3 Part One. The Predicate

The Structure of Covalent Compounds
The electronic structure of molecules ofcovalcnt compounds involving the principal groups of the periodic table can usually be written by counting the number of valence electrons in the molecule and then distributing the valence electrons as unshared electron pairs and shared electron pairs in such a way that each atom achieves an argonic structure.

For many molecules the covalence 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 covalence of the atoms is less simply related to the periodic tabic.

It is often necessary to have some experimental information about the way in which the atoms are bonded together. Thus, there are two compounds with the composition C₆H₆O ethyl alcohol and dimethyl ether. The chemical properties of these two substances 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 molecules are held tightly together by a very important sort of bond, the shared-electron-pair bond or the covalcnt bond. This bond is so important, so nearly universally present in substance that Professor Gilbert Newton Lewis of the University of California (1876— 1946), who discovered its electronic structure, called it the chemical bond.

It is the covalcnt bond that is represented by a dash in the valence- bond formulas, such as

Cl
Br—Br and Cl — С — Cl

Cl

that have been written by chemists for over a hundred years.

Modem chemistiy has been greatly simplified through the development of the theory o f 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 molecules, than was possible at the beginning of the century.
Words and Word-Combinations to Be Memorized

achieve, argon, such a, connect, ether, ethyl, experimental, greatly, infoimation, nearly, professor, simplify, sort, universally, university, covalent, in such a way, in this way

Ex. 5. Give the Uzbek equivalents for the following:

the principal groups of the periodic tabic, count the number o f 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:

kovalent birikma, bo'lingan elektron juft, shuning uchun sm.ga teng bo'ling, davriy jadvaldagi elementning holati, etil spirti, kovalent aloqa, Kaliforniya universiteti, Sankt-Peterburg universiteti, zamonaviy kimyo, soddalashtirish. faktlar, kimyoviy bog'lanishlar tabiati, molekulaning elektron tuzilishi, asr boshida

Ex. 7. Fill in the blanks with prepositions where necessary.

1.... many molecules the covalence ... cach ... atom is equal... the number... unpaired electrons ... its outer level. 2. There are some other molecules... which the covalence... the atoms is not dircctly related...the periodic table. 3 ethyl alcohol one hydrogen atom is attached... anoxygen atom. 4. Dimethyl ether does not contain ... a hydroxyl group.5. The atoms ... most molecules are held together ... a covalcnt bond.6. Modem chemistry was greatly simplified ... the development... the theory ... the covalcnt bond.

Ex. 8. Translate the sentences into Uzbek, paying attention to different functions of one.

1. There is some difference as regards solubility of gases in liquids, of two gases the onethat is more readily liquefied is the more soluble. 2. The transition elements are the onesthat have their inner shells partly filled.3. Ethyl alcohol contains onehydrogen atom attached to an oxygen atom.4. Now onecan understand chemical phenomena better by connccting chemical facts with the nature of the chemical bond. 5. Reliable experimental data enable oneto draw proper conclusions. 6. This approach is quite similar to the one just described. 7. The procedure is a simple one.8. Onefinds the reason for this similarity in the periodic table. 9. It takes« onemuch time to make all the calculations without a computer. 10. It is oneof the simplest methods and it is often recommended to the students.

Ex. 9. Translate the sentences into Uzbek.

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 expeetcd 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 presence of the catalyst causes the two dements 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 x. 16. He was made to stay in thelaboratory 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 forced 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. Mendeleyev’s periodic table allows the suppositions about the atomic structure to be made. 23. Experiments sometimes permit some useful information about the chemical bond to be obtained.

Ex. 10. Answer the following questions:

1. In what way is it possible to count the number of valence electrons in the molecule? 2. What rule helps us to determine the distribution of valence electrons in electron pairs? 3. What attracts the attention of the chemists as regards such compounds as ethyl alcohol and dimethyl ether?4. What kind of bond is usually called a covalent bond? 5. What is the usual way of writing structural formulas of molecules?