History of the Periodic Table of the Elements
Mendeleev and Chemical Periodicity
In September 1860, a group of chemists assembled at the First International
Congress of Chemists in Karlsruhe, Germany. The goal of the Congress was
to review various scientific matters on which there was little agreement. One
topic on the agenda was the measurement of atomic masses. Another was how
to determine the composition of compounds by using atomic masses.
Chemists hoped that communication on these matters, which had become
almost impossible, could be improved.
At the Congress, the Italian chemist Stanislao Cannizzaro (1826-1910)
presented a method for measuring atomic masses and interpreting the results
of the measurements. Cannizzaro's method enabled scientists to agree on
standard values for atomic masses. Scientists then began to search for
generalizations. Could any relationships be found among atomic masses and
other properties of the elements? Could predictions about such properties be
made based on these atomic masses?
Mendeleev noticed that when the elements were arranged in order of
increasing atomic masses, similar properties appeared at regular intervals.
Such a repeating pattern is referred to as "periodic." The second hand of a
watch, for example, passes over any given mark at periodic, 60-second
intervals.
Mendeleev created a table in which elements with similar properties were
grouped together--a periodic table of the elements. His first periodic table,
shown in Figure 5-2, was published in 1869.
Note that Mendeleev placed iodine (I, atomic mass 127) after tellurium
(Te, atomic mass 128), even though this meant not listing the elements in
order of increasing atomic mass. In this way, tellurium could be placed in the
same group as elements with similar properties such as oxygen (O), sulfur (S),
and selenium (Se). Iodine could also, then, be placed in the same group as
fluorine (F), chlorine (Cl), and bromine (Br), which have properties similar to
those of iodine.
Mendeleev's procedure left several empty spaces in his periodic table. In
1871, Mendeleev boldly predicted the existence and properties of the elements
that would fill three of these spaces. By 1886, all three had been discovered.
Their properties were found to be strikingly similar to those predicted by
Mendeleev, as shown in Table 5-1. The success of his predictions persuaded
most chemists to accept Mendeleev's periodic table and gave him historical
credit as the discoverer of periodicity. Two questions remained, however: (1)
Why was it that most elements could be arranged in the order of increasing
atomic mass, but few could not? (2) What is the reason for chemical
periodicity? Careful experimentation, observation, and analysis of data would
eventually reveal the answers to these questions.
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