Part I. Pre-history

THE DEVELOPMENT OF THE PERIODIC TABLE

1. The differentiation of chemical substances into two groups, elements and compounds, was achieved at the end of the eighteenth

century. Many researchers in the middle ages tried to classify elements according to the different systems. A long time was required for the recognition of the fact that the elements can be classified in the way now described by the periodic law.

2. The first step was taken in 1817, when the German chemist J.W. Dobereiner (1780 – 1849) showed that the combining weight of strontium lies midway between the combining weights of the two related elements calcium and barium. Some years later he recognized the existence of other "triads" of similar elements (chlo­rine, bromine, and iodine; lithium, sodium, and potassium). Other chemists then showed that the elements could be classified into groups consisting of more than three similar elements. Fluorine was added to the halogens, and magnesium to the alkaline-earth metals. Oxygen, sulfur, selenium, and tellurium had been classed as one family, and nitrogen, phosphorus, arsenic, antimony, and bismuth as another family of elements by 1854.

3. In 1862 the French chemist A.E.B. de Chancourtois arranged the elements in the order of atomic weight on a helical curve in space with corresponding points on the successive turns of the helix differing by 16 in atomic weight. He compared corresponding points and suggested that "the properties of elements are the properties of numbers".

4. The English chemist J.A.R. Newlands in 1863 proposed a system of classification of the elements in order of atomic weights in which the elements were divided into seven groups of seven elements each. He termed his relation the law of octaves by analogy with the seven intervals of the musical scale. His proposal was ridiculed and he did not develop it further.

9. Read the second part of the text and answer the following questions:

1. Do you use the first version of Mendeleyev’s Periodic Table?

2. Why did Mendeleyev revised his table?

3. Who created resemble system independently?

4. Why was the periodic law proposed by Mendeleyev accepted immediately?

Part II. Mendeleyev’s Peroidic System.

5. The final and most important step in the development of the periodic table was taken in 1869, when the Russian chemist Dmitriy Mendeleyev (1834 – 1907) made a thorough study of the relation between the atomic weights of the elements and their physical and chemical properties with special attention to valence. Mendeleyev proposed a periodic table containing seventeen columns, resembling in a general way the periodic table with the noble gases missing. In 1871 Mendeleyev then revised this table placing a number of elements in different positions, corresponding to revised values of the atomic weights.

6. At the same time (1871) he and the German chemist Lothar Meyer, who was working independently, each proposed a table with eight columns, obtained by split­ting each of the long periods into a period of seven elements, the eighth group containing the three central elements (such as Fe, Co, Ni), and a second period of seven elements. The first and the second periods of seven were later distinguished by use of the letters “a” and “b” attached to the group symbols which were the Roman numerals.

7. The periodic table in the second form proposed by Mendeleyev remained very popular, and is only now being replaced by the "long-period" form.

8. The periodic law was accepted immediately after its proposal by Mendeleyev, because of his success in making predictions with its use which were afterward verified by experiment. A very striking application of the periodic law was made by him. He was able to predict the existence of six elements which has not yet been discovered. He named these elements eka-boron, eka-aluminum, eka-silicon, eka-manganese, dvi-manganese, and eka-tantalum (Sanskrit: eka, first; dvi, second). Three of these elements were soon discovered (scandium, gallium and germanium), and it was found that their properties and the properties of their compounds are very close to those predicted ones. Since then the elements technetium, rhenium and polonium have been discovered or made arti­ficially (in the case of technetium) and have been found to have properties similar to those predicted for eka-manganese, dvi-manganese, and eka-tantalum.

10. Read the third part to complete your knowledge about the further development and changes with the information about:

a) the scientists contributed greatly to the development of the periodic table;

b) the groups of elements added;

c) correlations made later.

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