Academic Chemistry
Mr. Gensits
Class Notes 10/26/2015
Development of the Periodic Table
Before 1700 only 12 elements were known to exist
1800 27 elements
1860 60 elements
1900 80 elements
J. W. Döbereiner
1829
Classified some elements into groups of three – “triads”.
The elements in a triad had similar chemical properties.
The physical properties of the elements in a triad varied in an orderly way
according to their atomic masses.
Densities and Atomic Mass Predictions
Ca
1.55 g/cm3
Ca
40 u
Sr
?
Sr
?
Ba
3.62 g/cm3
Ba
137 u
Sr: 2.59 g/cm3 (predicted value)
Sr: _______ g/cm3 (actual value)
Sr: 88.5 u (predicted value)
Sr: _______ u
(actual value)
Suggested that there was a relationship between atomic mass and the
element’s properties.
John Newlands 1864
Suggested the elements could be ordered in octaves
Dmitri Mendeleev 1869
Ordered the elements by increasing atomic mass.
Elements that had similar properties were placed in the same column.
There was a repeating pattern of the properties as you go across a row on
the periodic table.
Mendeleev was able to predict the properties of elements that were not yet
discovered at the time: Sc, Ga, and Ge
Mendeleev’s Prediction
Eka-Silicon
72 g/mol
d = 5.5 g/cm3
high melting point
Actual Values
Germanium
72.59 g/mol
d = 5.36 g/cm3
m.p. = 958oC
Es forms EsO2
which will have a high m.p.
d = 4.7 g/cm3
Ge forms GeO2
m.p. = 1100oC
d = 4.70 g/cm3
EsCl4 will be volatile liquid
b.p. < 100oC
d = 1.9 g/cm3
GeCl4 is a volatile liquid
b.p. = 83oC
d = 1.88 g/cm3
The Modern Periodic Table
Noble gases, many transition metals, and most of the inner transition metals
have been added.
Elements are now ordered by increasing atomic number
(not atomic mass.)
Periodic Law
The physical and chemical properties of the elements repeat in a regular
pattern when they are arranged in order of increasing atomic number.
Final Note:
Lothar Meyer devised a classification scheme very similar to Mendeleev’s
at the same time (1870.) Mendeleev, however, is usually given the
credit since he applied his understanding of the periodic law to make
predictions as to the properties of yet undiscovered elements and their
compounds.