4. Exercise 6. Read the text.

5. 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.

6. 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.