Download Class 8: Introduction to VSEPR Theory

Chem 1011 – Intersession 2011
Class #8
16-May-11
1
Class 8: Introduction to VSEPR Theory
•
Sec 10.2 – VSEPR Theory: The Five Basic Shapes
▫
▫
▫
▫
▫
•
Two Electron Groups: Linear Geometry
Three Electron Groups: Trigonal Planar Geometry
Four Electron Groups: Tetrahedral Geometry
Five Electron Groups: Trigonal Bipyramidal Geometry
Six Electron Groups: Octahedral Geometry
Sec 10.3 – VSEPR Theory: The Effect of Lone Pairs
▫
▫
▫
Four Electron Groups with Lone Pairs
Five Electron Groups with Lone Pairs
Six Electron Groups with Lone Pairs
2
Shapes of Molecules
• While Lewis theory does an excellent job describing the
behavior of valence electrons in molecules, it does not
provide information about the shapes of molecules.
• Remember: Class #4
▫ Lewis theory predicts water to be
linear, however, the molecule is
actually bent
▫ Distinct molecular shapes arise due
to the distances between bonded atoms
and the angle between those bonds
104.5o
98.5 pm
3
Shapes of Molecules
•
Electron pairs repel each other, whether they
are in chemical bonds (bond pairs) or
unshared (lone pairs).
•
Electron pairs assume orientations about an
atom to minimize repulsions.
•
Each electron group wants to get as far away as
it possibly can from all other electron groups –
this is the basis for VSEPR theory
Introduction to VSEPR Theory
1
Chem 1011 – Intersession 2011
Class #8
16-May-11
4
VSEPR Theory
•
Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
▫
•
a theory used to predict probable shapes of molecules (or
polyatomic ions) based on the repulsions of electron pairs
found in the valence shell of the central atom of the
structure.
▫
VSEPR theory focuses on electron groups.
An electron group is defined as:





One lone pair
One single bond
One double bond
One triple bond
One odd (unpaired) electron
All of these are considered
to be equivalent when
determining the shape of a
molecule or ion
5
VSEPR Theory
6
Shapes
•
Electronic geometry (or shape)
▫
•
Is the shape that best describes the arrangement of all
electron groups (bonding and lone pairs) in the valence
shell of the central atom
Molecular geometry (or shape)
▫
Is the actual shape of the molecule or polyatomic ion –
only the atoms that make up the species are considered

▫
Any lone pairs are ignored
In a species where all the electron groups are bonding
groups, the molecular and electronic shapes will be the
same

Otherwise, the two are related, but not the same.
Introduction to VSEPR Theory
2
Chem 1011 – Intersession 2011
Class #8
16-May-11
7
Terminology
A = central atom
X = terminal atom
(or a group of
atoms) bonded
to the central
atom
E = lone pair of
electrons
8
The 5 Basic Shapes
•
The 5 basic arrangements of electron-groups
around a central atom are:
▫
▫
▫
▫
▫
Two electron groups = linear
Three electron groups = trigonal-planar
Four electron groups = tetrahedral
Five electron groups = trigonal-bipyramidal
Six electron groups = octahedral
9
The 5 Basic Shapes
Introduction to VSEPR Theory
3
Chem 1011 – Intersession 2011
Class #8
16-May-11
10
Two Electron Groups – Linear
• Two electron groups = linear = AX2
• In order for the two electron
groups around the central
atom to be as far apart as
possible, the atom form an
angle of 180o.
• Example: BeCl2
11
Three Electron Groups – Trigonal
Planar
• Three electron groups = trigonal
planar = AX3
• In order for the three electron
groups to be as far apart as
possible, they lie in a plane
with bond angles of 120o.
• Example: BCl3
12
Three Electron Groups – Trigonal
Planar
• We still call formaldehyde “trigonal planar”, however,
because the bonds are not identical, the observed angles
are slightly different from the ideal.
Introduction to VSEPR Theory
4
Chem 1011 – Intersession 2011
Class #8
16-May-11
13
Four Electron Groups -- Tetrahedral
• Four electron groups = tetrahedral
= AX4
• In order for the four electron groups
to be as far apart as possible, they
must point to the corners of a
tetrahedron (a 4-sided polyhedron).
• Example: CH4
14
Drawing 3D on Paper
Dashed wedge
indicates this bond is
pointing away from
you (into the paper)
Solid wedge indicates this bond
is pointing toward you (out of
the paper)
15
Five Electron Groups – Trigonal
Bipyramidal
• The positions above and below
the central atom are called the
axial positions (like the axis of
the Earth).
• The positions in the same base
plane as the central atom are
called the equatorial positions
(like the equator of the Earth).
• The bond angle between
equatorial positions is 120°.
• The bond angle between axial
and equatorial positions is 90°.
Introduction to VSEPR Theory
5
Chem 1011 – Intersession 2011
Class #8
16-May-11
16
Five Electron Groups – Trigonal
Bipyramidal
• Five electron groups
= trigonal bipyramidal
= AX5
• Example: PF5
17
Six Electron Groups – Octahedral
• Six electron groups = octahedral
= AX6
• All bonds are at 90o angles.
• All atoms point toward the corners
of an octahedron (8-sided polyhedron)
• Example: SF6
18
The Effect of Lone Pairs on Shape
•
The closer two groups of electrons are forced, the stronger the
repulsion between them
▫
•
Example: The repulsive force between two electron groups
separated by an angle of 90o is greater than it would be at 120o
or 180o.
Lone-pair electrons spread out more than bond-pair electrons do
▫
▫
•
Lone pairs occupy more space than bond-pairs
Lone pairs are held much closer to the central atom in comparison
to bond pair
So, the order of repulsive forces, from strongest to weakest, is:
▫
▫
▫
Lone-pair to Lone-pair
Lone-pair to Bond-pair
Bond-pair to Bond-pair
Introduction to VSEPR Theory
6
Chem 1011 – Intersession 2011
Class #8
16-May-11
19
Three Electron Groups with Lone Pairs
• AX2E
Electron geometry – Trigonal Planar
Molecular geometry – Bent
• Electron geometry reflects the
positions of all electron groups.
• Molecular geometry reflects the
positions of just the atoms.
20
Four Electron Groups with Lone Pairs
• AX3E
• Electron geometry – Tetrahedral
• Molecular geometry – Trigonal pyramidal
• The lone pairs repel the
bonding pairs more, so
bond angles between
atoms are < 109.5o.
21
Four Electron Groups with Lone Pairs
• AX2E2
• Electron geometry – Tetrahedral
• Molecular geometry – Bent
• The two lone pairs
further repel the
bonding pairs, so
bond angles are
approximately 104.5o.
Introduction to VSEPR Theory
7
Chem 1011 – Intersession 2011
Class #8
16-May-11
22
Five Electron Groups with Lone Pairs
• AX4E
• Electron geometry – Trigonal
Bipyramidal
• Molecular geometry – Seesaw
• The lone pairs occur in the
equatorial positions in order to
reduce repulsions.
• Example: IF2O2-
23
Five Electron Groups with Lone Pairs
• AX3E2
• Electron geometry – Trigonal Bipyramidal
• Molecular geometry – T-Shaped
24
Five Electron Groups with Lone Pairs
• AX2E3
• Electron geometry – Trigonal Bipyramidal
• Molecular geometry – Linear
Introduction to VSEPR Theory
8
Chem 1011 – Intersession 2011
Class #8
16-May-11
25
Six Electron Groups with Lone Pairs
•
•
•
•
AX5E
Electron geometry – Octahedral
Molecular geometry – Square Pyramidal
F(axial)-Br-F(equatorial) bond angle is less than 90o.
26
Six Electron Groups with Lone Pairs
• AX4E2
• Electron geometry: Octahedral
• Molecular geometry: Square Planar
• Example: ICl4-
27
Molecular Geometry - Summary
Introduction to VSEPR Theory
9
Chem 1011 – Intersession 2011
Class #8
16-May-11
28
On Wednesday
•
Sec 10.4 – VSEPR Theory: Predicting Molecular
Geometries
▫
Predicting the Shapes of Larger Molecules
•
Sec 10.5 – Molecular Shape and Polarity
•
And, of course, Western Conference Finals Game 2!
@
Introduction to VSEPR Theory
10