Chpt 9 Solutions

# Chpt 9 Solutions - CHAPTER 9 COVALENT BONDING ORBITALS...

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301 CHAPTER 9 COVALENT BONDING: ORBITALS Questions 9. In hybrid orbital theory, some or all of the valence atomic orbitals of the central atom in a molecule are mixed together to form hybrid orbitals; these hybrid orbitals point to where the bonded atoms and lone pairs are oriented. The sigma bonds are formed from the hybrid orbitals overlapping head to head with an appropriate orbital from the bonded atom. The π bonds, in hybrid orbital theory, are formed from unhybridized p atomic orbitals. The p orbitals overlap side to side to form the π bond, where the π electrons occupy the space above and below a line joining the atoms (the internuclear axis). Assuming the z-axis is the internuclear axis, then the p z atomic orbital will always be hybridized whether the hybridization is sp, sp 2 , sp 3 , dsp 3 or d 2 sp 3 . For sp hybridization, the p x and p y atomic orbitals are unhybridized; they are used to form two π bonds to the bonded atom(s). For sp 2 hybridization, either the p x or the p y atomic orbital is hybridized (along with the s and p z orbitals); the other p orbital is used to form a π bond to a bonded atom. For sp 3 hybridization, the s and all the p orbitals are hybridized; no unhybridized p atomic orbitals are present, so no π bonds form with sp 3 hybridization. For dsp 3 and d 2 sp 3 hybridization, we just mix in one or two d orbitals into the hybridization process. Which specific d orbitals are used is not important to our discussion. 10. The MO theory is a mathematical model. The allowed electron energy levels (molecular orbitals) in a molecule are solutions to the mathematical problem. The square of the solutions gives the shapes of the molecular orbitals. A sigma bond is an allowed energy level where the greatest electron probability is between the nuclei forming the bond. Valence s orbitals form sigma bonds, and if the z-axis is the internuclear axis, then valence p z orbitals also form sigma bonds. For a molecule like HF, a sigma-bonding MO results from the combination of the H 1s orbital and the F 2p z atomic orbital. For π bonds, the electron density lies above and below the internuclear axis. The π bonds are formed when p x orbitals are combined (side-to-side overlap) and when p y orbitals are combined. 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.

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302 CHAPTER 9 COVALENT BONDING: ORBITALS 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
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Chpt 9 Solutions - CHAPTER 9 COVALENT BONDING ORBITALS...

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