Chemical bonds Lewis diagrams

The formalism of connectivity indices is an embodiment of graph theory. Connectivity indices are intuitively appealing because each index can be calculated exactly from valence bond (Lewis) diagrams familiar to organic chemists, which depict molecular structure in terms of atoms, inner shell and valence shell electrons, valence shell hybridization, o and k electrons, bonds and lone pairs. The indices can then be correlated with physical or chemical properties of interest. Connectivity indices have, in the past, been very useful in treating molecular systems with well-defined chemical formulae and fixed numbers of atoms [24,25]. 

Lewis theory A simple theory for chemical bonding involving diagrams showing bonds between atoms as lines or dots. In this theory, atoms bond together to obtain stable octets (8 valence electrons). 

The term resonance does not mean that the molecule physically oscillates back and forth from one of these bonding structures to the other. Rather, within the limitations of the Lewis dot model of bonding, the best representation of the actual bonding is a hybrid diagram that includes features of each of the acceptable individual diagrams. This awkwardness can be avoided by using molecular orbital theory to describe chemical bonding (see Chapter 6). 

Urea is an important chemical fertilizer with the chemical formula (lT2N)CO(NH2)- The carbon atom is bonded to both nitrogen atoms and the oxygen atom. Draw a Lewis diagram for urea and use Table 3.6 to estimate its bond lengths. 

Methyl isocyanate, which was involved in the disaster in Bhopal, India, in 1984, has the chemical formula CH3NCO. Draw its Lewis diagram, including resonance forms. (Note The N atom is bonded to the two C atoms.)... 

The two-orbital interaaion diagrams (1-20 to 1-23) enable us to establish a link between the idea of a bonding pair in the Lewis sense and the MO description. The bondingpair corresponds to double occupation of the bonding MO with the antibonding MO empty. There is thus a single bond in H2 (identical orbitals, 1-20) and in HeH" " (different orbitals, 1-22). However, if four electrons are involved, the antibonding orbital is doubly occupied and no chemical bond exists between the two atoms. This is the situation in He2, for example (identical orbitals, 1-21), and HeH (different orbitals, 1-23), species where the two atoms remain separate. 

With the aid of computers, molecular orbital theory can be applied to polyatomic molecules and ions, yielding results that correlate very well with experimental measurements. These applications are beyond the scope of this text. However, the delocalization of electrons over an entire molecule is an important contribution of molecular orbital theory to our basic understanding of chemical bonding. For example, consider the Lewis structure and valence bond diagram of ozone ... 

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