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The periodic table of the elements is one of the most powerful tools in science; a single document that
consolidates our knowledge of chemistry.
You can find some version of the periodic table in every chemistry classroom or lab in the world. Without a
doubt, nothing else quite like it exists in the other disciplines.
The periodic table has been disputed, altered and improved as science has progressed and as new elements have
been discovered. But despite the dramatic changes that have taken place in science over the past century
(relativity and quantum mechanics) there has been no revolution in the basic nature of the periodic table.
On February 17, 1869, a Russian professor of chemistry; Dmitri Mendeleev; completed the first periodic chart. It
included 63 known elements arranged according to increasing atomic weight; Mendeleev also left spaces for elements
that he predicted would eventually be discovered.
The power of the periodic table lies in its two- or even three-dimensional display of all the known elements
(and even ones yet to be discovered) in a logical system of precisely ordered rows and columns.
The crucial characteristic of Mendeleev's system was that it illustrated a
The term "periodic" reflects the fact that the elements show patterns in their chemical properties in certain
regular intervals. These intervals or periods create groups or families of elements that have similar properties.
So, chemists only need to master the properties of a handful of typical elements; all the others fall into the
Mendeleev’s Periodic Law: The physical and chemical properties of the elements are periodic
functions of the atomic weight.
The periodic law was the basis for creating the periodic table but Mendeleev had no idea about subatomic
particles or the atomic nucleus. Thus, it was natural that he would base the periodicity of the elements on the
atomic mass rather than the positive charge (atomic number).
In 1913 English physicist Henry Moseley suggested that the ordering principle for the periodic
table was in the nuclear charge of each atom. Moseley tested his idea by photographing the x-ray spectrum of 12
elements, 10 of which occupied consecutive places in the periodic table. He discovered that the frequencies of
features called K-lines in the spectrum of each element were directly proportional to the squares of the integers
representing the position of each successive element in the table.
As Moseley put it, here was proof that "there is in the atom a fundamental quantity, which increases by regular
steps as we pass from one element to the next." This fundamental quantity first referred to as atomic number in 1920 by Ernest Rutherford is now identified as the number of protons in
Moseley's work provided a method that could be used to determine exactly how many empty spaces remained in the
periodic table. So, chemists turned to using atomic numbers as the fundamental ordering principle for the periodic
table. This change resolved many of the problems in the arrangement of the elements.
For example, when iodine and tellurium were ordered according to atomic weight (with iodine first), the two
elements appeared to be incorrectly positioned in terms of their chemical behavior. When ordered according to
atomic number (with tellurium first), however, the two elements were in their correct positions.
Mendeleev’s periodic law had to be rewritten to account for the Moseley’s discovery.
Modern Periodic Law: The physical and chemical properties of the elements are periodic
functions of the atomic number (positive charge).