VSEPR Theory:
VSEPR means Valence Shell Electron Pair Repulsion Theory
This theory is used to predict shapes of molecules.
The basic idea is that the shape of the molecule depends on the number of
electron pair that surround the central atom in the molecule.
The rule that is applied is that the electron pairs around the central atom in the
molecule will repel each other and go as far apart as possible.
Multiple bonds (double or triple) count as one bonding pair for VSEPR predictions.
Your knowledge of geometry will be used to determine how far apart the electron
pairs about the central atom are when they are as far apart as possible.
Number of and
type Atoms
(bonding or nonbonding atoms)
4 bonding pairs
Shape
Angles between
bonding pairs
Tetrahedral
109.5°
3 bonding pairs
Triangular Pyramid
and 1 non-bonding
107°
pair
2 bonding pairs
Bent or V-shaped
and 2 non-bonding
104.5°
pairs
2 bonding pairs
Linear
180°
3 bonding pairs
Triangular Planar
120°
Diagram/Example
VSEPR Theory:
VSEPR means Valence Shell Electron Pair Repulsion Theory. This theory is used to
predict shapes of molecules. The basic idea is that the shape of the molecule
depends on the number of electron pair that surround the central atom in the
molecule. The rule that is applied is that the electron pairs around the central
atom in the molecule will repel each other and go as far apart as possible.
Multiple bonds (double or triple) count as one bonding pair for VSEPR predictions.
Fill out the attached chart from the class Powerpoint to understand how VSEPR
theory determines shape, and consequently the polarity of covalent molecules.
Number of and
type Atoms
(bonding or nonbonding atoms)
4 bonding pairs
3 bonding pairs
and 1 non-bonding
pair
2 bonding pairs
and 2 non-bonding
pairs
2 bonding pairs
3 bonding pairs
Shape
Angles between
bonding pairs
Diagram/Example
Instructions:
1) Draw the molecule or compound in 3-D according to VSPER theory. (Use
Lewis dot structures to determine how many bonding and nonbonding
pairs are in the molecule or compound to determine the shape if needed.)
2) Name the shape.
3) Look up each atom’s electronegativity and calculate the ΔEN.
4) State if the bond type(s) as ionic, covalent polar, or covalent non-polar.
Write in charges or dipoles (partial charges) as necessary. The atom with
the higher EN will be negative (or partially negative) and the atom with
lower EN will be positive (or partially positive).
5) State if the molecule/compound is ionic, polar or nonpolar overall.
6) Build the molecule/compound
Examples:
1) CCl4
2) Na2O
3) H2O
4) BF3
5) NH3
Practice
1) Draw the molecule or compound in 3-D according to VSPER theory. (Use
Lewis dot structures to determine how many bonding and nonbonding
pairs are in the molecule or compound to determine the shape if needed.)
2) Name the shape.
3) Look up each atom’s electronegativity and calculate the ΔEN.
4) State if the bond type(s) as ionic, covalent polar, or covalent non-polar.
Write in charges or dipoles (partial charges) as necessary. The atom with
the higher EN will be negative (or partially negative) and the atom with
lower EN will be positive (or partially positive).
5) State if the molecule/compound is ionic, polar or nonpolar overall.
Click on the molecule's name to see the answer, but first try to do it yourself!
(Note: Trigonal pyramidal is listed as tetrahedral. This is because it really is just a variation on
the tetrahedral shape, and so is sometimes referred to as such. I expect you to differential
between trigonal pyramidal and tetrahedral.)
1. AlCl3 - Aluminum
Trichloride
2. CS2 - Carbon Disulfide
3. BeI2 - Beryllium Diiodide
4. NOCl - Nitrosyl Chloride
5. PO(OH)3 - Phosphoric Acid
6. SO2Cl2 - Sulfuryl Chloride
7. NOCl - Nitrosyl Bromide
8. BCl3 - Boron Trichloride
9. SiH4 - Silicon Tetrahydride
10. BeBr2 - Beryllium
Dibromide
11. CH2O - Formaldehyde
12. NH2Cl - Chloramine
13. CH4 - Methane
14. SO2 - Sulfur Dioxide
15. AlF3 - Aluminum
Trifluoride
16. NH3 - Ammonia
17. SeH2 - Hydrogen Selenide
18. H2O - Water
19. CO2 - Carbon Dioxide
20. SCl2 - Sulfur Dichloride
21. NO2F - Nitryl Fluoride
22. CSe2 - Carbon Diselenide
23. CCl4 - Carbon Tetrachloride
24. AlBr3 - Aluminum
Tribromide
25. BeCl2 - Beryllium
Dichloride
26. SO3 - Sulfur Trioxide
27. PCl3 - Phosphorus
Trichloride
28. BeF2 - Beryllium Difluoride
29. BF3 - Boron Trifluoride
(Source: http://www.tutorhomework.com/Chemistry_Help/Molecular_Geometry/Polar_Or_Nonpolar.html#problems )