1. Fluorine (L. fluo, flow) was not prepared
as an element until 1886, even
though the fluorine-containing mineral fluorspar or fluorite was described
in 1529 by Georg Bauer, the German physician and mineralogist better
known as Agricola. It appears likely that crude hydrofluoric acid was
first
prepared in the early 18th century by an unknown English glassworker
and
used to etch glass. Subsequently, in 1771, Swedish chemist Carl Wilhelm
Scheele obtained impure hydrofluoric acid—over a hundred years before
French chemist Henri Moissan isolated the element in 1886. The delay
was
largely due to the reactivity and toxicity of fluorine. After the nearly
anhydrous hydrofluoric acid was prepared in 1809, French scientist
Andre´-Marie
Ampe`re suggested (1811) that it was a compound of hydrogen with an
unknown element, analogous to chlorine, for which he suggested the
name
fluorine. Fluorspar was then recognized to be calcium fluoride.
Fluorine’s toxicity caused serious health problems for several chemists.
Sir
Humphry Davy tried to prepare the element by electrolytic decomposition
of
various fluorides, but succeeded only in ruining his health. French
chemist
Edmond Fre´my, as well as George and Thomas Knox of the Irish
Academy,
Gay-Lussac, and The´nard all suffered loss of health due to their
efforts to
make fluorine. At least two chemists—Paulin Louget and Jerome Nickles—
died as a direct result of attempts to isolate fluorine. Even Moissan,
who finally
prepared the gas by electrolysis of anhydrous hydrofluoric acid containing
KHF2
as an electrolyte, had earlier completed many unsuccessful experiments,
resulting in damage to his health. He received the Nobel prize in 1906,
when
he was 54, but died the following year. Prior to World War II, fluorine
remained
a laboratory curiosity. The use of uranium(VI) fluoride in the separation
of
uranium isotopes and development of organic fluorine compounds and
polymers has made fluorine an important and much more familiar element.
2. Chlorine (Gr. chlorous, greenish-yellow) was discovered
in 1772 by Scheele,
who thought it was an oxygen-containing substance. By 1810, Humphry
Davy
had completed a careful search for the oxygen, but, failing to find
any, changed
its name from oxymuriatic acid (muriatic acid is HCl and chlorine is
an oxidation
product of HCl) to chlorine, since he was convinced this was an element.
He had
some difficulty convincing the French of this, but his views eventually
prevailed.
Chlorine has been directly or indirectly involved in human history
for centuries.
At one time salt (NaCl) was more valuable than gold—wars were fought
over the
control of its sources. Chlorine gas was used as a weapon in World
War I, but by
contrast, in 1988 it became a federally required water disinfectant
in all U.S.
public drinking water supplies. Chlorine and other halogen-containing
hydrocarbons show a similar apparent dichotomy in their use in a wide
variety
of consumer, agricultural, and industrial products, while also being
banned
because of environmental and health concerns. Even so, halogen-containing
polymeric hydrocarbon derivatives continue to ease our everyday lifestyle.
3. Bromine (Gr. bromos, stench) This element’s discovery
is credited to French
chemist Antoine Je´rome Balard, who, in 1826, noted that bromine
could be
liberated from a solution of residues remaining after sodium chloride
had been
removed from sea water by bubbling chlorine through the solution. He
deduced
that this was a new element similar to chlorine because the distillation
of these
residues (containing MgBr2 ) with manganese dioxide and
sulfuric acid also
produces bromine and is analogous to a known procedure for producing
chlorine.
German chemist Justus von Liebig had actually isolated bromine prior
to its
discovery by Balard, but he interpreted its properties (between those
of
chlorine and iodine) as indicative of iodine chloride.
4. Iodine(Gr. iodes, violet) The discovery of iodine is
credited to French chemist
Bernard Courtois. In 1811, from the ashes of burnt seaweed, he observed
a
violet vapor that attacked his copper vessels. He called the condensed
black
crystalline product “substance X.” In 1813, Davy, who was passing through
Paris, recognized substance X as an element analogous to chlorine and
suggested the name iodine. Historically, iodine has been prepared by
a number
of methods and sources ranging from extraction from sea kelp and sea
animals
to various methods of I– oxidation and IO3
– reduction.
5. Astatine First prepared in 1940 by bombardment of bismuth
with alpha particles.
