WVU CHEM 111 - VSEPR (3 pages)

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VSEPR



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VSEPR

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Pages:
3
School:
West Virginia University
Course:
Chem 111 - Survey of Chemistry
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Predicting Molecular Shape and Bond Angles The Lewis Structure of a molecule or ion shows only the placement of valence electrons Note that the Lewis Structure does not give information on the three dimensional shape of the molecule or ion VSEPR theory Valence Shell Electron Pair Repulsion must be used to predict the three dimensional shape of a molecule or ion To use VSEPR theory follow the steps outlined below 1 Draw a correct Lewis Structure for the molecule or ion 2 Count up the total number of regions of electron density surrounding the central atom this includes both bonding and nonbonding pairs of electrons 3 Arrange the regions of electron density in one of the five arrangements shown below Note Since the regions of e density are negatively charged and therefore repel each other they will adopt a geometry in which they are located as far apart as possible Total Electron Pairs 2 Arrangement of Electron Pairs Linear 180 Angle Between Electron Pairs 180 E A E E 120 A 3 Trigonal Planar 120 E E E 109 5 A 4 Tetrahedral 109 5 E E E axial position E E 90 E A E 120 5 Trigonal Bipyramidal 90 120 180 E E E 6 4 Octahedral 90 180 equatorial positions A 90 E E E E The Molecular Shape is the resulting arrangement of the atoms in the molecule or ion The various molecular shapes are tabulated on the next page VSEPR Theory Summary Regions of electron density Regions of bonding e pairs Arrangement of regions of e density Molecular Shape AX2 2 2 0 linear linear AX3 AX2U1 3 3 3 2 0 1 trigonal planar trigonal planar trigonal planar bent AX4 AX3U1 AX2U2 4 4 4 4 3 2 0 1 2 tetrahedral tetrahedral tetrahedral tetrahedral trigonal pyramidal bent AX5 AX4U1 AX3U2 AX2U3 5 5 5 5 5 4 3 2 0 1 2 3 trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal see saw shaped T shaped linear AX6 AX5U1 AX4U2 6 6 6 6 5 4 0 1 2 octahedral octahedral octahedral octahedral square pyramidal square planar Formula Key A central atom X bonding region of electron density U nonbonding region of electron density i e lone pairs Regions of nonbonding e pairs Polarity of Molecules and Ions A polar molecule is one where the center of positive and negative charge partial charge does not coincide One end of the molecule or ion has a permanent partial positive charge while the other end of the molecule has a permanent partial negative charge Polar molecules are obtained when the vector sum of the bond dipoles is nonzero Nonpolar molecules are obtained when the center of positive and negative charge coincide Nonpolar molecules are obtained when the vector sum of bond dipoles is zero A molecule or ion with polar covalent bonds can be either polar or nonpolar depending on the three dimensional arrangement of the atoms An easy way to tell if a molecule is polar or nonpolar is to look at the symmetry of the molecule Highly symmetric molecules are nonpolar while unsymmetric molecules are generally polar For example BeCl2 is nonpolar while BeFCl is polar Both molecules have a linear molecular shape but BeCl2 is highly symmetric while BeFCl is unsymmetric See the diagram below 180 Cl Be 180 Cl Sum of bond dipoles equal zero Highly symmetric molecular shape NONPOLAR F Be Cl Sum of bond dipoles is nonzero because F more electronegative than Cl Unsymmetric molecular shape POLAR To determine whether or not a molecule or ion is polar perform the following steps 1 2 3 Draw the Lewis Structure Predict the Molecular Shape using VSEPR theory If the molecular shape is symmetric then the molecule is nonpolar If the molecular shape is unsymmetric then the molecule is polar Example Is SF4 polar or nonpolar


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