aqueous solution A solution in which water is the
solvent or dissolving medium, such as salt water, rain,
or soda.
Archaea A group of prokaryotes that can be subdi-
vided into three groups (methanogenic, halophilic,
thermoacidophilic) and are characterized by special
constituents, such as ether-bonded lipids and special
COENZYMEs. The Archaea are members of a separate
kingdom that falls in between eubacterial and eukary-
otic organisms.
See alsoEUKARYOTE; METHANOGEN.
aromatic, aromaticity
- In the traditional sense, “having a chemistry typified
by benzene.” - A cyclically conjugated MOLECULAR ENTITYwith a
stability (due to DELOCALIZATION) significantly
greater than that of a hypothetical localized struc-
ture (e.g., KEKULÉ STRUCTURE) is said to possess
aromatic character. If the structure is of higher
energy (less stable) than such a hypothetical classical
structure, the molecular entity is “antiaromatic.”
The most widely used method for determining
aromaticity is the observation of diatropicity in the
(^1) H NMR spectrum.
- The terms aromaticand antiaromatichave been
extended to describe the stabilization or destabi-
lization of TRANSITION STATEs of PERICYCLIC REAC-
TIONs. The hypothetical reference structure is here
less clearly defined, and use of the term is based on
application of the HUCKEL(4N+2) RULEand on
consideration of the topology of orbital overlap in
the transition state. Reactions of molecules in the
GROUND STATEinvolving antiaromatic transition
states proceed, if at all, much less easily than those
involving aromatic transition states.
See alsoCONJUGATED SYSTEM.
aromatic hydrocarbons Benzene and its derivatives.
Arrhenius, Svante August(1859–1927) Swedish
Chemist, physicist Svante August Arrhenius was born
in Vik (or Wijk), near Uppsala, Sweden, on February
19, 1859. He was the second son of Svante Gustav
Arrhenius and Carolina Christina (née Thunberg).
Svante’s father was a surveyor and an administrator of
his family’s estate at Vik. In 1860, a year after Arrhe-
nius was born, his family moved to Uppsala, where his
father became a supervisor at the university. He was
reading by the age of three.
Arrhenius received his early education at the
cathedral school in Uppsala, excelling in biology,
physics, and mathematics. In 1876 he entered the Uni-
versity of Uppsala and studied physics, chemistry, and
mathematics, receiving his B.S. two years later. While
he continued graduate classes for three years in
physics at Uppsala, his studies were not completed
there. Instead, Arrhenius transferred to the Swedish
Academy of Sciences in Stockholm in 1881 to work
under Erick Edlund to conduct research in the field of
electrical theory.
Arrhenius studied electrical conductivity of dilute
solutions by passing electric current through a variety
of solutions. His research determined that molecules
in some of the substances split apart or dissociated
from each other into two or more ions when they
were dissolved in a liquid. He found that while each
intact molecule was electrically balanced, the splitArrhenius, Svante August 17Nobel Prize–winning Swedish chemist, at work in his laboratory.
Svante Arrhenius was an infant prodigy and taught himself to
read at age three. He studied physics and chemistry at the Uni-
versity of Uppsala before transferring to Stockholm to begin
research on aqueous solutions of acids, bases, and salts. He dis-
covered that such solutions conduct electricity because the
solute divides into charged ions in the water—a finding that was
at first scoffed at but which later won him the 1903 Nobel Prize in
chemistry. Subsequent achievements include his elucidation of
the effect of temperature on reaction rates (Arrhenius equation)
and discovery of the greenhouse effect.(Courtesy of Science
Photo Library)