The Periodic Table of Chemical Elements Originated about 200 Years Ago!
The Modern Periodic Table
Chemist Julius Lothar Meyer of Breslau
University in Germany, while
in the process of revising his chemistry
textbook in 1868, produced a periodic
table that turned out to be remarkably
similar to Mendeleev's famous 1869
version; although Lothar Meyer failed
to classify all the elements correctly.
Unlike his predecessors, Mendeleev had sufficient confidence in his periodic table to use it to predict several new elements and the properties of their compounds.
The table did not appear in print until
1870 because of a publisher's delay; a
factor that contributed to an acrimonious
dispute for priority that ensued
between Lothar Meyer and Mendeleev.
Around the same time, Mendeleev
assembled his own periodic table while
he, too, was writing a textbook of chemistry.
Unlike his predecessors, Mendeleev
had sufficient confidence in his periodic
table to use it to predict several
new elements and the properties of their
compounds.
He also corrected the atomic
weights of some already known elements.
Interestingly, Mendeleev admitted
to having seen certain earlier tables;
such as, those of Newlands, but claimed
to have been unaware of Lothar Meyer's
work when developing his chart.
Although the predictive aspect of
Mendeleev's table was a major advance,
it seems to have been overemphasized by
historians, who have generally suggested
that Mendeleev's table was accepted
especially because of this feature.
These scholars have failed to notice that the
citation from the Royal Society of London
that accompanied the Davy Medal
(which Mendeleev received in 1882)
makes no mention whatsoever of his
predictions.
Instead, Mendeleev's ability to accommodate the already known elements may have contributed as much to the acceptance of the periodic system as did his striking predictions.
Although numerous scientists helped to develop
the periodic system, Mendeleev receives
most of the credit for discovering chemical
periodicity because he elevated the
discovery to a law of nature and spent
the rest of his life boldly examining its
consequences and defending its validity.
Defending the periodic table was no
simple task because its accuracy was frequently
challenged by subsequent discoveries.
One notable occasion arose in 1894,
when William Ramsay of University College
London and Lord Rayleigh (John
William Strutt) of the Royal Institution
in London discovered the element argon;
over the next few years, Ramsay
announced the identification of four
other elements: helium, neon, krypton
and xenon; known as the noble gases.
The last of the known noble gases, radon,
was discovered in 1900 by German
physicist Friedrich Ernst Dorn.
The name "noble" derives from the
fact that all these gases seem to stand
apart from the other elements, rarely
interacting with them to form compounds.
As a result, some chemists suggested
that the noble gases did not even
belong in the periodic table.
These elements had not been predicted by Mendeleev
or anyone else, and only after six
years of intense effort could chemists
and physicists successfully incorporate
the noble gases into the table.
In the new arrangement, an additional column was
introduced between the halogens (the
gaseous elements fluorine, chlorine, bromine,
iodine, and astatine) and the alkali
metals (lithium, sodium, potassium,
rubidium, cesium, and francium).
A second point of contention surrounded
the precise ordering of the elements.
Mendeleev's original table positioned
the elements according to atomic
weight, but in 1913, Dutch amateur
theoretical physicist Anton van den
Broek suggested that the ordering principle
for the periodic table lay instead in
the nuclear charge of each atom.
Understanding the Atom
The periodic table inspired the work
not only of chemists but also of
atomic physicists struggling to understand
the structure of the atom.
In 1904, working at Cambridge, physicist
J. J. Thomson (who also discovered the
electron) developed a model of the atom,
paying close attention to the periodicity
of the elements.
The periodic table inspired the work
not only of chemists but also of atomic physicists struggling to understand the structure of the atom.
He proposed that the
atoms of a particular element contained
a specific number of electrons arranged
in concentric rings.
Furthermore, according
to Thomson, elements with similar
configurations of electrons would
have similar properties; Thomson's
work thus provided the first physical
explanation for the periodicity of the
elements.
Although Thomson imagined
the rings of electrons as lying inside the
main body of the atom, rather than circulating
around the nucleus as is believed
today, his model does represent
the first time anyone addressed the arrangement
of electrons in the atom, a
concept that pervades the whole of
modern chemistry.
Danish physicist, Niels Bohr, the first
to bring quantum theory to bear on the
structure of the atom, was also motivated
by the arrangement of the elements
in the periodic system.
In Bohr's model of the atom, developed in 1913, electrons
inhabit a series of concentric shells
that encircle the nucleus.
Bohr reasoned that elements in the same group of the
periodic table might have identical configurations of electrons in their outermost shell and that the chemical properties
of an element would depend in
large part on the arrangement of electrons
in the outer shell of its atoms.
Bohr's model of the atom also served
to explain why the noble gases lack reactivity:
noble gases possess full outer
shells of electrons, making them unusually
stable and unlikely to form compounds.
Indeed, most other elements
form compounds as a way to obtain
full outer electron shells.
More recent analysis of how Bohr arrived at these
electronic configurations suggests that
he functioned more like a chemist than
has generally been credited.
Bohr did not derive electron configurations from
quantum theory but obtained them
from the known chemical and spectroscopic
properties of the elements.
Quantum mechanics
can only reproduce Mendeleev's original discovery by the use of mathematical approximations; it cannot predict
the periodic system.
Variations on a Theme
After evolving for
over 200 years through the work of
many people, the periodic table remains
at the heart of the study of chemistry.
It ranks as one of the most fruitful ideas
in modern science, comparable perhaps
to Charles Darwin's theory of evolution.
Unlike theories such as Newtonian mechanics,
it has not been falsified or revolutionized
by modern physics but has
adapted and matured while remaining
essentially unscathed.
—Compiled from a summary of
"The Evolution of the Periodic System From its origins some 200 years ago,
the periodic table has become a vital tool for modern chemists" by Eric R. Scerri;
as seen in the Scientific American; September, 1998; pages 78-83.
The knowledge in this article is applicable to the Chemical Elements List where you will find considerable information about their histories and other facts.
Chemical Elements Chart History, Part 1 of 2 is available here, if you want it again.
