Lecture 11sf

Lecture 11sf - Methane CH4 has a Lewis structure This seems...

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Methane, CH 4 , has a Lewis structure This seems like an obvious structure, but let’s analyze the bonding of carbon in detail. Carbon has four valence electrons with a ground state outer shell configuration of 2s 2 2p 2 . This atom has an electron dot structure with one pair (2s 2 ) and two single electrons (2p 2 ) This configuration, with two singly-occupied orbitals, suggests that carbon can form two bonds, pairing up these two orbitals with hydrogen or some other element. But carbon normally forms four bonds, such as in CH 4 . It appears that, when bonding, carbon has a dot structure with four unpaired electrons ready to bond. This can happen if one of the paired 2s electrons gets promoted to the vacant 2p orbital, giving a configuration of 2s 1 2p 3
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If these four electrons bonded with four hydrogens, we would get three equivalent bonds from the 2p orbitals, directed 90to each other, since p x , p y , p z orbitals are pointed along the x,y,z axes. A fourth bond with the 2s electron would be different from the other three. This is not in accord with the facts. Methane has four equivalent bonds, pointed towards the corners of a tetrahedron with the tetrahedral angle of 109.5. We can explain this by blending the 2s 1 2p 3 configuration into four equivalent orbitals called sp 3 hybrid orbitals. The sp 3 hybrid orbitals are all equivalent and point to the corners of a tetrahedron satisfying the principles of VSEPR--they spread out in space as much as possible to minimize mutual repulsion. Thus each of the C-H bonds in CH 4 results from an interaction between an sp 3 hybrid orbital on carbon and a 1s orbital on hydrogen. Boron, the element just before carbon in the periodic table, typically forms three bonds, such as boron trifluoride, BF 3 , with a trigonal planar geometry. The description of the bonding with boron is similar to that for carbon.
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The ground state electron configuration for boron is 2s 2 2p 1 , which would have a pair of 2s electrons and only a single unpaired 2p electron available for bonding. It appears that, when bonding, carbon has a dot structure with three unpaired electrons ready to bond. This can happen if one of the paired 2s electrons gets promoted to the vacant 2p orbital, giving a configuration of 2s 1 2p 2 Since boron forms three equivalent bonds, they must form from three equivalent orbitals. We again use the hybridization argument, blending the 2s 1 2p 2 configuration into 3 equivalent orbitals, known as sp 2 hybrid orbitals.
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The sp 2 hybrid orbitals point in a direction to satisfy VSEPR. They minimize their mutual repulsion. The geometry is trigonal planar, and the bond angle is 120. Beryllium, the element just before boron in the periodic table, typically forms
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Lecture 11sf - Methane CH4 has a Lewis structure This seems...

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