Part III. P.S

9. In 1871 Mendeleyev found that by changing seventeen elements from the positions indicated by the atomic weights which had been accepted for them into new positions their properties could be better correlated with the properties of other elements. He pointed out that this change indicated the existence of small errors in the previously accepted atomic weights of several of the elements, and large errors for several others, for which the atomic weight had been assumed to be correct. Experimental work verified that Mendeleyev revisions were correct. Most of the elements occur in the periodic table in the order of increasing atomic weight. However, there still remain three pairs of elements in the inverted order of atomic weight: argon and potassium, cobalt and nickel, and tellurium and iodine. The nature of the isotopes of these elements is such that the atomic weight of the naturally occurring mixture of isotopes is greater for the element of lower atomic number in each of these pairs than for the element of higher atomic number. This inversion of order from that of ato­mic weight caused much concern before the atomic numbers of the elements were discovered, but has now been recognized as having little significance.

10. The "zero" group was added to the periodic table after the discovery of the inert gases helium, neon, argon, etc., by Lord Rayleigh (1842 – 1919) and Sir William Ramsay (1852 – 1916) in 1894 and the following years. A similar form of the periodic table was devised in 1895 by the Danish chemist Julius Thomsen (1826 – 1909). After the discovery of the electron by the English physicist Sir J.J. Thomson (1856 – 1940) and the development of the theory of the nuclear atom by Ernest Rutherford (1871 – 1937), it was suggested in 1911 by the Dutch physicist. A. Van den Brock supposed that the nuclear charge of an element, which we now call its atomic number, might be equal to the ordinal number of the element in the periodic system. The young British physicist H.G.J. Moseley then determined the correct values of the atomic numbers of many elements by the study of their X-ray spectra.

11. After the discovery of helium and argon the existence of neon, krypton, xenon and radon was clearly indicated by the periodic law and the search for these elements in air led to the discovery of the first three of them; radon was then discovered during the investigation of the properties of radium and other radioactive substances. While studying the relations between atomic structure and the perio­dic law Niels Bohr pointed out that element 72 would be expected to be similar in its properties to zirconium. G.von Hevesy and D. Coster were led by this observation to examine ores of zirconium and to discover the missing element which they named hafnium.

Notes to the text:

antimony - сурьма

helical curve - спиральная кривая

valence - валентность

multiple - множитель

dilution - разбавление

11. Look through some paragraphs again and decide:

1. Which word in paragraph 4 means "laugh at"?

2. Which word in paragraph 5 means "to examine something and make corrections in it"?

3. Which word in paragraph 9 means "mistake"?

4. Which word in paragraph 9 means "changed in order or position"?

5. Which word in. paragraph 9 means "anxiety, worry"?

6. Which word in paragraph 9 means "importance"?

12.Complete the sentences with the right variant:

1. The first step in classification of elements was taken by.

a) Newlands

b) Dobereiner

c) Chancourtois

2. Chancourtois arranged elements.

a) in accordance with the law of octaves

b) into triads

c) on a helical curve

d) into columns

3. The final and most important step in the development of the
periodic table was made by.

a) Meyer ^:'

b) Mendeleyev

c) Ramsey

4. The Dutch physicist Van den Brock suggested that.

a) the determination of the correct values of the atomic numbers may be done by the study of the X-ray spectra of the elements.

b) the nuclear change of an element might be equal to the ordinal number of the element in the periodic system ….

c) the periodic table may be interpreted in terms of electronic structure of atoms.

5. The periodic law was accepted immediately after its proposal by Mendeleyev because ….

a) of the existence of small errors in the previously accepted atomic weights of several of the elements

b) many of the elements occur in the periodic table in the order of increasing atomic numbers

c) of his success in making predictions with its use which were afterward verified by experiment.

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