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11-18-11 - Chemistry 1A F11 11.18.11 Dr Shaka Trigonal...

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269 Chemistry 1A F11 11.18.11 Dr. Shaka Course Code 40000 Trigonal Bipyramidal Five pairs of electrons has the three pairs around the equator at 120° angles and the two at the poles, with 90° angles. Lone pairs tend to occupy the equatorial positions. So, in ClF 3 there is one fluorine at each pole, and one at the equator. The other two lone pairs occupy the other equatorial positions. The structure then looks like a “T”.
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270 Chemistry 1A F11 11.18.11 Dr. Shaka Course Code 40000 Exceptions to VSEPR When we compare the bond angles of NH 3 (ammonia) with PH 3 (phosphine) or AsH 3 (arsine), we notice a systematic shift. NH 3 H-N-H angle = 107.3°, length 102 pm. PH 3 H-P-H angle = 93.5°, length 142 pm. AsH 3 H-As-H angle = 91.8°, length 152 pm. This occurs because the ns electrons get pulled down in energy compared to the np . The bonding becomes almost pure “ p ” (90° angles) with the s forming an “inert pair”.
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271 Chemistry 1A F11 11.18.11 Dr. Shaka Course Code 40000 Exceptions to VSEPR We observe the same trends with H 2 O. H 2 O H-O-H angle = 104.5°. H 2 S H-S-H angle = 92°. H 2 Se H-Se-H angle = 91°. The explanation is the same. N.B. Water has the highest BP by far, because only the first short period shows hydrogen bonding (N, F, or O). H 2 O = 100°C, H 2 S = 60°C, H 2 Se = 42°C.
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