Ensayo

1. Do a conceptual map based in the reading have on count the connectors and make that everything is clear.
2. Make three questions based in the reading whit answers.
3. makesure the workshop have (APA) .
4. Don´t forget to give me the workshop on Monday 27 August, 2012.

Covalent Bonds and Lewis Structures
Clearly the explanation of the chemical bond given by Kossel cannot applyto homonuclear molecules such as C12. Almost simultaneously with the publication of Kossel's theory, Lewis published a theory that could account for such molecules. Like Kossel, Lewis was impressed with the lack of reactivity of the noble gases. But he was also impressed by the observation that the vast majority of molecules have an even number of electrons, which led him to suggest thatin molecules, electrons are usually present in pairs. In particular, he proposed that in a molecule such as Cl2 the two atoms are held together by sharing a pair of electrons because in this way each atom can obtain a noble gas electron arrangement, as in the following examples:

Diagrams: of this type are called Lewis diagrams or Lewis structures. The bond between the two atoms could be called ashared-electron-pair bond but it is now universally called a covalent bond-a term introduced by Irving Langmuir (1919). In drawing Lewis structures, the core of the atom is represented by the symbol of the element and the valence shell electrons by one to eight dots, the first four arranged singly around the symbol for the core, with additional electrons used to form pairs as follows:

The completesymbol for each element can be called its Lewis symbol. The number of unpaired electrons in the symbol equals the number of bonds that the atom can form, that is, its valence. Each unpaired electron can be paired with an unpaired electron in the Lewis symbol of another element to form a shared pair or covalent bond. In this way the atoms of the elements in groups 14-17, such as C, N, 0 and F, canattain a noble gas electron arrangement as shown by the Lewis structures. The elements in groups 1, 2, and 13 such as Li, Be, and B do not, however, achieve a noble gas electron arrangement even when they form the maximum number of bonds. A covalent bond (a shared electron pair) is usually designated by a bond line rather than by a pair of dots (Figure 1.8b). As we noted earlier, and as we will discussin detail later, some elements have more than one valence. The valence given by the number of unpaired electrons in the Lewis symbol for an element, as illustrated above, is called its principal valence.
In a Lewis diagram, the pairs of electrons that are not forming bonds are called nonbonding pairs or more usually lone pairs. A lone pair is usually designated by a pair of dots but lesscommonly by a single line. In the Lewis diagrams for the CF4, NF3, OF2, and F2 molecules, each fluorine atom has three lone pairs, oxygen two, and nitrogen one. Lewis called the apparent tendency of atoms to acquire a noble gas electron arrangement, either by forming ions or by sharing electron pairs, the rule of eight. Later Langmuir called it the octet rule, and this is the term that is now generallyused. Lewis did not regard the rule of eight as being as important as the rule of two, according to which electrons are present in molecules in pairs, because he found more exceptions to the octet rule than to the rule of two. There are only a few exceptions to the rule of two, such as molecules with an odd number of electrons (free radicals), whereas there are a large number of exceptions to theoctet rule. Because CX4 molecules have a tetrahedral geometry, Lewis postulated that the four pairs of electrons in the valence shell of the carbon atom have a tetrahedral arrangement, thus giving  the four covalent bonds a tetrahedral geometry. Later, when the angular geometry of the OX2 molecules and the pyramidal geometry of NX3 molecules were established, it became clear that the directed...