CH2. Molecules and covalent bonding Lewis Structures VSEPR MO Theory

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Lewis structure H3PO4 •

Skeleton is:

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Count total valence electrons: 1P= 5 3H= 3 4 O = 24 Total = 32 e- or 16 valence e- pairs.

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7 e- pairs needed to form s skeleton.

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Lewis structure H3PO4

• Add remaining e- pairs:

• Left has a formal charge of +1 on P and -1 on one O, right has 5 e- pairs around P (hypervalence) • Analysis of phosphoric acid shows purely Td phosphate groups, which requires something beyond either simple Lewis model.

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Resonance in NO3-

experimental data - nitrate is planar with 3 equivalent N-O bonds

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VSEPR model • Count e- pairs about the central atom (draw Lewis structure if needed). Include non-bonding pairs, but not multiple bonds. • Geometry maximizes separation: # e pairs geometry example 2 3 4 5 6 7 8

linear equilateral triangular tetrahedral (Td) trigonal bipyramidal (TBP) octahedral (Oh) pentagonal bipyramidal square antiprismatic

HF2BF3 CF4 PF5 SF6 IF7 TaF835

Drawing Oh and Td molecules It's often useful to draw octahedra and tetrahedra with a cubic framework

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Deviations from ideal geometries: unshared pairs and multiple bonds require larger bite ex: CH4, NH3, H2O