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Unformatted text preview: 1 Molecular Structure I. Valence Bond Theory A. General 1. We will consider the covalent compounds formed by the interactions of nonmetals. 2. They interact by sharing electrons between them. 3. Two theories, the valence bond theory and the molecular orbital theory , have been developed to analyze these interactions and rationalize some important properties of covalent compounds. B. Valence Bond: Orbital overlap and the covalent bond. The H 2 molecule 1. Consider two hydrogen atoms forming an H 2 molecule. Originally each H has an electron in a spherically symmetric 1s orbital. Consider what happens as the two H atoms approach one another. Overlap region H A 1s A 1 H B 1s B 1 H 2 Lewis diagram H H H : H or HH 2. When the two atoms get close enough, their electron clouds can interpenetrate one another, that is, the orbitals can overlap so that there are common regions where both 1s wave functions are sizable. a. When an electron enters this region of overlap, it will lose its identify and could just as well move about nucleus A or B. In this way the two electrons are shared by both nuclei. b. This sharing results in a lowering of the energies of the electrons, because 1) the potential energies decrease because the electrons can be close to two nuclear centers. 2) the kinetic energies of the electrons decrease because they can occupy larger regions in space. c. The energy lowering gives rise to a directional force of attraction between the two atoms, called a covalent bond or an electron pair bond . 3. We can think of a covalent bond being formed by the overlap of two half-filled orbitals, one on each atom. It is also possible to form a covalent bond by the overlap of a vacant orbital on one atom with a filled orbital on another, such a covalent bond is called a 2 coordinate covalent bond . In either case, the covalent bond involves two orbitals and two electrons. 4. Since the two electrons in a bond can occupy the same orbital, their spins must be paired. C. Consider two F atoms forming an F 2 molecule. 1. The electron configuration of F is 1s 2 2s 2 2p 5 . The valence electrons are distributed as 2s 2p 2. The two half-filled p orbitals can overlap to form a FF bond. 3. In the F 2 molecule, each F atom will have four electron pairs associated with it, one shared pair and three other pair that are not shared. 4. The unshared pairs are called lone pairs . D. Summary. 1. Consider only the valence electrons. 2. Molecule is held together by adjacent atoms sharing pairs of electrons, giving a series of electron pair bonds. The shared electrons are said to be delocalized about the two atoms. 3. The sharing is accomplished by the overlap of an orbital on one atom and an orbital on an adjacent atom (two half-filled or one vacant and a completely filled orbital). The greater the overlap of the orbital, the stronger the bond. Since both electrons can occupy the same orbital, their spins must be paired. same orbital, their spins must be paired....
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