Tham khảo sách 'chemistry part 14, julia burdge,2e (2009)', khoa học tự nhiên, hoá học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 318 CHAPTER 9 Chemical Bonding II Molecular Geometry and Bonding Theories molecular geometries where there are one or more lone pairs on the central atom. Note the positions occupied by the lone pairs in the trigonal bipyramidal electron-domain geometry. When there are lone pairs on the central atom in a trigonal bipyramid the lone pairs preferentially occupy equatorial positions because repulsion is greater when the angle between electron domains is 90 or less. Placing a lone pair in an axial position would put it at 90 to three other electron domains. Placing it in an equatorial position puts it at 90 to only two other domains thus minimizing the number of strong repulsive interactions. All positions are equivalent in the octahedral geometry so one lone pair on the central atom can occupy any of the positions. If there is a second lone pair in this geometry though it must occupy the position opposite the first. This arrangement minimizes the repulsive forces between the two lone pairs they are 180 apart instead of 90 apart . In summary the steps to determine the electron-domain and molecular geometries are as follows 1. Draw the Lewis structure of the molecule or polyatomic ion. 2. Count the number of electron domains on the central atom. 3. Determine the electron-domain geometry by applying the VSEPR model. 4. Determine the molecular geometry by considering the positions of the atoms only. Sample Problem shows how to determine the shape of a molecule or polyatomic ion. Determine the shapes of a SO3 and b ICI4. i Strategy Use Lewis structures and the VSEPR model to determine first the electron-domain geometry and then the molecular geometry shape . Setup a The Lewis structure of SO3 is I There are three electron domains on the central atom one double bond and two single bonds. b The Lewis structure of IClf is C1 I J I s I I I I j Ï I i CI C1 CU f I T s There are six electron domains on the central atom in IC14 four single bonds and two lone pairs. is Solution
