zumdahl_chemprin_6e_csm_ch14

zumdahl_chemprin_6e_csm_ch14 - CHAPTER 14 COVALENT BONDING:...

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540 CHAPTER 14 COVALENT BONDING: ORBITALS The Localized Electron Model and Hybrid Orbitals 9. The valence orbitals of the nonmetals are the s and p orbitals. The lobes of the p orbitals are 90 E and 180 E apart from each other. If the p orbitals were used to form bonds, then all bond angles shoud be 90 E or 180 E . This is not the case. In order to explain the observed geometry (bond angles) that molecules exhibit, we need to make up (hybridize) orbitals that point to where the bonded atoms and lone pairs are located. We know the geometry; we hybridize orbitals to explain the geometry. Sigma bonds have shared electrons in the area centered on a line joining the atoms. The orbitals that overlap to form the sigma bonds must overlap head to head or end to end. The hybrid orbitals about a central atom always are directed at the bonded atoms. Hybrid orbitals will always overlap head to head to form sigma bonds. 10. Geometry Hybridization Unhybridized p atomic orbitals l i n e a r s p 2 trigonal planar sp 2 1 tetrahedral sp 3 0 The unhybridized p atomic orbitals are used to form π bonds. Two unhybridized p atomic orbitals each from a different atom overlap side to side, resulting in a shared electron pair occupying the space above and below the line joining the atoms (the internuclear axis). 11. We use d orbitals when we have to, i.e., we use d orbitals when the central atom on a molecule has more than eight electrons around it. The d orbitals are necessary to accom- modate the electrons over eight. Row 2 elements never have more than eight electrons around them, so they never hybridize d orbitals. We rationalize this by saying there are no d orbitals close in energy to the valence 2s and 2p orbitals (2d orbitals are forbidden energy levels). However, for Row 3 and heavier elements, there are 3d, 4d, 5d, etc. orbitals that will be close in energy to the valence s and p orbitals. It is Row 3 and heavier nonmetals that hybridize d orbitals when they have to. For phosphorus, the valence electrons are in 3s and 3p orbitals. Therefore, 3d orbitals are closest in energy and are available for hybridization. Arsenic would hybridize 4d orbitals to go with the valence 4s and 4p orbitals, whereas iodine would hybridize 5d orbitals since the valence electrons are in n = 5.
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CHAPTER 14 COVALENT BONDING: ORBITALS 541 12. Rotation occurs in a bond as long as the orbitals that go to form that bond still overlap when the atoms are rotating. Sigma bonds, with the head-to-head overlap, remain unaffected by rotating the atoms in the bonds. Atoms that are bonded together by only a sigma bond (single bond) exhibit this rotation phenomenon. The π bonds, however, cannot be rotated. The p orbitals must be parallel to each other to form the π bond. If we try to rotate the atoms in a π bond, the p orbitals would no longer have the correct alignment necessary to overlap.
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zumdahl_chemprin_6e_csm_ch14 - CHAPTER 14 COVALENT BONDING:...

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