Chemistry

Historic changes to the Periodic Table of Elements



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In 1803, John Dalton fathered the science of chemistry by establishing that all matter was made up of indivisible units called atoms. From that point forward, the race was on to discover and define all the different atomic configurations and subsequent elements. As more and more elements were discovered the relation ships between them began to become apparent and  a method of organizing them a necessity.

The Russian scientist Dmitri Mendeleyev, drawing on the work of other scientist making elemental discoveries in the first half of the nineteenth century, noticed certain relationships between elements which he referred to as “a periodicity of properties.” In 1869, Mendeleyev presented what amounts to as the original attempt at defining the periodic table of elements to the Russian Chemical Society. Mendeleyev’s table divided the elements into eight periods corresponding to the atomic weights of the elements. Dmitri new that there were more elements that had not been discovered and left empty spaces for them in the table.

In 1895, Lord Rayleigh discovered a new inert element he called “argon” which didn't fit in any of the existing periods of Mendeleyev's periodic chart. An eight period was added to accommodate this and the other inert gases yet to be discovered.

Then, in 1913, the British scientist Henry Moseley published a paper on measurements he had made using X-ray spectroscopy. His process made it possible to analyze elements by atomic number (number of protons and neutrons in the atoms nucleus) rather than atomic weight and was a more accurate way of classifying elements. Moseley's new method of classification ultimately caused the first major redefinition of Mendeleyev’s periodic table of elements. After Moseley's discovery, the periodic table was reorganized by atomic number instead of atomic weight. Each element in its assigned position listed the Elements abbreviation, atomic weight and atomic number.

In the nineteen twenties, the Periodic table was updated once more based on Niels Bohr’s electron shell theory. A vertical sidebar was added on the right hand side of each element block showing the number of electrons in each orbital shell.

The final change to the periodic table of elements resulted from the work of Glenn Seaborg at the University of California at Berkeley. Beside discovering most of the known elements in the Actinide series, he rearrange the periodic table to accommodate both the Lanthanide and Actinide series of elements in Group III B            

Over time, the periodic table of elements has evolved to accommodate human understanding of atomic structure. Today, the modern periodic table of elements is a key quick reference tool for chemists physicists and students of these scientific disciplines.


References:

http://www.wou.edu/las/physci/ch412/perhist.htm

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