electrolytes conduct because of the
presence of ions. He also worked on
kinetics and proposed the *Arrhe-
nius equation. He was theÜrst to pre-
dict the greenhouse effect resulting
from carbon dioxide. Arrhenius was
awarded the 1903 Nobel Prize for
chemistry.Arrhenius equationAn equation
of the form
k = Aexp(–Ea/RT)
where k is the rate constant of a
given reaction and Eathe *activation
energy. A is a constant for a given re-
action, called the pre-exponential
factor(or A-factor). Often the equa-
tion is written in logarithmic form
lnk = lnA – Ea/RT
A graph of lnk against 1/T is a straight
line with a gradient –Ea/R and an in-
tercept on the lnk axis of lnA. It is
named after Svante *Arrhenius.Arrhenius theorySee acid.arsenate(III)See arsenic(iii) oxide.arsenate(V)See arsenic(v) oxide.
arsenicSymbol As. A metalloid el-
ement of *group 15 (formerly VB) of
the periodic table; a.n. 33; r.a.m.
74.92; r.d. 5.7; sublimes at 613°C. It
has three allotropes – yellow, black,
and grey. The grey metallic form is
the stable and most common one.
Over 150 minerals contain arsenic
but the main sources are as impuri-
ties in sulphide ores and in the min-
erals orpiment (As 2 S 3 ) and realgar
(As 4 S 4 ). Ores are roasted in air to
form arsenic oxide and then reduced
by hydrogen or carbon to metallic ar-
senic. Arsenic compounds are used in
insecticides and as doping agents in
semiconductors. The element is in-
cluded in some lead-based alloys to
promote hardening. Confusion can
arise because As 4 O 6 is often sold as
white arsenic. Arsenic compounds
are accumulative poisons. The el-ement will react with halogens, con-
centrated oxidizing acids, and hot al-
kalis. Albertus Magnus is believed to
have been theÜrst to isolate the el-
ement in 1250.
A- Information from the WebElements site
arsenic acidSee arsenic(v) oxide.
arsenic(III) acidSee arsenic(iii)
oxide.arsenic hydrideSee arsine.arsenic(III) oxide (arsenic trioxide;
arsenious oxide; white arsenic)A
white or colourless compound,
As 4 O 6 , existing in three solid forms.
The commonest has cubic or octahe-
dral crystals (r.d. 3.87; sublimes at
193 °C) and is soluble in water,
ethanol, and alkali solutions. It oc-
curs naturally as arsenolite. A vitre-
ous form can be prepared by slow
condensation of the vapour (r.d.
3.74); its solubility in cold water is
more than double that of the cubic
form. The third modiÜcation, which
occurs naturally as claudetite, has
monoclinic crystals (r.d. 4.15). Ar-
senic(III) oxide is obtained commer-
cially as a byproduct from the
smelting of nonferrous sulphide ores;
it may be produced in the laboratory
by burning elemental arsenic in air.
The structure of the molecule is simi-
lar to that of P 4 O 6 , with a tetrahedral
arrangement of As atoms edge linked
by oxygen bridges. Arsenic(III) oxide
is acidic; its solutions were formerly
called arsenious acid (technically, ar-
senic(III) acid). It forms arsenate(III)
salts (formerly called arsenites). Ar-
senic(III) oxide is extremely toxic and
is used as a poison for vermin; trace
doses are used for a variety of medici-
nal purposes. It is also used for pro-
ducing opalescent glasses and
enamels.arsenic(V) oxide (arsenic oxide)AArrhenius equation 44a