Unit 8
•Historical development of the periodic
table (2.5)
•Current state of the periodic table (2.5)
Historical Status (2.5)
•By the mid-1800’s about 60 elements were
known
•Due to the work done with the laws of definite
proportions and multiple proportions, among
others, the relative masses of atoms of the
elements were fairly well-known
•Because science has a tendency toward
organization and classification, it was
necessary to find a method to catalogue these
elements.
Developments Prior to the Current Periodic Table
Dobereiner’s Triads (1816)
•Johann Dobereiner noticed similarities in trios of
some elements. For example, lithium, sodium, and
potassium have many similar properties
Table from http://www.corrosion-doctors.org
Developments Prior to the Current Periodic Table
De Chancourtois’s “Telluric Helix” (1862)
•Beguyer de Chancourtois observed that if the elements
were arranged by relative atomic masses and wound in a
spiral fashion, elements with similar properties would be
aligned vertically.
Images from http://www.corrosion-doctors.org
Developments Prior to the Current Periodic Table
Newland’s Law of Octaves (1863)
•John Newlands noticed that when the elements were listed in
order of atomic mass every eight element had similar
properties. This only worked well through calcium and placed
iron in the same group as oxygen and sulfur even though their
properties are very different.
Table from http://www.corrosion-doctors.org
Developments Prior to the Current Periodic Table
Lothar Meyer (1868)
•Lothar Meyer came up with a periodic chart very similar to
the one used today. His work was at the same time as that of
the ultimate developer of the periodic table, Dmitri Mendeleev,
but Mendeleev manage to publish his first and is typically
given the most credit.
Lothar Meyer’s Periodic Table - 1870
The Current Periodic Table (2.5)
Mendeleev (1869)
•The current periodic table is widely attributed to Dmitri Mendeleev. Meyer’s table
was very similar but Mendeleev published his a few months before Meyer and
Mendeleev’s had recognized predictive value.
•Two points regarding Mendeleev’s:
•The elements were arranged mostly by atomic masses, but he switched a couple
around to keep elements with similar properties together.
•Mendeleev left a couple of openings with the belief that those elements had not yet
been discovered. He predicted the existence of “eka-silicon” and its properties –
eventually this element was discovered and named germanium.
Property
Predicted for ekasilicon by Mendeleev
(1871)
Observed by Winkler
for Germanium (1876)
Atomic mass
72
72.6
Density (g/cc)
5.5
5.47
Color
Dirty gray
Grayish white
Density of oxide (g/cc)
4.7
4.703
Boiling point of chloride
Below 100 °C
86 °C
Density of chloride (g/cc)
1.9
1.887
The Current Periodic Table (2.5)
Mendeleev (1869)
•Below is Mendeleev’s first periodic table
•Compare this to today’s periodic table (front flap of your book
or next slide) and you will see a striking similarity.
Historical Discovery of Elements
Table from IUPAC.org
The number of known elements has grown considerably since Mendeleev’s time.
The chart above gives a sense of the time period of elemental discovery – in
Mendeleev’s time he basically had the gray and green boxes above to work with.
The Current Periodic Table (2.5)
Much of our study of chemistry will revolve around the elements, their
combinations, and properties based on position in the periodic table.