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Exercise 1. Read the text





ATOMS, MOLECULES AND COMPOUNDS

Water is abundant on Earth today - as a gas, as a liquid, and as a solid. The physical state of water depends on temperature. If the temperature is high enough, water is a gas. If the temperature is low enough, water freezes to ice. Although the forms of water may

vary, its chemical compositions remains the same. Water has some unusual physical and chemical properties that have had a powerful effect on the evolution of life. A close look at the chemistry of water can provide information about the chemical nature of matter and about the function of biological organisms.

 

Suppose you found a way to subdivide a drop of water

into smaller and smaller droplets until you could not even see them under a microscope. No matter how small the water droplet, it would still be made of identical units called molecules. Molecules of water

are the smallest units into which water can be subdivided and still have the essential chemical properties of water. When an electric current flows through water under proper conditions, a remarkable change occurs: water becomes two gases. One is the lightest gas, hydrogen, which burns with a very hot flame in air. The other gas is oxygen. Any burning object thrust into

oxygen will continue to burn with a more brilliant flame. Thus, under certain conditions, water molecules can break down into two different substances, hydrogen and oxygen. Neither hydrogen nor oxygen has the appearance or any other property of water.

Molecules are made of atoms that have been chemically combined. Molecules may be made from more than one kind (as in water), or they may be made from atoms of the same kind. For example, hydrogen gas consists of hydrogen atoms that exists in combination with each other. A molecule of hydrogen has two hydrogen atoms. The same arrangement is true of an oxygen molecule, which has two oxygen atoms (Oxygen also forms molecules of ozone that contain three atoms of oxygen).

A substance made of only one kind of atom is called an element. More than 100 different elements are known today. A substance made of two or more different kinds of atoms, such as water, is called a compound. Elements can combine chemically in many ways to form the millions of compounds that give Earth variety of materials. Chemists have given each element a symbol of letters from the element’s name. H stands for hydrogen, O for oxygen, C for carbon, and N for nitrogen. Iron, however, is Fe, derived from the word ferrum, reflecting that some symbols come from an element’s Latin or Greek name.

Exercise 2. Master the active vocabulary:

hydrogen [´haidr∂d (∂)n] - водород

nitrogen [´naitr∂d (∂)n] - азот

oxygen [´oksid (∂)n] - кислород

carbon [´ka:b(∂)n] - углерод

ozone [´∂ z∂ n] - озон

compound [´kompa nd] - состав, соединение

 

Exercise 3. Answer the questions:

 

1) What does the physical state of water depend on?

2) Under what conditions does water freeze to ice?

3) What are molecules?

4) What remarkable change occurs when an electric current flows through water?

5) What does hydrogen gas consist of?

6) What is an element?

7) How many elements are known today?

Exercise 4. Give English equivalents for the following:

жидкость; химические свойства воды; мельчайшие частицы; электрический ток; водород; кислород; соединение; отражать.



 

Exercise 5. Give Russian equivalents for:

to freeze to ice; to have a powerful effect on; biological organisms; under proper conditions; to derive.

Exercise 6. Read the text.

CHEMICAL REACTIONS AND CHEMICAL BONDS.

When two or more atoms react to form compounds, chemical bonds are formed by

the attraction, sharing, or transfer to outer electrons from one atom to other. Such bonds between atoms can be broken, the atoms rearranged, and new bonds formed. A chemical reaction involves the making and breaking of chemical bonds. Chemical reactions are always occurring in living organisms. Living organisms are sometimes compared to chemical factories, bat the number and complexity of chemical reactions that occur in living organisms are vastly greater than in factory.

In chemical reactions, substances interact and form new bonds and new substances. These events are important in a cell for two reasons.

First, chemical reactions are the only way to form new molecules that the cell requires for such things as growth and tissue maintenance.

 

Second, the making and bracing of bonds involves changes in energy. As a result of chemical reactions in a cell, energy may be stored, used

to do work, or released. When atoms interact, they can form several types of chemical bonds. One type forms when electrons are transferred from one atom to another. This type of chemical bond occur in many substances, including table salt, also known as sodium chloride (NaCl). As the latter name suggests, table salt is made of two elements, sodium (Na) and chlorine(Cl). When atoms of these two elements react, an electron passed from a sodium atom to a chlorine atom. The resulting sodium atom is positively charged, for it has one less electron than protons. It becomes a sodium ion, written as Na+. The chlorine atom is negatively charged, for it has one more electron than protons. It becomes a chloride ion, written as Cl-. Note the change in name from chlorine to chloride.

As these examples suggest, an ion can be defined as an atom or a group of atoms that has acquired a positive or negative charge as a result of gaining or losing one or more electrons. By this definition molecules as well as atoms can become ions by gaining or losing electrons. Table salt consists of positively charged sodium ions and negatively charged chloride ions, each strongly attracted to the other. The attraction between oppositely charged ions forms an ionic bond.


 
 
Sodium and chlorine can react to form the salt sodium chloride (NaCl). By losing one electron, sodium becomes a positive ion, and by gaining one electron, chlorine becomes a negative ion, chlorine.


 

The chemical behavior of water indicates that the atoms do not share the electrons equality. The larger oxygen atom attracts the electrons more strongly than the smaller hydrogen atoms do. In the electrons of a bond are not shared equally, the bond is called a polar covalent bond. In contrast, the electrons in a molecule of hydrogen gas are shared equally, and the resulting covalent bond is said to be nonpolar. Polar molecules may form still another type of chemical bond. An attraction can occur between a slightly positive hydrogen atom in a molecule and a nearby slightly negative atom of another molecule (or of the same molecule if it is large enough).This type of attraction is called a hydrogen bond.

Exercise 7. Master the active vocabulary:

bond [bond ] - связь, соединение

electron [i´lektron ] - электрон

tissue [´tiu:] - ткань

chloride [´kl :raid] - хлорид

sodium [´s∂ di∂m] -натрий

chlorine [´kl :ri:n ] - хлор

ion [´ai∂n] -ион

proton [´pr∂ ton] - протон

Exercise 8. Answer the questions:

1) What does a chemical reaction involve?

2) In what organisms are chemical reactions always occuring?

3) What do substances form in chemical reactions?

4) What does the making of bonds involve?

5) How many types of chemical bonds can atoms form when they interact? Enumerate them.

6) What does table salt consist of?

7) What does the chemical behavior of water indicate?

8) What type of attraction is called a hydrogen bond?

Exercise 9. Put an appropriate expression in each blank.

 

1. When two or more atoms ............. compounds, chemical bonds are formed by ............., or transfer to outer electrons from one atom to other.

2. In chemical reactions, substances ............. and ............. new bonds and new substances.

3. As a result of chemical reactions in a cell, energy ............. used to do work, or released.

4. The resulting sodium atom ............, for it has one less electron than protons.

5. The attraction between ............. forms an ionie bond.

ORGANIC COMPOUNDS AND LIFE

Many chemical compounds besides water are needed for life to exist. The most important are organic compounds .Chemically speaking organic compounds are carbon compounds in which the carbon atoms are combined with hydrogen and usually oxygen .Organic compounds also frequently contain nitrogen, sulfur, or phosphorus .A few carbon compounds have never been included with organic compounds: carbon dioxide (C O ),carbon monoxide (C O),and carbonic acid (H C O ).

The name «organic» was coined long ago when it was thought these compounds could be formed only by living cells .Since then millions of different organic compounds have been synthesized in the laboratory. There is no longer any reason to call these compounds «organic»,but the name is so well established that it is widely used to describe nearly all carbon compounds ,essential for life or not . Carbon can combine in long chains that form the backbone of large ,complex molecules or macromolecules. The backbone of carbon atoms is called the carbon skeleton. Other atoms and molecules can attach to the carbon skeleton ,giving each macromolecule a particular structure and, therefore ,a particular function .The following sections of the chapter discuss the characteristics of the four most important classes of macromolecules-carbohydrates, lipids ,proteins, and nucleic acids.

Exercise 10. Master the active vocabulary:

compound [´kompa nd] - состав, соединение

hydrogen [´haidr∂d (∂)n] - водород

oxygen [´oksid (∂)n ] - кислород

nitrogen [´naitr∂d (∂)n] - азот

carbon [´ka:b(∂)n] - углерод

 

Exercise 11. Answer the questions:

1) What are organic compounds?

2) What elements do organic compounds contain?

3) When was the name «organic» coined?

4) What is a carbon skeleton?

Exercise 12. Find English equivalents for:

органические соединения; существовать; углеродные соединения; создавать (новые слова, выражения); основа.





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