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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