What’s coming up???
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Oct 25
Oct 27
Oct 29
Nov 1
Nov 3,5
Nov 8,10
Nov 12
Nov 15
Nov 17
Nov 19
The atmosphere, part 1
Midterm … No lecture
The atmosphere, part 2
Light, blackbodies, Bohr
Postulates of QM, p-in-a-box
Hydrogen and multi – e atoms
Multi-electron atoms
Periodic properties
Periodic properties
Valence-bond; Lewis structures
Ch. 8
Ch. 8
Ch. 9
Ch. 9
Ch. 9
Ch.9,10
Ch. 10
Ch. 10
Ch. 11
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Nov 22
VSEPR
Ch. 11
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Nov 24
Nov 26
Nov 29
Dec 1
Dec 2
Hybrid orbitals; VSEPR
MO theory
MO theory
bonding wrapup
Review for exam
Ch. 11, 12
Ch. 12
Ch. 12
Ch. 11,12
Electron cloud probability
distributions for different
types of bond
Dipole moment will
align molecules in
an electric field
Nitrate anion NO3- Put a pair between each atom
O
O
N
O
nitrogen does not have noble gas structure!!!
form a double bond by sharing a pair from one
of the oxygen atoms……….
FORM A DOUBLE BOND BETWEEN O AND N
-
Here is one
O
O
N
O
Here is another!
Here is another!
-
-
O
O
N
O
O
O
N
O
Experiment shows all three bonds are the same.
All bond lengths 128 pm
O
N
All bond angles 120 0
O
O
Any one of the structures suggests one is different!
O
O
N
Double Bond
Single Bond
O
Should be different!
So…….
RESONANCE
We use a double headed arrow between the
structures..
O
O
O
O
N
N
N
O
O
O
O
The electrons involved are said to be
DELOCALIZED over the structure.
The blended structure is a
RESONANCE HYBRID
O
Elements in rows 3 and following can exceed the
octet rule:
When it is necessary to exceed the octet rule the extra
electrons go on the central third row element.
F
F
SF6
F
S … 12
S
F
F
F
I3−
I
I
I
Central I … 10
FREE RADICALS
Molecules which have unpaired electrons.
NO2
Is a free radical
Total number of valence electrons = 5+6+6 = 17
O
N
O
Form double bond to get N close to octet
O
N
O
O
RESONANCE
N
O
PREDICTING THE SHAPES
OF MOLECULES
from the Lewis electron dot structure using
the principle that electron pairs stay as far apart
as possible.
Electron pairs
BOND PAIRS
LONE PAIRS
H
O
H
VALENCE SHELL ELECTRON PAIR REPULSION:
VSEPR
Based on the idea that all electron pairs repel
each other.
The bonding and lone pairs push apart as far as
possible……..
This means that atoms bound to a central atom
are as far apart as possible…….
we can find the molecular shape!
Lets see how it works…...
Ronald J. Gillespie
Professor Emeritus
B.Sc., Ph.D., D.Sc. (London), F.R.S., F.R.S.C., F.R.S.C.
(U.K.), F.C.I.C.
Molecular Geometry
Contact Information
Curriculum Vitae
My currents interests are in the field of Molecular Geometry. I have
long been interested in further developing the VSEPR model and at the
same time trying to understand why certain molecules appear to be
exceptions to the model. Partly in collaboration with my colleague Richard
Bader, I have been making use of the analysis of calculated electron
density distributions to better understand the VSEPR model and molecular
geometry in general. We have shown that the Laplacian of the electron
density provides evidence for the localized lone pairs of the VSEPR model
and we have developed the Lennard-Jones function which also provides
evidence for lone pairs on a different basis.
One of the largest classes of exceptions to the VSEPR model are
certain molecules of the transition metals. We have shown that the
deviations of the geometry of these molecules from the VSEPR model can
be related to the distortion of the metal atom core from a spherical shape
which we have been able to study by means of the Laplacian electron
density. This investigation is continuing. Recently I have shown that the
intramolecular distance between two given ligands is remarkably constant
over a wide variety of molecules which led me to suggest that interligand
interactions are much more important in determining geometry than has
previously generally been supposed. This observation has led me to
develop the ligand close packing (LCP) model.
SeO2
SO2
O
Se
O
Resonance (like SO2)
LEWIS STRUCTURE
S
O
S
O
O
O
Experiment shows that both S-O bonds are
equivalent.
We say that the real SO2 molecule is a hybrid of the
two resonance forms.
SeO2
O
Se
O
ELECTRON PAIR GEOMETRY
THREE ELECTRON PAIRS AROUND THE
SELENIUM ATOM.
Se
VSEPR treats double
bonds like a single bond
TRIGONAL PLANAR
Now place the oxygen atoms
SeO2
O
Se
O
Electron Pair Geometry is trigonal planar
ADD OXYGENS
Se
Se
O
SeO2 IS V-SHAPED, OR BENT
O
H
CH4
H
C
H
H
There are four
electron pairs
around the
carbon atom.
The best arrangement for four electron pairs:
109.5°
TETRAHEDRAL
C
4 electron pairs
tetrahedral electron
pair geometry
Put on the H-atoms…….
There is a better arrangement for four electron
pairs:
TETRAHEDRAL
H
109.5°
C
C
H
H
H
4 electron pairs
tetrahedral EPG
The shape of CH4 is tetrahedral.
NOW LOOK AT AMMONIA
NH3
H
N
H
H
There are four
electron pairs
around the nitrogen
atom.
The electron pair geometry around the nitrogen is
tetrahedral:
PUT ON THE 3 H ATOMS
N
NH3
H
N
H
H
There are four electron pairs
around the nitrogen atom.
The electron pair geometry around the nitrogen is
tetrahedral:
PUT ON THE 3 H ATOMS
N
N
H
H
H
The shape of NH3 is trigonal pyramidal.
H2O
H
O
H
There are four electron pairs
around the oxygen atom.
The electron pair geometry around the oxygen is
tetrahedral:
PUT ON THE 2 H-ATOMS
O
O
H
The shape of H2O is V-shaped or bent.
H
VALENCE BOND THEORY
A covalent bond is formed by an
overlap of two valence atomic
orbitals that share an electron pair.
The better the overlap the stronger the bond
The orbitals need to point along the bonds
Lets look at methane
METHANE: a tetrahedral molecule
H
CH4
C
What orbitals are used?
H
H
H
Hydrogen atoms bond using their 1s orbitals.
Carbon needs four orbitals to bond with.
[He] 2s22p2
Try
2s, 2px , 2py and 2pz
The electronic configuration of carbon is:
[He] 2s22p2
The orbital diagram is: [He]
The Lewis dot structure is
.
.C .
.
Promote one of the 2s electrons
PROMOTE AN ELECTRON
[He]
[He]
[He] 2s22p2
excited state
The Lewis dot structure is still
[He] 2s12p3
C
Four unpaired electrons
We can use these to form chemical bonds
A covalent bond is formed by an overlap of two
valence atomic orbitals that share an electron pair.
Bonds formed with s orbitals will be different to
bonds formed with p orbitals.
Experiment shows that all four bonds are identical.
The three p orbitals are mutually perpendicular,
suggesting 90° bond angles.
Experiment shows that methane has 109.5° bond
angles.
We get round this by combining the orbitals
We need four orbitals pointing to the vertices of
a tetrahedron….
We remember that orbitals
are just algebraic functions
and so we can combine them
H
C
H
H
H
Combining orbitals is called
HYBRIDIZATION
COMBINING ORBITALS TO FORM
HYBRIDS
HYBRIDIZATION :
the combination of two or more “native” atomic
orbitals on an atom to produce “hybrid” orbitals
RULE: the number of atomic orbitals that are
combined must equal the number which are
formed
All resulting hybrid orbitals are identical.
HYBRIDIZATION
Combine one s and one p
a sp- hybrid
+
ADD the orbitals
+
Ψ2s+ Ψ2p
The positive part adds to positive part
CONSTRUCTIVE INTERFERENCE
Ψ2s+ Ψ2p
+
+
The positive part cancels negative part
DESTRUCTIVE INTERFERENCE
Combine one s and one p to give an sp- hybrid
Ψ2s+ Ψ2p
+
REMEMBER IF WE MIX TWO WE MUST GET TWO BACK
The other combination is s - p
The positive part adds to positive part
Ψ2s- Ψ2p
CONSTRUCTIVE INTERFERENCE
+
+
The positive part cancels negative part
DESTRUCTIVE INTERFERENCE
Ψ2s- Ψ2p
+
We get two equivalent sp orbitals
ORIENTED AT 1800
sp-HYBRIDIZATION
The s and p orbitals
The two sp-hybrids
Directed at 1800