Text A. Brief history of periodic classification

More than one hundred and nine elements are known today. The periodic table of elements is an important landmark in the history of chemistry. It would be difficult to study individually the chemistry of all the elements and their numerous compounds. The periodic table provides a systematic and extremely useful framework for organizing a lot of information available on the chemical behaviour of the elements into a few simple and logical patterns. Several chemists have for long tried to classify the elements and to find patterns in their properties.

In 1829, John Dobereiner (German Chemist) classified elements having similar properties into groups of three. These groups were called triads. According to this law when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the other two elements. For example lithium, sodium and potassium constituted one triad. However, only a limited number of elements could be grouped into triads.

In 1865, John Newlands (English Chemist) observed that if the elements were arranged in order of their increasing atomic weights, the eighth element starting from a given one, possessed properties similar to the first, like the eighth note in an octave of music. He called it the law of octaves. It worked well for the lighter elements but failed when applied to heavier elements.

In 1869, J. Lother-Meyer in Germany gave a more detailed and accurate relationship among the elements. Lother-Meyer plotted atomic volumes versus atomic weights of elements and obtained a curve. He pointed out that elements occupying similar positions in the curve possessed similar properties.

In 1869, Dimitriy Mendeleev (Russian Chemist) arranged the 63 chemical elements, then known, according to their increasing order of atomic weights. He gave his famous scheme of the periodic classification of elements known as the periodic law. The law states that “the properties of the elements are the periodic function of their atomic weights”. It means that when elements are arranged in order of increasing atomic weights, the elements was similar properties recur after regular intervals. On the basis of this periodic law Mendeleev constructed a periodic table in such a way that the elements were arranged horizontally in order of their increasing atomic weights. Mendeleev, while studying his Periodic Table, had found that in certain cases the regularity in behaviour between two succeeding elements was not observed. In order to overcome this he had kept gaps between such elements and had predicted that the gaps would be filled by new elements, to be discovered in future.

Mendeleev put the elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table. In order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, so breaking his own rules.

Using atomic number instead of atomic mass as the organizing principle was first proposed by the British chemist Henry Moseley in 1913, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium. This observation led to the development of modern periodic law. The modern periodic law states that “the physical and chemical properties of the elements are periodic function of their atomic numbers.”

5. Comprehension check. Answer the following questions:

1. Why is the periodic table of elements an important landmark in the history of chemistry?

2. What was the main principle of forming a triad in John Dobereiner’s classification?

3. What was the problem with triads?

4. Why did John Newlands call his classification the law of octaves?

5. What was the main principle of organizing elements according to J. Lother-Meyer?

6. What does Mendeleev’s periodic law state?

7. How did Mendeleev overcome the cases when regularity of behavior between two succeeding elements was not observed?

8. What problem did Mendeleev have with tellurium and iodine?

9. What did Henry Moseley use in his classification instead of atomic mass?

10. What does the modern Periodic law state?

6. Study Text A and find the English equivalents of the following words and phrases:

1) быть важным этапом в истории; 2) имеющаяся информация; 3) приблизительно равный; 4) ограниченное; 5) обладать похожими свойствами; 6) расположенный в порядке; 7) oставить пустое место.