The Periodic Table

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Claire Garfield        Open-Book Paper        Candidate No. 3257        

The Periodic Table was developed in stages; the first person that attempted to classify elements in relation to their atomic mass was Johann Döbereiner.  Döbereiner noticed similar properties between known elements.  Theses similarities occurred in groups of threes and were known as ‘triads’.  The atomic weight of the middle element in each triad is approximately an average of the others.  In 1863 John Newlands put the known elements in order of atomic weight and noticed that every eighth element had similar properties, he called this the Law of Octaves.  After about 20 elements the table became ragged and some elements had identical places whilst others were incorrect because of inaccurate weights.  Furthermore Newlands left no gaps for any unknown elements.  Dimitri Mendeleev amended some atomic weight values and left gaps for any undiscovered elements.  Mendeleev predicted properties of five elements that should be discovered, within 15 years of his predictions three of these elements had been discovered.  One of the unknown elements was called Eka-aluminium today known as Gallium.  Below a table compares the predictions made by Mendeleev about gallium with what is now known.  

Table 1 Comparing Mendeleev’s predictions

 with the properties of element 31, gallium

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Mendeleev’s prediction was close to what is known about gallium and therefore supports what he thought about the Periodic Table.  The modern Periodic Table is based largely on what Mendeleev drafted, however, it is now complete with noble gases having been discovered, and furthermore the elements are now ordered by increasing atomic number opposed to relative atomic mass.  (1+2)


Gallium is considered an unusual element as it has physical and chemical properties that suggest it could be either metal or non-metal.  The first physical property suggests that gallium is non-metal because of its low melting point.  Although ...

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