diation. Light and ultraviolet radia-
tion from the sun are able to pen-
etrate the atmosphere and warm the
earth’s surface. This energy is re-radi-
ated as infrared radiation, which, be-
cause of its longer wavelength, is
absorbed by such substances as car-
bon dioxide. Emissions of carbon
dioxide from human activities have
increased markedly in the last 150
years or so. The overall effect is that
the average temperature of the earth
and its atmosphere is increasing (so-
called global warming). The effect is
similar to that occurring in a green-
house, where light and long-wave-
length ultraviolet radiation can pass
through the glass into the green-
house but the infrared radiation is
absorbed by the glass and part of it is
re-radiated into the greenhouse.
The greenhouse effect is seen as a
major environmental hazard. Aver-
age increases in temperature are
likely to change weather patterns
and agricultural output. It is already
causing the polar ice caps to melt,
with a corresponding rise in sea
level. Carbon dioxide, from fossil-fuel
power stations and car exhausts, is
the main greenhouse gas. Other con-
tributory pollutants are nitrogen
oxides, ozone, methane, and
chloroÛuorocarbons.
greenhouse gas See greenhouse
effect.
greenockite A mineral form of
cadmium sulphide, CdS.green vitriol See iron(ii) sulphate.
Griess testA test for nitrates or ni-
trites, once widely used to detect the
possible existence of gunshot
residue.Grignard reagents A class of
organometallic compounds of mag-
nesium, with the general formula
RMgX, where R is an organic group
and X a halogen atom (e.g. CH 3 MgCl,C 2 H 5 MgBr, etc.). They actually have
the structure R 2 Mg.MgCl 2 , and can be
made by reacting a haloalkane with
magnesium in ether; they are rarely
isolated but are extensively used in
organic synthesis, when they are
made in one reaction mixture. Grig-
nard reagents have a number of reac-
tions that make them useful in
organic synthesis. With methanal
they give a primary alcohol
CH 3 MgCl + HCHO →CH 3 CH 2 OH
Other aldehydes give a secondary al-
cohol
CH 3 CHO + CH 3 MgCl →
(CH 3 ) 2 CHOH
With alcohols, hydrocarbons are
formed
CH 3 MgCl + C 2 H 5 OH →C 2 H 5 CH 3
Water also gives a hydrocarbon
CH 3 MgCl + H 2 O →CH 4
The compounds are named after
their discoverer, the French chemist
Victor Grignard (1871–1935).Grotrian diagram A diagram that
summarizes the energy levels and
the *allowed transitions between
these energy levels in an atom. The
energy is plotted vertically with a
horizontal line for each energy level.
The intensity of the transition can be
represented on a Grotrian diagram
by allowing the thickness of the line
to represent the transition propor-
tional to the intensity. Grotrian dia-
grams are named after the German
spectroscopist W. Grotrian, who in-
vented them in 1928.Grottius–Draper law A law in
photochemistry stating that only the
light absorbed by a substance or sub-
stances is effective in bringing about
chemical change. Not all the light
falling on the substances will neces-
sarily bring about chemical change,
since some of it can be re-emitted in
the form of heat or light. The lightgreenhouse gas 252g