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VSEPR Theory
Valence Bond Theory
Molecular Orbital Theory
Molecular Geometry
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 theoryDraw a plausible Lewis structureDetermine the # of lone pairs and bonding pairs
around the central atomEstablish 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 electronpairs in determining the structure.
Nonbonding electrons take up more space in the valence shell than thebonding electrons.
If one or more of the electron pairs are lone pairs, the distribution ofelectron pair and the geometrical shape of the molecule must bedifferent.
The bond angles decrease as the number of nonbonding electron pairsIncreases.
Repulsion strengths
lone pair −lone pair lone pair − bond pair bond pair −bond pair
Structures and Formal ChargeFormal 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] – [non-bonded electrons + number of bonds]
Examples of Resonance & Formal Charge
Valence Bond TheoryThe 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
Examplesfind the hybridization type and
geometry of CH4
PCl5SF6
NH3
BeF2