Transcript ppt
Molecular Geometry VSEPR Theory Valence Bond Theory Molecular Orbital Theory What is Molecular Geometry Molecules of different substances have diverse shapes. Atoms attach to one another in various geometric arrangements. The overall molecular shape of a molecule is determined by its bond angles in three dimensions. The shape of a molecule is very important for its physical and chemical properties Applying the VSEPR theory Draw a plausible Lewis structure Determine the # of lone pairs and bonding pairs around the central atom Establish the geometrical orientation of the electron pairs around the central atom as linear, trigonal planar, tetrahedral, trigonal bipyramid or octahedral Describe the molecular geometry The nonbonding electron pairs are as important as bonding electron pairs in determining the structure. Nonbonding electrons take up more space in the valence shell than the bonding electrons. If one or more of the electron pairs are lone pairs, the distribution of electron pair and the geometrical shape of the molecule must be different. The bond angles decrease as the number of nonbonding electron pairs Increases. Repulsion strengths lone pair −lone pair lone pair − bond pair bond pair −bond pair Structures and Formal Charge Formal charge helps to determine which resonance structure is most stable, as well as charges on individual atoms. Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons] OR Formal Charge = [# of valence electrons] – [nonbonded electrons + number of bonds] Examples of Resonance & Formal Charge Valence Bond Theory The covalent bonds are formed by overlap of atomic orbitals each of which contains one electron of opposite spin. The valence bond method predicts molecule shapes from the shapes and orientation of the atomic orbitals and their overlap regions when two atoms approach. In most cases the orbitals that overlap are reconfigured orbitals, called hybrid orbitals, having different shapes and orientations than pure orbitals. The process of hybridization corresponds to a mathematical mixing of the valence-shell atomic orbitals. Valence Electron Pair Geometry Number of Orbitals Hybrid Orbitals Linear 2 sp Trigonal Planar 3 sp2 Tetrahedral 4 sp3 Trigonal Bipyramidal 5 sp3d Octahedral 6 sp3d2 Examples find the hybridization type and geometry of CH4 PCl5 SF6 NH3 BeF2