Whats coming up

1
Whats coming up???

• Oct 25 The atmosphere, part 1 Ch. 8
• Oct 27 Midterm No lecture
• Oct 29 The atmosphere, part 2 Ch. 8
• Nov 1 Light, blackbodies, Bohr Ch. 9
• Nov 3,5 Postulates of QM, p-in-a-box Ch. 9
• Nov 8,10 Hydrogen and multi e atoms Ch. 9
• Nov 12 Multi-electron atoms Ch.9,10
• Nov 15 Periodic properties Ch. 10
• Nov 17 Periodic properties Ch. 10
• Nov 19 Valence-bond Lewis structures Ch. 11
• Nov 22 VSEPR Ch. 11
• Nov 24 Hybrid orbitals VSEPR Ch. 11, 12
• Nov 26 MO theory Ch. 12
• Nov 29 MO theory Ch. 12
• Dec 1 bonding wrapup Ch. 11,12
• Dec 2 Review for exam

2
Electron cloud probability distributions for
different types of bond
3
(No Transcript)
4
Dipole moment will align molecules in an electric
field
5
(No Transcript)
6
Nitrate anion NO3-
Put a pair between each atom
nitrogen does not have noble gas structure!!!
form a double bond by sharing a pair from one of
the oxygen atoms.
7
FORM A DOUBLE BOND BETWEEN O AND N
Here is one
Here is another!
Here is another!
8
Experiment shows all three bonds are the same.
All bond lengths 128 pm
N
All bond angles 120 0
Any one of the structures suggests one is
different!
Double Bond
Single Bond
Should be different!
So.
9
RESONANCE
We use a double headed arrow between the
structures..
The electrons involved are said to be DELOCALIZED
over the structure.
The blended structure is a RESONANCE HYBRID
10
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.
S 12
SF6
Central I 10
I3-
11
FREE RADICALS
Molecules which have unpaired electrons.
NO2
Is a free radical
Total number of valence electrons 566 17
O
N
O
Form double bond to get N close to octet
RESONANCE
12
PREDICTING THE SHAPES OF MOLECULES
from the Lewis electron dot structure using
the principle that electron pairs stay as far
apart as possible.
BOND PAIRS
Electron pairs
LONE PAIRS
13
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...
14
     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.
15
Resonance (like SO2)
SeO2
O
Se
O
LEWIS STRUCTURE
SO2
Experiment shows that both S-O bonds are
equivalent.
We say that the real SO2 molecule is a hybrid of
the two resonance forms.
16
SeO2
ELECTRON PAIR GEOMETRY
THREE ELECTRON PAIRS AROUND THE SELENIUM ATOM.
VSEPR treats double bonds like a single bond
TRIGONAL PLANAR
Now place the oxygen atoms
17
SeO2
Electron Pair Geometry is trigonal planar
ADD OXYGENS
SeO2 IS V-SHAPED, OR BENT
18
There are four electron pairs around the carbon
atom.
CH4
19
The best arrangement for four electron pairs
109.5
TETRAHEDRAL
tetrahedral electron pair geometry
4 electron pairs
Put on the H-atoms.
20
There is a better arrangement for four electron
pairs
H
TETRAHEDRAL
109.5
H
H
H
tetrahedral EPG
4 electron pairs
The shape of CH4 is tetrahedral.
NOW LOOK AT AMMONIA
21
There are four electron pairs around the nitrogen
atom.
NH3 The electron pair geometry around the
nitrogen is tetrahedral
H
N
H
H
PUT ON THE 3 H ATOMS
22
NH3 The electron pair geometry around the
nitrogen is tetrahedral
There are four electron pairs around the nitrogen
atom.
PUT ON THE 3 H ATOMS
N
H
H
H
The shape of NH3 is trigonal pyramidal.
23
H2O The electron pair geometry around the oxygen
is tetrahedral
There are four electron pairs around the oxygen
atom.
H
O
H
PUT ON THE 2 H-ATOMS
O
H
H
The shape of H2O is V-shaped or bent.
24
(No Transcript)
25
(No Transcript)
26
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
27
METHANE a tetrahedral molecule
CH4
What orbitals are used?
Hydrogen atoms bond using their 1s orbitals.
Carbon needs four orbitals to bond with.
He 2s22p2
Try 2s, 2px , 2py and 2pz
28
The electronic configuration of carbon is
He 2s22p2
He
The orbital diagram is
The Lewis dot structure is
Promote one of the 2s electrons
29
PROMOTE AN ELECTRON
He
He
He 2s22p2
He 2s12p3
excited state
The Lewis dot structure is still
Four unpaired electrons
We can use these to form chemical bonds
30
(No Transcript)
31
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
32
We need four orbitals pointing to the vertices of
a tetrahedron.
Combining orbitals is called
HYBRIDIZATION
33
(No Transcript)
34
COMBINING ORBITALS TO FORM HYBRIDS
HYBRIDIZATION
the combination of two or more native atomic
orbitals on an atom to produce hybrid orbitals
the number of atomic orbitals that are combined
must equal the number which are formed
RULE
All resulting hybrid orbitals are identical.
35
HYBRIDIZATION
Combine one s and one p
a sp- hybrid

ADD the orbitals

?2s ?2p
36
The positive part adds to positive part
CONSTRUCTIVE INTERFERENCE
?2s ?2p
37
Combine one s and one p to give
an sp- hybrid
?2s ?2p

REMEMBER IF WE MIX TWO WE MUST GET TWO BACK
The other combination is s - p
38
The positive part adds to positive part
?2s- ?2p
CONSTRUCTIVE INTERFERENCE
The positive part cancels negative part
DESTRUCTIVE INTERFERENCE
39
?2s- ?2p

We get two equivalent sp orbitals
ORIENTED AT 1800
40
sp-HYBRIDIZATION
The s and p orbitals
The two sp-hybrids
Directed at 1800