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THE PERIODIC LAW OF CHEMICAL ELEMENTS

For a long time the possibilities of discovering some regularities in the properties of elements occupied the minds of many scientists all over the world. As a consequence in the middle of the 18th century the French chemist, Lavoisier, for the first time started the quantitative analysis of the then known chemical elements, in this way setting up the corner-stone of the chemical science.

Following his example, other chemists turned their attention to the quantitative analysis too. Among them was an English chemist, John Dalton, who, early in the 19th century, took the first steps towards the establishment of his atomic theory. Having assumed that different elements possess unlike atoms, he started investigating their relative weights Dalton's emphasis on the weights of the atoms was, indeed, his greatest contribution to science, since there was nothing new in the atomic hypothesis itself.

By the middle of the 19th century chemical elements had been classified into two general groups: metals and non-metals. But studying the properties of the elements belonging to each of the two categories, chemists arrived at the conclusion that there existed some "borderline" elements possessing the properties of both metals and non-metals. While looking for relations among the properties of elements within metal and non-metal groups, chemists were quick to realise the importance of a new concept called "valence", which expresses the relative capacity of atoms to combine with one another.

When proved experimentally the new concept was universally adopted as a reliable means for classifying the properties of elements. Thus, helped by the valence concept chemists were able to build up a numerical scale of valences of the elements, due to which they revealed an important phenomenon: despite the fact that no two elements were identical some of them did display similarities so strong that they seemed to belong to the same "family".

Thus, using further the valence concept as a guide, scientists succeeded in separating these groups and in exploring the extent of similarity within them. The growing list of elemental families combined with the evidence of atomic weight regularities within them made it possible for the investigators to correlate, systematically, the atomic weights and properties of all the then known elements. An English experimentator, J. New-lands, having compared the atomic weights and properties of the elements, proposed a law, according to which "properties are repeated at equal intervals when the elements are arranged in order of increasing atomic weight". But having anticipated the periodicity of properties, Newlands was unable, however, of systematising the properties of more than a few of the elements.

It was Dmitri Ivanovich Mendeleyev (18341907), a Russian scientist, professor of St. Petersburg University, who succeeded in building up a workable periodic classification of all the then known elements by

 

arranging them in a table. The best statement of the Periodic law is that of Mendeleyev himself : "The properties of the elements are in periodic dependence upon their atomic weights."

So being based on atomic weights of elements, Mendeleyev'sTable correlated the whole chemistry of an element to the weight of its atom. It proved that the properties of the atoms were the source of all the chemical properties of the elements, though it was some thirty years before there was any evidence a that the atom was a composite body. In the course of improving4 his Table, D. I. Mendeleyev made bold predictions of the properties of yet undiscovered elements for which he left blank spaces.

His predictions of the missing elements were based on the group characters of the families the anticipated elements were to join, on the observed regularity of properties within these families, and on the expected dissimilarities between the neighbouring elements within the periods involved.5 In direct consequence the element gallium was discovered in 1875, filling the gap immediately below zinc, scandium in 1879, filling the gap below calcium, germanium in 1886, filling the second gap below zinc. By 1900 the Table had become an indispensable part of chemical science. At present all spaces between 1 and 104 are filled. As a tribute to the originator of the Periodic law, the 101st element discovered by American scientists in 1951 was named mendeleyevum.

Having won recognition all over the world Mendeleyev's Periodic law is regarded as one of the most important achievements in the history of science. It is invariably referred to in the works of the 20th century investigators dealing with new developments in chemistry, physics, atomic theory and theoretical metallurgy.

Memorize the following words and word-combinations:

1. then known

2. they seemed to belong to the same "family" , 쒿

3. it was some thirty years before there was some evidence

4. In the course of improving

5. within the periods involved

6. properties

7. atomic theory

8. relative weight

9. possess

10.exist -

Answer the questions:

1.What occupied the minds of many scientists?

2.Who began the quantitative analysis of the chemical elements?

3.Who was John Dalton?

4.When were the chemical elements classified into two general groups?

5.What is valence?

 

TEXT 5

Read and translate the following text:

THREE STATES OF MATTER

We know that the cohesion between the molecules of a solid body is very great. A solid body retains its form.

When a solid body is heated, the motion of its molecules incomes more rapid. The cohesion among the molecules weakens; the body expands on heating. On further heating, the movement of the molecules becomes still more rapid and the attraction between them diminishes.

Finally, when the motion of the molecules has attained some velocity, the molecules begin to move among other molecules in various directions, chaotically, in disorder. The cohesion has become very much weaker. The body is no longer a solid. It has been transformed into a liquid; it has melted.

On cooling, the above described phenomena occur in exactly the reverse order.

At some temperature the density is so great that the liquid becomes a solid body.

During melting the bond between the molecules becomes considerably weaker than in the solid body. In order to separate the molecules from one another, it is necessary to overcome the attraction between them, it is necessary to perform work to destroy the bonds between the molecules. Overcoming the attraction, the molecules situated on the surface of a liquid escape from the liquid into the air. These are molecules of vapour.

The higher the temperature of the liquid, the greater the number of molecules escape from the liquid. Hence, when a liquid is heated, the rate of evaporation is increased.

The larger the surface of the evaporating liquid, the greater the number of molecules that can escape from the liquid at the same time.

The velocity of the molecules increases in proportion to the temperature of the liquid and, finally, the velocity becomes so great that the formation of vapour goes on not only on the surface but through the liquid.

Liquids whose molecules are weakly attracted to one another are easily evaporated. Such liquids are called volatile liquids. Less energy is required for their evaporation.

Attraction between molecules of vapour is practically nonexistent. As a result of this, molecules of vapour move in all directions, collide with one another and occupy a vast volume in comparison with the volume of the liquid from which they were formed.

The state of substance depends on the velocity of its molecules.

Render and memorize the following words and word-combinations:

1.cohesion

2.a solid body

3.to heat

4.rapid

5.to weaken

6.attraction

7.to diminish

8.velocity

9.to melt

10.vapour -

 

Answer the questions:

1.What is the cohesion between the molecules of the solid body?

2.When does the motion of the molecules become more rapid?

3.When has it been transformed into the liquid?

4.What are the molecules of vapour?

5.What liquids are easier evaporated?

 

TEXT 6

Read and translate the following text:

 

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