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Unformatted text preview: 1 Molecular Shapes Molecular Shapes 1. Areas of high electron density repel each other , 2. The e- domains arrange themselves as far apart as possible on the central atom. 3. The molecule adopts whichever 3D geometry minimized this repulsion . • This controls the geometry around the central atom • We call this V alence S hell E lectron P air R epulsion ( VSEPR ) theory. Molecular Geometries: The VSEPR Model Molecular Geometries: The VSEPR Model There are 11 shapes that are important to us: • linear (3 atoms, AB 2 ); bent (3 atoms, AB 2 ); • trigonal planar (4 atoms, AB 3 ); trigonal pyramidal (4 atoms, AB 3 ); T-shaped (4 atoms, AB 3 ); • tetrahedral (5 atoms, AB 4 ); square planar (5 atoms, AB 4 ); see-saw (5 atoms, AB 4 ); • trigonal bipyramidal (6 atoms, AB 5 ); square pyramidal (6 atoms, AB 5 ); and • octahedral (7 atoms, AB 6 ). •When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). • BUT when naming the molecular geometry, we focus only on the relative positions of the atoms. Predicting Molecular Geometries Predicting Molecular Geometries 2 • To determine the “Electron Pair” geometry: • Draw the Lewis structure, • Count the total number of electron pairs around the central atom, • Arrange the electron pairs in one of the above geometries to minimize e −-e − repulsion, and count multiple bonds as one bonding pair. Predicting Molecular Geometries Predicting Molecular Geometries The Effect of Nonbonding Electrons and The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Multiple Bonds on Bond Angles By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs....
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