JOHNN WOLFGANG DOBEREINER
Johnn Wolfgang
Dobereiner (13 December 1780 – 24, March 1849) was a German Chemist who is best
known for the work that foreshadowed the periodic law for the chemical
elements.
LIFE AND WORK
As a coachman’s
son, Doberiner had little opportunity for former schooling, so he was
apprenticed to an apothecary, reading widely and attending science
lectures. He eventually became a
professor at the University of Jena in 1810, he also studied chemistry at
Strasbourg. In work beginning 1829,
Doberiner discovered trends in certain properties of elements. For example, the average atomic mass of
Lithium and Potassium was close to the atomic mass of Sodium. A similar pattern was found with Calcium,
Strontium, and Barium. With Sulphur, Selenium
and Tellurium and also with Chlorine, Bromine and Iodine.
DOBERINER’S TRIADS
As the number of
elements increased, chemists inevitably began to find patterns in their properties. In 1829, Johnn Wolfgang Dobereiner discovered
the existence of families of elements with similar chemical properties.
Because there
always seemed to be three elements in these families he called them
triads. Each of the vertical columns in
table 1 represent one of this triads
Li Ca S Cl Mn
Na Sr Se Br Cr
K Ba Te I Fe
Dobereiner’s
triads grouped elements with similar chemical properties. Consider Lithium, Sodium and Potassium for
example.
·
These elements
all react with water at room temperature
·
They react with
Chlorine to form compounds with similar formulas: LiCl, NaCl, KCl
·
They combine
with Hydrogen to form compounds with similar formulas: LiH, NaH, KH
·
They form
hydroxide with similar formulas: LiOH, NaOH, KOH
Dobereiner also
found patterns in the physical properties of the elements in a triad. He noted, for example that the atomic weight
of middle element in each triad is about equal to the average of atomic weights
of the first and third elements. The
atomic weight of Sodium (22.99 g/mol) for example, is remarkably close to the
average of the atomic weights of Lithium (6.94 g/mol) and Potassium (39.10
g/mol). Dobereiner also found that the
density of the middle element in most triads is roughly equal to the average of
the densities of the other elements. The
density of Strontium (2.60 g/cm3), for example, is close to the
average of the densities of calcium (1.55 g/cm3) and Barium (3.51
g/cm3).
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