1. Дайте відповіді на запитання:

1. What certain natural groupings are there among the chemical elements? 2. What have chemists tried to do in order to understand better these groupings? 3. By whom was one of the first attempts to fit the relationships among the elements made? 4. By whom was developed the concept of the periodic repetition of properties? 5. What columns are there in the Periodic Table? 6. What did Mendeleyev predict? 7. What are the ways the Periodic Table can be used?

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The story of how D. I. Mendeleyev established the Periodic System of Elements has long been a matter of great interest to research workers.

When Mendeleyev began to teach at St. Petersburg University, chemistry was still far from being the well-ordered and harmonious branch of science that we know today.

The great majority of scientists were firmly convinced that atoms of different elements were in no way connected with each other, and that they were quite independent particles of nature. Only a few advanced scientists realized that there must be a general system of laws which regulates the behaviour of atoms of each and every element. However, the few attempts made by Beguyer de Chancourtois, Newlands, Lother Meyer and others to find a system of laws controlling the behaviour of atoms were unsuccessful and exercised no influence on Mendeleyev, the future founder of the Periodic System of Elements.

"Mendeleyev was a man who could not bear any kind of disorder and chaos," writes Academician A. A. Boikov. "This is why at the beginning of his course in chemistry at St. Petersburg University, where he had been appointed to the department of chemistry, D. I. had to establish order in the chemical elements."

By comparison of chemical properties of different elements researchers had long ago discovered that elements could be placed in several groups according to similarity in their properties.

Mendeleyev applied in his system the principles that he developed and included in his table the listing of the elements according to increasing weights.

Because he had the insight to see that many elements had not yet been discovered, he left open spaces in the Periodic Table. For example, he predicted that an unknown element with atomic weight of 44 would be found for the space following calcium. And in 1879 the Swedish chemist Lars Fredric Nilson discovered scandium.

Mendeleyev's table developed into the modern Periodic Table, one of the most important tools in chemistry. The vertical columns of the modern Periodic Table are called groups and the horizontal rows are called periods. The atomic number of an element is the number of protons in the nucleus of the atom of that element. The modern Periodic Table not only clearly organizes all the elements, it lucidly illustrates that they form "families" in rational groups, based on their characteristics.

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1. Ще раз продивіться текст та знайдіть речення, в яких автор описує наступні факти:

1. Mendeleyev could foresee the existence of new elements because he was very gifted.

2. Scientists of Mendeleyev's time didn't believe that elements are connected with each other.

3. Mendeleyev's character made him order the elements.

4. Mendeleyev's work on the Periodic Table and the Periodic Law has long interested scientists.

5. There were some scientists' attempts to find a system to order the elements but they failed.

6. Thanks to Mendeleyev modern chemistry uses the clearly developed Periodic System as the main instrument.