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Unformatted text preview: Molecular Orbitals and Curved Arrows 1 Molecular Orbitals and Curved Arrows Introduction You have already seen the Lewis theory of covalent bonding, in which covalent bonds result from the sharing of electrons between atoms, and all atoms seek to complete an octet of electrons (or, in the case of hydrogen, a duet of electrons). This theory is remarkably successful in explaining why various compounds have the structures that they do and in predicting the geometry and reactivity of many molecules. However, the Lewis theory was developed before the advent of quantum mechanics, and some of the basic results of the Lewis theory are in direct conflict with quantum mechanics. For instance, consider the formation of a hydrogen molecule from two hydrogen atoms. According to quantum mechanics, every electron must reside in an orbital, and each orbital can contain either zero, one, or two electrons. Thus, each H atom has a single electron in a 1s orbital, and the resulting H 2 molecule has a pair of electrons that must reside in some sort of shared orbital: H + H H H a 1s orbital that contains a single electron a 1s orbital that contains a single electron a shared orbital that contains a pair of electrons This interpretation, however, violates one of the basic principles of quantum mechanics, which is that in any process, the total number of orbitals cannot change . We cant have a process that starts with two orbitals (one on each atom) and ends up with one shared orbital. Somehow, our final product (the H 2 molecule) must have two orbitals. It turns out that the missing orbital is extremely important in chemical reactions, but Lewis theory gives no indication that this missing orbital exists. Moreover, although the octet rule does give excellent predictions of the structures of the vast majority of all chemical compounds, there is no explanation within Lewis theory of why the presence of eight electrons is preferred. Lewis theory cannot Molecular Orbitals and Curved Arrows 2 truly explain , for instance, why we cant form a molecule He 2 by combining the electrons in two He atoms: He + He He He a 1s orbital that contains a pair of electrons a 1s orbital that contains a pair of electrons two orbitals that each contains a pair of electrons Indeed, the combination of two He atoms to form an He 2 molecule (with a double bond) seems perfectly reasonable from the simple vantage of sharing electrons. One would think that if sharing one pair of electrons (in H 2 ) is good, then sharing two pairs of electrons (in He 2 ) must be even better. Of course, the molecule He 2 is not stable, but Lewis theory cannot explain why this molecule does not exist....
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