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Science

The periodic table

Mendeleev's periodic table

Another table in order of relative atomic mass

Dmitri Mendeleev (1834 - 1907)

In 1869, just five years after John Newlands put forward his law of octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements known at the time in order of relative atomic massrelative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom., but he did some other things that made his table much more successful.

He realised that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.

Part of Mendeleev's periodic table

RowGroup IGroup II Group IIIGroup IVGroup VGroup VIGroup VIIGroup VIII
1H       
2LiBeBCNOF 
3NaMgAlSiPSCl 
4KCa?TiVCrMn Fe, Co, Ni, Cu

Gaps and predictions

Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered.

He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below aluminium in his table. When this element, called gallium, was discovered in 1875, its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.

Back to Patterns in properties index

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