1. 10.2-10.4 VSEPR Theory: Shapes, The Effect of Lone Pairs, and Predicting Molecular Geometries

2. VESPR Theory
Valence shell electron pair repulsion (VESPR) theory – electron pair (whether paired or unpaired), and even single electrons, repel one another through Coulombic forces.
The repulsion between electron groups on interior atoms of a molecule, determine molecular geometry.
Maximum separation between electron groups is the preferred geometry.
If the molecule has one central atom, we must: a.) determine the number of electron groups around that atom and b.) indicate how many pairs are shared or unshared.

3. Molecular Geometry: No Unshared Electrons on the Central Atom
Binary molecules made up of three atoms, and that have two electron groups of either single or double bonds, have linear geometry.
Examples: BeCl Cl─Be─Cl ((Be) does not often
follow the octet rule)
CO O=C=O
Both examples have linear geometry, and have a bond
angle of?
180o

4. Molecular Geometry: No Unshared Electrons on the Central Atom
For molecules with three electron groups around the central atom, the bond angle will be around 120o, and the geometry is said to be trigonal planar.
Example: BCl3
Even formaldehyde, CH2O, whose double bond has a stronger repulsion on the single bonds, and has bond angles that aren’t exactly 120o is said to be trigonal planar.

5. Molecular Geometry: No Unshared Electrons on the Central Atom
Molecules with four electron groups around the central atom have tetrahedral geometry, with bond angles of 109.5o
Examples would be methane (CH4), or carbon tetrachloride (CCl4) which both have the following structure.

6. Quick Question!
What is the geometry of hydrocyanic acid (aka hydrogen cyanide or prussic acid) HCN? The Lewis structure is: H─C≡N Linear

7. Molecular Geometry: No Unshared Electrons on the Central Atom
Molecules with 5 electron groups around the central atom have trigonal bipyramidial geometry. The three terminal atoms that lie in a single plane (equatorial positions) are 90o apart from each other, while the two that lie above and below this plane (axial positions) are 120o apart from each other.
To picture this, tie 5 balloons together. The knot acts as your central atom.
An example of this would be PCl5 (phosphorus pentachloride)

8. Molecular Geometry: No Unshared Electrons on the Central Atom
Molecules that have 6 electron groups around a central atom have octahedral geometry with 90o bond angles (has 6 points of connection, but 8 faces or sides).
An example would be SF6 (sulfur hexafluoride)

9. Let’s Try a Sample Problem!
Determine the molecular geometry of NO3- Trigonal Planar.

10. The Effect of Lone Pairs
Lone pair repulsion is greater than bonded pair repulsion, and as a result will bend bonded pairs slightly. Lone pairs position themselves to minimize lone pair repulsion. Strength of repulsion goes as follows:
Lone pair-lone pair > loan pair-bonding pair > bonding pair-bonding pair
Molecules with 4 electron groups on the central atom, where one is an unshared pair, has trigonal pyramidal geometry (the bond angle is 107o).
Example: NH3 (ammonia)
If two of the four electron groups are unshared pairs, the molecule has bent geometry (bond angle 104.5o).
Examples: H2O (water) and SCl2 (sulfur dichloride)

11. The Effect of Lone Pairs
Molecules with 5 electron groups around the central atom where one is a lone pair have seesaw geometry.
Example: SF4 (sulfur tetrafluoride)
If two of the electron groups are unshared, the molecule has T-shaped geometry.
Example: BrF3 (bromine trifluoride)
When three of the five electron groups are unshared pairs, the molecule has linear geometry.
Example: XeF2 (xenon difluoride)

12. The Effect of Lone Pairs
Molecules with 6 pairs of electron pairs around the central atom where one pair is unshared, has square pyramidal geometry.
Example: BrF5 (bromine pentafluoride)
When two of the electron pairs are unshared, the molecule has square planar geometry.
Example: XeF4 (xenon tetrafluoride)

13. Let’s Try a Practice Problem
Which statement is always true according to VSEPR theory? a.) The shape of a molecule is determined only by repulsion among bonding electron groups. b.) The shape of a molecule is determined only by repulsions among nonbonding electron groups. c.) The shape of a molecule is determined by the polarity of its bonds. d.) The shape of a molecule is determined by repulsions among all electron groups on the central atom (or interior atoms, if there are more than one).

14. Let’s Predict the Shapes of a Few Molecules
Predict the molecular geometry and bond angle of ClNO (nitrosyl chloride). Bent When predicting the shape of large molecules with more than one interior atom, we predict the shape of each interior atom. Predict the geometry about each interior atom in acetic acid and make a sketch of the molecule. The skeleton structural formula is: O II H3C─ C─OH
Atom
# of e- groups
# of lone pairs
Molecular geometry
C (left) 4
Tetrahedral
C (right) 3
Trigonal Planar O 2
Bent

15. 10. 2-10. 4 pg. 475 #’s 34, 36 (no electron geometry), 38, 40 & 44
pg. 475 #’s 34, 36 (no electron geometry), 38, 40 & 44. Read pgs