Lecture 20sf

# Lecture 20sf - VSEPR Valence Shell Electron Pair Repulsion...

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VSEPR Valence Shell Electron Pair Repulsion Electron pairs surrounding a central atom keep as far away from each other as possible in order to minimize mutual repulsion. Four electron pairs around central atom Let’s first apply this principle to methane, CH 4 . The Lewis structure is simple: There are four pairs of electrons in the four bonds surrounding the central carbon atom. If methane were constrained to two dimensions, it would look like the above drawing, a cross-like structure with H-C-H bond angles of either 90 or 180 degrees. In three dimensions, we can construct a tetrahedral structure where every bond angle is the same and greater than 90. In this structure, carbon is at the center of a tetrahedron, with each hydrogen atom is at the corner of the tetrahedron. The tetrahedral bond angle—from the center to any two corners — is 109.5. Ammonia, NH 3 , also has a simple Lewis structure.

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Both methane and ammonia have four pairs of electrons surrounding the central atom. While methane has four bonds, ammonia has three bonds and a lone pair surrounding the central nitrogen atom. In both cases, the four pairs of electrons will spread out with tetrahedral geometry to keep as far away from each other as possible. In NH 3 , one arm of the tetrahedron will be occupied by the lone pair of electrons, giving rise to a structure described as trigonal pyramidal. The H-N-H bond angle in NH 3 is 107, slightly less than the ideal tetrahedral angle of 109.5. This is because the lone pair-bonded pair repulsion is greater than the bonded pair- bonded pair repulsion. This means that the lone pair spreads out and takes up a bit more room, making the bond angles slightly smaller. Water has the Lewis structure: Like the central carbon atom in CH 4 and the central N atom in NH 3 , the central O atom in water has 4 electron pairs around it. These electron pairs will have a tetrahedral geometry. The two lone pairs occupy two arms of the tetrahedron, leaving an overall bent shape for H 2 O. Since the lone pair-bonded pair repulsion is greater than then bonded pair-bonded pair repulsion, the bond angle is a little less than that of NH 3 since there are two lone pairs in water. The bond angle in water is 104.5.
Summarizing for molecules with four electron pairs surrounding the central atom Molecule Bonds Lone Pairs Electron Pair Molecular Geometry Geometry CH 4 4 bonds Tetrahedral Tetrahedral NH 3 3 bonds 1 lone pair Tetrahedral Trigonal pyramidal H 2 O 2 bonds 2 lone pairs Tetrahedral Bent Three Electron Pairs Around Central Atom Boron trifluoride has the structure There are three electron pairs around the central atom. To satisfy the principles of VSEPR, three electron pairs have a trigonal planar geometry. The molecule is planar, and all bond angles are 120. Two Electron Pairs around Central Atom

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## This note was uploaded on 11/09/2011 for the course CHEM 161 taught by Professor Vacillian during the Fall '08 term at Rutgers.

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Lecture 20sf - VSEPR Valence Shell Electron Pair Repulsion...

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