Download Chemical Periodicity - Celina City Schools

Chemical Periodicity
From ancient Greece until 1700 AD, only 14
elements had been correctly identified.
Numerous other substances (compounds) were
incorrectly thought to be elements.
Chemistry began to improve it’s theories and
methods during the 1700’s so that between 1800
and 1810 14 new elements were discovered.
By 1830, there were 45 correctly identified
elements. This is half of those that naturally
occur on the earth.
With ½ of the pieces to the element puzzle,
chemists begin to see patterns emerge.
1829
Johann Wolfgang Döbereiner
Triads
He was apprenticed to an
apothecary (pharmacist),
reading widely, and attending
science lectures. He eventually
became a professor at the University of Jena in
1810. Döbereiner discovered trends in certain
properties of selected groups 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 sulfur, selenium, and
tellurium, and also with chlorine, bromine, and
iodine. Moreover, the densities for some of these
triads followed a similar pattern. These sets of
elements became known as "Döbereiner's
triads".
Döbereiner also is known for his discovery of
furfural, for his work on the use of platinum as a
catalyst, and for a lighter, known as Döbereiner's
lamp.
1858
Stanislao Cannizzaro
Element mass & molecular mass
Established a table of “correct”
atomic weights for the elements.
Apart from his work extensive
work in organic chemistry,
Cannizzaro rendered great
service to chemistry with his 1858
paper in which he insisted on the distinction,
previously hypothesised by Avogadro, between
atomic and molecular weights. For these
achievements, of fundamental importance to
atomic theory, he was awarded the Copley
Medal by the Royal Society in 1891.
1860
The Karlsruhe Congress was called so that
European chemists could discuss matters of
chemical nomenclature, notation, and atomic
weights.
As an example of
the problems
facing the
delegates, Kekulé
presented nineteen
different formulas
used by chemists
for acetic acid, as
shown in the figure
at the right.
The Karlsruhe
meeting ended
with no firm
agreement on the
problem of atomic and molecular weights.
However, on the meeting's last day reprints of
Cannizzaro's 1858 paper on atomic weights, in
which he utilized earlier work by Avogadro, were
distributed. Cannizzaro's efforts exerted a heavy
influence on the delegates.
An important long-term result of the Karlsruhe
Congress was the adoption of the now-familiar
atomic weights (actually, atomic masses). Prior
to the Karlsruhe meeting, and going back to
Dalton's work in 1803, several systems of atomic
weights were in use.
The Elements were from now on organized by
increasing mass.
1862
Alexandre-Emile Béguyer de Chancourtois
Telluric Helix
De Chancourtois devised a spiral
graph that was arranged on a
cylinder which he called his telluric
helix because tellurium was the
element in the middle of the graph.
De Chancourtois ordered the
elements by increasing atomic weight and similar
elements lined up vertically.
1864
John Newland
Law of Octaves
In music, an octave is the interval
between one musical pitch and another with half or
double its frequency. The octave relationship is a
natural phenomenon which has been referred to as
the "basic miracle of music."
Newland arranged many of the known elements
in the increasing order of their atomic masses.
He noticed that the eighth element was similar in
properties to the first element, just like the eighth
note in music.
The eighth element after lithium is sodium. In
many of its chemical properties it is similar to
lithium. Similarly, the eighth element after sodium
is potassium, whose properties are similar to
sodium. The eighth element from fluorine is
chlorine both of which are similar in their
properties. The eighth element from nitrogen is
phosphorus and both these elements are similar
in properties.
Based on this observation, Newland stated his
law of octaves thus 'when elements are arranged
in increasing order of their atomic mass, the
eighth element resembles the first in physical
and chemical properties just like the eighth note
on a musical scale resembles the first note'. As a
result a very important conclusion was made that
there is some systematic relationship between
the order of atomic masses and the repetition of
properties of elements. This gave rise to a new
term called 'periodicity' which signifies the
recurrence of characteristic properties of
elements arranged in a table, at regular intervals
of a period.
The law of octaves was the first logical attempt to
classify elements on the basis of atomic weights.
The periodicity of elements was recognized for
the first time.
The big limitation was that this law could be best
applied, only up to the element calcium.
Also, newly discovered elements could not fit into
the octave structure.