Multimedia Periodic Table/970521_0945.iso/STTUT/PERIOD8.TXT

Text File | 1993-07-27 | 2KB | 26 lines

There is a profusion of different chemical behaviour amongst the elements.
Much of this diversity is still unresolved, but if you look carefully some patterns can
be discerned. In the early 1860s a Chemistry conference was held at Kahrlsruhe in
Germany. A paper was given by the Italian chemist Stanislao Cannizaro
on the atomic weights of the elements, now called relative atomic mass.
|Mendeleev| was at the conference and he became convinced that these atomic weights
held the key. Afterwards he gathered chemical and physical data for all the elements and
listed these on individual cards - their atomic weight, their physical state, melting point,
boiling point and their reactions with oxygen, hydrogen and chlorine. He
arranged the cards in rows according to increasing atomic weight, and columns according
to similarities in their properties.
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If an element in atomic weight sequence did not fit he moved it to the next group.
In this way he identified gaps in his table; these were missing elements
for which he was able to predict properties.
One such gap was after zinc.
In 1869 he published his proposal and a few years later gallium was
discovered; its measured and predicted properties were an almost perfect match.
Simply stated the periodic law says that if the elements are arranged in order of
their increasing atomic weight then at certain recurring intervals or periods there occur
elements with similar properties. But the law is seldom perfect; some
properties follow it better than others and there are often anomalies. It cannot be used
for precise prediction but serves well for mental organisation, and it is invaluable in
encouraging a structured approach to the study of chemistry.