Transcript Valence Shell Electron Pair Repulsion (VSEPR) Theory

Molecular Geometry
Molecules of different subtances 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. Molekül geometrisini açıklamak
için farklı kuramlar mevcuttur. modeller
VSEPR
Valence Bond
Theory
Molecular Orbital
Theory
Valence Shell Electron Pair Repulsion
(VSEPR) Theory
It is a method for predicting the shape of a molecule from the
knowledge of the groups of electrons around a central atom.
Electron pairs (bonding and nonbonding electrons) repel one
another, as a result , the electron pairs remain as far apart as
possible from another as possible to minimize the repulsion.
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Two electron pairs in the valence orbital are arranged linearly
Three electron pairs are organized in a trigonal planar arrangement
Four electron pairs are organized in a tetrahedral arrangement
Five electron pairs are arranged in a trigonal bipyramid
Six electron pairs are organized in an octahedral arrangement
The repulsion of lone pair electrons is grater than the repulsion of bond pair electrons
Electron pairs assume orientations about an atom to minimize repulsions.
Applying the VSEPR theory.
• draw a plausible Lewis structure of the molecule or polyatomic
ion.
• Determine the number of unshared electron pairs and numbers
of bonds around the central atom ( multiple bonds count as a
single bond)
• B + No
• 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
question: predict the shape of CO2 HCN CH4, NH3 SO2 PCl5,
SF6 and H2O by using VSEPR theory.
question: predict the shape of CO2 HCN CH4, NH3 SO2 PCl5,
SF6 and H2O by using VSEPR theory.
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 e-bond pair  bond pair-bond pair
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 valenceshell atomic orbitals.
Bonding in Methane
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
question : find the hybridazation type and geometry of
CH4, PCl5, SF6. NH3 BeF2
Multiple Covalent Bonds
Molecular Orbital Theory
A molecular orbital describes a region of space in a molecule where
electrons are most likely to be found.
Molecular orbitals are formed by combining atomic orbitals on different
atoms.
Combining Atomic Orbitals
No. e- in bonding MOs - No. e- in antibonding MOs
Bond Order =
2
Bonding in Metals
Electron sea model
Band theory.
12-7 Bonding in Metals
• Electron sea
model
Bonding in Metals
Band theory.
Band Theory
Semiconductors
Photovoltaic Cells