The Periodic Table of Elements - DOC

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					                       The Periodic Table of Elements

Element -    a substance that cannot be broken down into simpler substances
             by ordinary chemical means.

Compound - A substance of two or more elements in fixed proportions.
           Compounds can be decomposed into their constituent elements.

The development of the periodic table is generally
attributed to this man, Dmitri Ivanovich Mendeleev.
Scientists had identified over 60 elements by
Mendeleev’s time (today over 110 elements are
known.)

First Mendeleev created a chart that listed the known
elements according to increasing atomic weight. He
then organized the table in the rows and a pattern
became apparent – but only if he left blanks. When he
left blanks elements with similar chemical properties
appeared in vertical columns on the table. He was bold
enough to suggest that new elements not yet discovered would be found to fill in
the blanks and they were soon discovered. These new elements were found to
match the description that Mendeleev predicted for them. Below is a copy of
what the original periodic table he formed looked like.
However, Mendeleev is NOT the only one to realize this periodicity of the
elements. At the same time Lothar Meyer made a chart that plotted atomic
volumes against atomic weight and this is the result he saw:


                                                              He noticed there
                                                              was a repeating
                                                              pattern, but the
                                                              length of that
                                                              pattern changed.
                                                              Hydrogen by itself,
                                                              then 7, then 7, then
                                                              17, then 17. This
                                                              was prior to the
                                                              discovery of noble
                                                              gases – so the
                                                              current length of
pattern is 2, 8, 8, 18, 18.


Both men came up with the same idea, but Mendeleev is generally given credit.
Even though both men were able to organize and predict the properties of
elements still undiscovered – neither one knew why the atoms exhibited this
periodicity.

The reason elements have periodicity is that their properties are based off the
number of electrons in their valence electron. Elements with only one atom in
their valence electron will tend to lose it so they can drop down to a lower outer
shell. Noble gases, which have full valence shells and are very stable and
unreactive, were not discovered until 1894 by William Ramsay.

				
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posted:8/9/2012
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