The clcctronic structure of molcculcs of covalcnt compounds involving the principal groups of the periodic table can usually be written by counting the number of valcncc electrons in the molcculc and then distributing the valcncc electrons as unshared electron pairs and shared electron pairs in such a way that cach atom achieves an argonic structure.
For many molcculcs the covalcncc of cach atom is equal to the number of unpaired electrons in its outer shell, and is, thus, simply related to the position of the clement in the periodic table. For other molecules and ions the covalcncc of the atoms is less simply related to the periodic table.
It is often ncccssary to have some experimental information about the way in which the atoms arc bonded together. Thus, there are two compounds with the composition C2H60: ethyl alcohol and dimethyl ether. The chcmical properties of these two substanccs show one of them, ethyl alcohol, to contain one hydrogen atom attached to an oxygen atom, whereas dimethyl ether doesn't contain such a hydroxyl group.
The atoms of most molcculcs are held tightly together by a very important sort of bond, the sharcd-clcctron-pair bond or the covalcnt bond. This bond is so important, so nearly universally present in substancc that Professor Gilbert Newton Lewis of the University of California (1876- 1946), who discovered its electronic structure, callcd it the chemical bond.
It is the covalcnt bond that is represented by a dash in the valcnce- bond formulas, such as
CI
I
Br—Br and CI — С — CI
I
CI
that have been written by chemists for over a hundred years.
Modern chemistry has been greatly simplified through the development of the theory of the covalcnt bond. It is now easier to understand and to remember chemical facts, by connecting them with our knowledge of the nature of the chemical bond and the electronic structure of molcculcs, than was possible at the beginning of the ccntury.
Words and Word-Combinations to Be Memorized
achieve, argon, such a, connect, ether, ethyl, experimental, greatly, information, nearly, professor, simplify, sort, universally, university, covalent, in such a way, in this way
Ex* 5. Give the Russian equivalents for the following:
the principal groups of the periodic table, count the number of valence electrons, distribute the electrons, unshared electron pairs, in the outer shell, be related to, it is often necessary to, be bonded together, attach one atom to another, a hydroxyl group, be held together, represent the bond by a dash, over a hundred years, it is now easier to understand, the theory of the covalcnt bond
Ex* 6. Give the English equivalents for the following:
ковалснтнос соединение, поделенная электронная пара, таким образом, быть равным чему-л., положение элемента в периодической таблице, этиловый спирт, присоединить к, ковалентная связь, Калифорнийский университет, Санкт-Петербургский университет, соврс- мснная химия, упростить что-л., запомнить факты, природа химической связи, электронная структура молекулы, быть возможным, в начале столетия
Ex. 7. Fill in the blanks with prepositions where necessary*
1.... many molcculcs the covalcncc ... each ... atom is equal... the number ... unpaired electrons ... its outer level. 2. There are some other molecules ... which the covalcncc ... the atoms is not dircctly related ...
the periodic table. 3_ ethyl alcohol one hydrogen atom is attached... an
oxygen atom. 4. Dimethyl ether does not contain ... a hydroxyl group.
5. The atoms ... most molecules arc held together ... a covalcnt bond.
6. Modern chemistry was greatly simplified ... the development... the thcoiy ... the covalcnt bond.
Ex. 8. Translate the sentences into Russian, paying attention to different functions of one.
1. There is some difference as regards solubility of gases in liquids, of two gases the one that is more readily liquefied is the more soluble. 2. The transition elements arc the ones that have their inner shells partly filled.
3. Ethyl alcohol contains one hydrogen atom attached to an oxygen atom.
4. Now one can understand chcmical phenomena better by connccting chemical facts with the nature of the chcmical bond. 5. Reliable experimental data enable one to draw proper conclusions. 6. This approach is quite similar to the one just described. 7. The procedure is a simple one. 8. One finds the reason for this similarity in the periodic table. 9. It takes
Ь one much time to make all the calculations without a computer. 10. It is one of the simplest methods and it is often recommended to the students.
Ex. 9. Translate the sentences into Russian.
1. Everybody considers him to be an expert in his field. 2. Mendeleyev believed some elements to be missing in his periodic table and he even predicted their properties. 3. The teacher expected us to finish the work at 5, but we couldn't solve the problem. 4.1 suppose the paper to have been already typed. 5. They saw him pour the liquid into the test-tube and then heat the tube over the burner. 6. She heard somebody call her and went to the door. 7. We waited for the solution to boil. 8.1 knew him to have passed the exam in mathematics successfully, that is why I asked him to help me. 9. We should expect the atomic weight of sulphur to be greater than that of oxygen. 10. The ancicnt scientists believed earth, water, air and fire to be elements. 11. Dalton thought a water molecule to consist of one hydrogen and one oxygen atom. 12. Certain conditions must be observed to make nitrogen react with other elements. 13. Sometimes, the prcscncc of the catalyst causes the two elements to unite and form a compound. 14. Dr. N thought high stability of the compound to be due to the prcscncc of an admixture ions. 15. Let's assume the volume of the gas to be equal to jc. 16. He was made to stay in the
laboratory and finish cleaning. 17. Pressure causes gas to comprcss.
18. The obtained results led us to draw the following conclusion.
19. Under ordinary conditions it is not always easy to get some elements to combine with other elements. 20. They were forccd to use an old apparatus because the new one was out of order. 21. Qualitative analysis enables the composition of a sample to be determined. 22. Mcndclcycv's periodic table allows the suppositions about the atomic structure to be made. 23. Experiments sometimes permit some useful information about the chcmical bond to be obtained.
Ex. 10. Translate the sentences into English.
1. Знание электронной структуры атомов позволяет определить характер связи. 2. Ковалснтность каждого атома в молекуле может быть в большей или меньшей степени связана с его положением в периодической таблице. 3. Существуют соединения, имеющие одинаковый состав, но разную структуру. 4. Одни из примеров таких соединений — это этиловый спирт и димстиловый эфир, оба они могут быть написаны одной и той же формулой — С2Н60.5. В молекулах атомы удерживаются вместе посредством связей, одна из наиболее распространенных связей — ковалентная. б. Эта связь очень шггсрссна с точки зрения ее электронной структуры.
Ex. 11. Answer the following questions:
1. In what way is it possible to count the number of valcncc electrons in the molcculc? 2. What rule helps us to determine the distribution of valcncc electrons in electron pairs? 3. What attracts the attention of the chcmists as regards such compounds as ethyl alcohol and dimethyl ether? 4. What kind of bond is usually called a covalent bond? 5. What is die usual way of writing structural formulas of molcculcs?
Section П
Упр. 1. Назовите значения следующих интернациональных слов:
conduct, electricity, indication, orbital, spin, energy, control, configuration, stability, characteristic
Упр. 2. Определите значения выделенных слов по контексту.
1. Wc saw her on the opposite side of the street and crossed the street to meet her. 2. In terms of modem chcmistry it is more correct to speak about the clcctronic levels than clcctronic shells. 3. The main duty of a student is to study.
Сюва к тексту:
loose—высвобождать; desire—желать; goal — цель; seek—искать, стремиться; in turn — в свою очередь
Text 20 В
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