The periodic table

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history:

The first scientific discovery of an element was the discovery of Phosphorus in 1649 by Hennig Brand.  By 1869, 63 elements were known, and as this number grew, patterns in the properties of these elememnts were noticed and a number of classification systems were devised.

The Law Of Octaves:

In 1863 John Newlands classified the known 56 elements into 11 groups based on similar physical properties.  He noticed that many pairs of similar elements existed with atomic weights that differed by a multiple of eight, and hence he proposed the Law of Octaves, based on an analogy with the musical scale.

The Law Of Octaves -- Any given element will exhibit analogous behaviour to the eighth element following it in the table.

The Law Of Triads:

Johann Dobereiner, in 1817, noticed that Strontium had similar chemical properties to Calcium and Barium, and that its atomic weight fell midway between the two.  More work in this area led to him discovering the halogen triad of Chlorine, Bromine and Iodine, and the alkali metal triad of Lithium, Sodium and Potassium.  In 1829 he proposed the Law Of Triads.

The Law Of Triads -- Nature contains triads of elements where the middle element has properties that are an average of the other two members of the triad when ordered by the atomic weight.

Extensive research into this Law of Triads found that these kind of chemical relationships extended beyond the triad, Fluorine was added to the halogen group, and Oxygen, Sulphur, Selenium and Tellurium were all grouped together, as were Nitrogen, Phosphorus, Arsenic and Bismuth.

Dmitri Mendeleev developed the periodic table while working as a professor of Chemistry at St. Petersburg University.  He noticed patterns in the properties and atomic weights of halogens, alkali metals and alkaline metals.  He also spotted similarities between the series of Cl - K - Ca, Br - Rb - Sr, and I - Cs - Ba.  To extend this pattern to other elements he made up cards, each card had an element symbol, its atomic weight and its properties.  He had a card for each of the 63 known elements, and he arranged the cards in order of ascending atomic weight, and grouping elements of similar properties together.  This led to Mendeleev publishing his table and a statement of periodic law in 1869.