cal compound that rotates the plane
of polarization of plane-polarized
light to the right (clockwise as ob-
served by someone facing the on-
coming radiation). See optical
activity.dextrose See glucose.diagonal relationship A relation-
ship within the periodic table by
which certain elements in the second
period have a close chemical similar-
ity to their diagonal neighbours in
the next group of the third period.
This is particularly noticeable with
the following pairs.
Lithium and magnesium:
(1) both form chlorides and bromides
that hydrolyse slowly and are soluble
in ethanol;
(2) both form colourless or slightly
coloured crystalline nitrides by direct
reaction with nitrogen at high tem-
peratures;
(3) both burn in air to give the nor-
mal oxide only;
(4) both form carbonates that decom-
pose on heating.
Beryllium and aluminium:
(1) both form highly refractory ox-
ides with polymorphs;
(2) both form crystalline nitrides that
are hydrolysed in water;
(3) addition of hydroxide ion to solu-
tions of the salts gives an amphoteric
hydroxide, which is soluble in excess
hydroxide giving beryllate or alumi-
nate ions [Be(OH) 4 ]2–and [Al(OH) 4 ]–;
(4) both form covalent halides and co-
valent alkyl compounds that display
bridging structures;
(5) both metals dissolve in alkalis.
Boron and silicon:
(1) both display semiconductor prop-
erties;
(2) both form hydrides that are unsta-
ble in air and chlorides that hydrol-
yse in moist air;
(3) both form acidic oxides with cova-
lent crystal structures, which arereadily incorporated along with
other oxides into a wide range of
glassy materials.
The reason for this relationship is a
combination of the trends to increase
size down a group and to decrease
size along a period, and a similar, but
reversed, effect in electronegativity,
i.e. decrease down a group and in-
crease along a period.
dialysisA method by which large
molecules (such as starch or protein)
and small molecules (such as glucose
or amino acids) in solution may be
separated by selective diffusion
through a semipermeable mem-
brane. For example, if a mixed solu-
tion of starch and glucose is placed in
a closed container made of a semi-
permeable substance (such as Cello-
phane), which is then immersed in a
beaker of water, the smaller glucose
molecules will pass through the
membrane into the water while the
starch molecules remain behind. The
cell membranes of living organisms
are semipermeable, and dialysis
takes place naturally in the kidneys
for the excretion of nitrogenous
waste. An artiÜcial kidney (dialyser)
utilizes the principle of dialysis by
taking over the functions of diseased
kidneys.diamagnetismSee magnetism.
diaminobenzene(phenylenedi-
amine)Any one of three isomeric
aromatic compounds, C 6 H 4 (NH 2 ) 2 ,
with strong basic properties. 1,2-
Diaminobenzeneis a yellow-brown
crystalline solid, m.p. 104°C, used as
a photographic developer and in
making dyes. 1,3-Diaminobenzeneis
a colourless crystalline solid, m.p.
63 °C, which turns brown on standing
in air. It is made by reducing 1,3-
dinitrobenzene with iron and hy-
drochloric acid, and is used for mak-
ing dyes. 1,4-Diaminobenzeneis a
white crystalline solid, m.p. 147°C,dextrose 172d