The discovery of the elements.

Chemistry Coursework

Open Book Paper

‘The nineteenth century was a golden age for the discovery of elements [1]’. This was when scientist began to look for patterns and behaviours of elements. The early periodic table was arranged by the elements relative atomic mass rather than atomic number, which is now seen in the modern periodic table. Johann Dobereiner was the first to attempt to classify elements using their RAM (relative atomic mass). He identified that in a set of three elements (then known) in a group have similar properties (e.g. Cl, Sr and Ba [1]). He calculated that the RAM of the middle element is approximately the average of the other two elements (this was before the discovery of the noble gases). But after 20 elements his table became ragged and incorrect because he had to put two elements in one space and the RAM values were incorrectly measured.

It was in 1869 when Russian chemist Dimitri Mendeleev had produced a better version of the table (see below). He altered some of the RAM values and left gaps for undiscovered elements. The gaps were a prediction for new elements and were taken as a bold policy by other scientists because if proven wrong, his explanation* will fail.

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The discovery of gallium by the French scientist, Paul. E. L. de Boisbaudran supported Mendeleev’s ideas about the periodic table because gallium fitted into one of the gaps he had left for undiscovered elements. Mendeleev called gallium eka-aluminium (see table on next page) before it was discovered, as it was one element below aluminium in his periodic table. He even predicted its chemical and physical properties. This then gave a big boost to Mendeleev's idea of the periodic table.

Table showing Mendeleev’s predictions with properties of element 31, gallium [2]

The physical properties of gallium are considered to be unusual ...