Tuesday, April 15, 2014

Periodic Table Creation

Have you ever wondered who created the Periodic Table of Elements? As it happens to be, that man is a Russian by the name of Dmitri Mendeleev.   Although he is often regarded as the father of the Periodic Table, Mendeleev himself called his table, the Periodic System. He put elements into their correct places in the table. In some cases the relative atomic mass had been wrongly calculated by others. By correcting the relative atomic mass he put the element in the correct place.  At the time, relative atomic masses (then called atomic weights) were laboriously determined using the formula

atomic weight = equivalent weight x valency
The combining weights were generally accurate but sometimes an element was given the wrong valency. Thus beryllium, combining weight 4.6, was given the valency 3 because it was chemically similar to aluminum. This gave an atomic weight of 13.8, placing it between carbon and nitrogen where there was no space. Mendeleev said the valency was 2; the problem was solved - it fitted into the space between lithium and boron.
Mendeleev sometimes decided that atomic weights must be wrong because the elements simply appeared in the wrong place. For example he placed tellurium before iodine although its atomic weight is greater simply because iodine’s properties are so similar to those of fluorine, chlorine and bromine and tellurium’s to those of oxygen, sulfur and selenium rather than the other way round. We now know that it is atomic number, not relative atomic mass that governs an element’s position in the Periodic Table but in most cases the two result in the same order.The greatness of Mendeleev was that not only did he leave spaces for elements that were not yet discovered but he predicted properties of five of these elements and their compounds.

1 comment:

Eric Scerri said...

As a matter of fact Mendeleev predicted the existence of 16 elements altogether but only 8 were ever found.
A success rate of 50-50 is not hugely impressive by any means.

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