1.3 chemical bonding_2 ppt - Introductory University...

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Introductory University Chemistry I Unit 2 Chemical Bondings II (VSEPR)
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Introduction An important aspect of covalent bonding is the shape of the covalent molecules. Understanding their shape is important – For drug design – For understanding metabolism
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The Shape of Covalent Molecules Lewis structures do not give the true shape of a covalent molecules The Valence Shell Electron-Pair Repulsion V alence S hell E lectron-P air R epulsion (VSEPR) model – predicts molecular geometries
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Some basic definitions An electron group is a group of electron of any number that occupies a local region around an atom. An electron group can be a single, double or triple bond . It can also be a lone pair . The molecular shape ( molecular geometry ) is the X Y X Y X An electron group X Y relative position of the atomic nuclei of the molecule. The bond angle is the angle formed by the nuclei of two surrounding atoms with the nucleus of the central atom. A B B Bond angle
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Basic postulates of the VSEPR model : The repulsion of the electron groups should be minimized . Electron groups around a central atom is located as far away as possible from the others. The repulsion between electron group decreases in the order of: Lone pair to lone pair > lone pair to bond pair > bond pair to bond pair A A X A X X > > Repulsions
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Some important points in the construction of the molecular shape : To determine the molecular shape (molecular geometry) - only the arrangement of bond pairs is considered. Never take lone pairs into consideration. Multiple bonds are treated as a single bond in considering the structure.
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Electron group arrangement and molecular shapes Two electron group AB 2 With no lone pair in the central atom Shape: linear (bond angle =180 o ) A B B Example: BeCl 2 , CO 2 C O O Be Cl Cl
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Linear structure: CO 2
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A B B B Three electron group AB 3 With no lone pair in the central atom : Shape: trigonal planar , bond angle = 120 o B F F F C O O O Example: BF 3 , CO 3 2-
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Trigonal Planar structure: CO 3 2-
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With lone pair in the central atom : AB 2 E Shape: V-shape , bond angle < 120
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