2. Decide what books the texts come from. What helped you to make up your mind? Choose from the following:

1. General, Organic, and Biochemistry;

2. Organic Chemistry;

3. Chemistry: the science in context.

3. How do these texts differ? What do they have in common?

4. Entitle the texts.

5. Tell what was said in the texts about:

Foundation of organic chemistry, "organic" and "inorganic" substances, vitalistic theory, chemistry today, organic chemistry, carbon, organic compounds, cotton, plastics, Kevlar, Styrofoam, chemical signals, enzymes, matter, biochemistry, inorganic chemistry, analytical chemistry, physical chemistry.

Section B

1. Modern picture of chemical bonding is the title of the text in this section. Look carefully at the title and choose the description you think is closest to the likely content of the article:

- Ionic bonds;

- Covalent bonds;

- Molecular orbital theory;

- Development of chemical bonding theory.

2. The first sentences of each paragraph of the text are printed below. Read the sentences to get an impression of the main ideas of the text.

	The simplest kind of chemical bonding is that between an electropositive element (low IE) and an electronegative element (large negative EA).
	We know through empirical observation that eight electrons (an octet) in the outermost electron shell impart special stability to the inert-gas elements in Group 0: Ne (2 + 8); Ar(2 + 8 + 8); Kr (2 + 8 + 18 + 8).
	A simple shorthand way of indicating covalent bonds in molecules is to use what are known as Lewis structures, or electron-dot structures.
	The amount of energy it takes to pull an electron away from an atom is called the ionization energy (IE)of the element.
	Elements on the left and right sides of the periodic table form ionic bonds by gaining or losing an electron to achieve an inert-gas configuration.
	Lewis structures are valuable because they make electron "bookkeep­ing" possible and constantly remind us of the number of outer-shell electrons (valence electrons)involved.
	What is the modern picture of chemical bonding?
	Just as the electropositive alkali metals at the left of the periodic table have a tendency to form positiveions by losingan electron, the halogens (Group VIIA elements) at the right of the periodic table have a tendency to form negativeions by gainingan electron.

3. Which sentence could be the opening sentence of the text?

4. Think about the first sentences above and decide which you think are likely to introduce a paragraph with:

- electropositive elements;

- electron affinity;

- Kekule structures.

5. Read the text and match the first sentences with the paragraphs. For this read the missing sentences and look for ideas/topics that might link them to the paragraph. Look for words and phrases that are repeated or 'echoed' in the paragraphs or the missing sentences.

1_____________________Why do atoms bond together, and how does the quantum-mechanical view of the atom describe bonding? The whyquestion is relatively easy to answer: Atoms form bonds because the compound that results is more stable (has less energy) than the alternative arrangement of isolated atoms. Energy is always releasedwhen a chemical bond is formed. The howquestion is more difficult. To answer it, we need to know more about the properties of atoms.

2_____________________We also know that the chemistry of many elements with nearlyinert-gas con­figurations is dominated by attempts to achieve the stable inert-gas elec­tronic makeup. The alkali metals in Group I, for example, have single s electrons in their valence shells. By losing this electron, they can achieve an inert-gas configuration.

3_____________________ Alkali metals, at the far left of the periodic table, give up an electron easily, have low ionization energies, and are thus said to be electropositive. Elements at the middle and far right of the periodic table hold their electrons more tightly, give them up less readily, and therefore have higher IE's. In other words, a low IE corresponds to the ready loss of an electron, and a high IE corresponds to the difficult loss of an electron.

4______________________ By so doing, the halogens can achieve an inert-gas configuration. The measure of this tendency to gain an electron is called the electron affinity (EA).Energy is released when an electron is added to most elements, and EA's are therefore negative numbers. Elements on the right of the periodic table have a much greater tendency to add an electron than elements on the left side and are said to be elec­tronegative.Thus, the halogens release a large amount of energy when they react with an electron and have much larger negative electron affinities than the alkali metals.

5______________________ For example, when sodium metal [IE = 118 kcal/mol(494 kJ/mol)] reacts with chlorine gas [EA = -83.2 kcal/mol(-348 kJ/mol)], sodium donates an electron to chlorine forming positively charged sodium ions and negatively charged chloride ions. The product, sodium chloride, is said to have ionic bonding.That is, the ions are held together purely by electrostatic attrac­tion between the two unlike charges. A similar situation exists for many other metal salts such as potassium fluoride (K⁺F), lithium bromide (Li⁺Br⁻), and so on. This picture of the ionic bond, first proposed by Walter Kossel in 1916, satisfactorily accounts for the chemistry of many inorganic compounds.

6______________________ How, though, do elements in the middle of the periodic table form bonds? Let's look at the carbon atom in methane, CH₄, as an example. Certainly the bonding in methane isn't ionic, since it would be very difficult for carbon (ls²2s²2p²) either to gain or to lose four electrons to achieve an inert-gas configuration. In fact, carbon bonds to other atoms, not by donating elec­trons, but by sharingthem. Such shared-electron bonds, first proposed in 1916 by G. N. Lewis, are called covalent bonds.The covalent bond is the most important bond in organic chemistry.

7_______________________In this method, the outer-shell electrons of an atom are represented by dots. Thus, hydrogen has one dot representing its 1s electron, carbon has four dots (2s²2p²), oxygen has six dots (2s²2p⁴), and so on. A stable molecule results whenever the inert-gas configuration is achieved for all atoms.

8_____________________ Simpler still is the use of "Kekule" struc­tures, also called line-bond structures, in which a two-electron covalent bond is indicated simply by a line drawn between atoms. Pairs of nonbonding valence electrons are often ignored when drawing line-bond structures, but you must still be mentally aware of their existence.