History of the Periodic Table of the Elements
(continued)
Moseley and the Periodic Law
The first question was not answered until over forty years after Mendeleev's
first periodic table had been published. In 1911, the English scientist Henry
Gwyn-Jeffreys Moselet (1887-19150) discovered why a few elements could not
be arranged according to their atomic masses. Moseley, who was working
with Rutherford, performed a series of experiments on 38 different metals.
By analyzing his data, he discovered a previously unrecognized pattern: The
positive charge of the nucleus increases by one unit from one element to the
next when the elements are arranged as they are in the Periodic Table.
Moseley's work led to both the modern definition of atomic number and the
recognization that atomic number, not atomic mass, is the basis for the
organization of the Periodic Table.
Moseley's studies provided experimental justification for Mendeleev's
ordering of the Periodic Table by properties rather than strictly by atomic
mass. For example, the order Mendeleev chose for tellurium and iodine is
justified by the atomic numbers of 52 for tellurium and 53 for iodine. A
similar reversal of argon (Ar, atomic mass 39.95, atomic number 18) and
potassium (K, atomic mass 39.10, atomic number 19) can be seen in the
modern Periodic Table.
Today, Mendeleev's principle of chemical periodicity is correctly stated in
what is known as the periodic law: the physical and chemical properties of
the elements are periodic functions of their atomic numbers. In other words,
when the elements are arranged in order of increasing atomic number,
elements with similar properties recur at regular intervals.
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