often crystalline
and are used in organic chemistry for
characterizing aromatic amines (by
preparing the Schiff’s base and measuring the melting point). They are
named after the German chemist
Hugo Schiff (1834–1915).
Schiff’s reagent A reagent, devised by Hugo Schiff, used for testing
for aldehydes and ketones; it consists
of a solution of fuchsin dye that has
been decolorized by sulphur dioxide.
Aliphatic aldehydes restore the pink
immediately, whereas aromatic ketones have no effect on the reagent.
Aromatic aldehydes and aliphatic ketones restore the colour slowly.
SchoenÛies system A system for
categorizing symmetries of molecules. C n groups contain only an nfold rotation axis. C nv groups, in
addition to the n-fold rotation axis,
have a mirror plane that contains the
axis of rotation (and mirror planes associated with the existence of the nfold axis). C nh groups, in addition to
the n-fold rotation axis, have a mirror
plane perpendicular to the axis. S n
groups have an n-fold rotation–reÛection axis. D n groups have an n-fold rotation axis and a two-fold axis
perpendicular to the n-fold axis (and
two-fold axes associated with the existence of the n-fold axis). D nh groups
have all the symmetry operations of
D n and also a mirror plane perpendicular to the n-fold axis. D nd groups contain all the symmetry operations of
D n and also mirror planes that contain the n-fold axis and bisect the angles between the two-fold axes. In
the SchoenÛies notation C stands for
‘cyclic’, S stands for ‘spiegel’ (mirror),
and D stands for ‘dihedral’. The
subscripts h, v, and d stand for horizontal, vertical, and diagonal respectively, where these words refer to
the position of the mirror planes
with respect to the n-fold axis (considered to be vertical). In addition to
the noncubic groups referred to so
far, there are cubic groups, which
have several rotation axes with the
same value of n. These are the tetrascavenger
474
s
www.AzShimi.ir www.AzShimi.com