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 elec-
tron density of the surface and the
distance between the tip and the sur-
face itself. The electric current pro-
duced is kept constant by moving the
tip up or down as moves across the
surface.scavengerA reagent that removes
a trace component from a system or
that removes a reactive intermediate
from a reaction.
SCFSee 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 chlo-
rine. He also discovered manganese,
glycerol, hydrocyanic (prussic) acid,
citric acid, and many other sub-
stances.scheeliteA mineral form of cal-
cium tungstate, CaWO 4 , used as an
ore of tungsten. It occurs in contact
metamorphosed deposits and vein
deposits as colourless or white tetrag-
onal crystals.Schiff base An imine with the gen-
eral 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 meas-
uring the melting point). They arenamed after the German chemist
Hugo Schiff (1834–1915).Schiff’s reagent A reagent, de-
vised 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 ke-
tones have no effect on the reagent.
Aromatic aldehydes and aliphatic ke-
tones restore the colour slowly.
SchoenÛies systemA system for
categorizing symmetries of mol-
ecules. Cngroups contain only an n-
fold rotation axis. Cnvgroups, in
addition to the n-fold rotation axis,
have a mirror plane that contains the
axis of rotation (and mirror planes as-
sociated with the existence of the n-
fold axis). Cnhgroups, in addition to
the n-fold rotation axis, have a mirror
plane perpendicular to the axis. Sn
groups have an n-fold rotation–reÛec-
tion axis. Dngroups have an n-fold ro-
tation axis and a two-fold axis
perpendicular to the n-fold axis (and
two-fold axes associated with the ex-
istence of the n-fold axis). Dnhgroups
have all the symmetry operations of
Dnand also a mirror plane perpendic-
ular to the n-fold axis. Dndgroups con-
tain all the symmetry operations of
Dnand also mirror planes that con-
tain the n-fold axis and bisect the an-
gles 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 hori-
zontal, vertical, and diagonal respec-
tively, where these words refer to
the position of the mirror planes
with respect to the n-fold axis (con-
sidered 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 tetra-scavenger 474s