2.7 - VSEPR
Molecular Geometry
►
3D arrengement of atoms in a molecule – physical and chemical
properties - melting, boiling pointing, density, types of reactions it
undergoes.
►
Bond lenghts and bond angles – experiment determined
►
Simples procedure to predict the geometry – using the number of
electrons surrounding a central atom in its Lewis structure.
►
Base: electrons pairs in the valence shell of an atom repel one
another.
VSEPR Model (valence-shell
electron-pair repulsion)
► The
repulsion between electrons in different
bonding pairs causes them to remain as far
apart as possible.
► The
structure around a given atom is
determined principally by minimazing
electron pair repulsions.
Predicting a VSEPR Structure
► Draw
► Put
Lewis Structure
pairs as far apart as possible.
► Determine
positions of atoms from the way
electron pairs are shared.
► Determine
the name of molecular structure from
positions of the atoms.
Predicting a VSEPR Structure
Couple of things to keep in mind:
►
Lone pairs take up more space than shared pairs
►
Multiply bond count as one shared pair
►
We divide the molecules into two categories: according to whether
or not the central atom has lone pairs.
PHET Simulation
►
https://phet.colorado.edu/sims/html/moleculeshapes/latest/molecule-shapes_en.html
Remember that lone pairs take up
more space than shared pairs
Any 2-atom
molecule with a
polar bond
Wich
molecules
have a dipole
moment?
With three or more
atoms there are two
considerations:
1. There must be a
polar bond.
2. Geometry cannot
cancel it out.
Geometry and Dipole Moment
►
Examples of Bond that cancel polarity of bonds:
Linear
Trigonal Planar
Obs: Terminal atoms are the same.
120
Geometry and Dipole Moment
►
Examples of Bond that not cancel polarity of bonds:
Bent
Trigonal Pyramidal
VSEPR Practice
►
Predict the molecular geometry, bond angles and polarity for each
of the following molecule or ion:
Hybridization
►
The mixing of atomic orbitals to form special orbitals for bonding.
►
Blending Orbitals
►
The atoms are responding as needed to give the minimum energy
for the molecule.
Weakness of Atomic Orbitals
►
… to explains molecular geometry
►
In methane, CH4, the shape is tetrahedral.
►
The valence electrons of carbon should be two in s, and two in p.
►
The atomic orbitals change when making a molecule.
Methane Atomic Orbitals
3
sp
Hybridization
3
sp
Energy
Energy
How do we kow the hybridization?
►
We know the geometry from experiment
►
We know the orbitaks of the atom
►
Hybridizing atiomic orbitaks explains the geometry
►
If the geometry requires a tetrahedral shape, it is sp3 hybridized
(bond angle 109.5)
►
This includes bent and trigonal pyramiidal.
Hybridization depends upon stearic number only!!
Stearic number = electron domains = number of electrons pair
surrounding the atom
2
sp Hybridization
►
Ex. C2H4
►
Double bond acts as one pair
►
Trigonal planar geometry
►
We need three blended orbitals
►
Use one s and two p orbitals to make sp2 orbitals
►
Leaves one p orbital perpendicular
2
sp Hybridization
2
sp Energy
Energy
3rd p orbital
Located perpendicular to the sp2 orbitals
2 types of Bonds
►
A sigma (𝞂) bond centers along the internuclear axis (formed from
overlapping hybridized orbitals)
►
A pi (𝛑 ) bond occupies the space above and below the
internuclear axis (formed from unhybridized orbitals)
A sigma bond and a pi bond together
make a double bond
2
sp Hybridization
sp Hybridization
►
Ex. CO2
►
Linear geometry
►
We need two blended orbitals
►
Use one s and one p orbital to make sp
orbitals
►
Leaves two p orbitals perpendicular
sp Energy
Energy
2nd and 3rd p orbitals
►
Both are located perpendicular to the sp orbital
sp Hybridization
►
If the geometry requires a linear shape, it is sp hybridized bond
angle 180)
Hybridization Practice
►
Using the diagram below…
►
1. Determine the hybridization around
the atomns a-e
►
2. A student claims that all the atoms in
the molecule exist in a single plane. Do
you agree with the student`s claim?
►
Explains why or why not..
Resonance Vocabulary
►
𝞂 bonds can described as being localized
►
A resonating 𝛑 bond must be treated as being delocalized
►
Ex. No2 _ and NO3 _