1. Bonding Unit Part B) Structures and Shapes
VSEPR-Shapes Notes
Bonding Unit
Part B) Structures and Shapes

2. EXCEPTIONS TO THE OCTET RULE
Octet rule can be used to predict the structural formals for millions of molecules..
But there are exceptions!
Atoms with less than an octet
Atoms with more than an octet
Molecules with an odd # of electrons
Some of the geometric shapes represent these exceptions

3. EXCEPTION: having less than octet
Compounds with BORON
Has 6 valence electrons
Ex. BF3, BBr3
Write this one down… Shape: TRIGONAL PLANAR SHAPE

4. EXCEPTION: having more than octet
Compounds with Sulfur and Phosphorous can form bonds that give more than an octet
Usually 10 or 12 electrons
Ex. SF SF PCl5
Trigonal Bipyramidal
Octahedral

5. EXCEPTION: having an odd number of electrons
If odd # of valence electrons, octet is impossible
Ex. NO
Total of 11 valence electrons
….it is an unstable molecule

6. VSEPR Model
The shape of a molecule determines physical & chemical properties
Electron densities created by the overlap of the orbitals of shared electrons determine molecular shape
The molecular geometry (shape) can be determined once a lewis dot structure is drawn.

7. VSEPR Model The model used to determine molecular shape
Valence Shell Electron Pair Repulsion model
Central principle: electrons repel each other
Based on an arrangement that minimizes the repulsion of shared and unshared electron pairs around the central atom

8. Shapes
The shape of a molecule depends a number of factors. These include:
Atoms forming the bonds
Bond distance
Bond angles

9. Types of electrons
Bonding electrons: the valence electrons are shared between atoms to form covalent bonds.
Nonbonding electrons: valence electrons may not be shared with other atoms
Also called lone pairs

10. Shape: Linear Atoms connected in straight line
All molecules with only 2 atoms and also many with 3 atoms
Bond angle is 180°
Draw:

11. Shape: Trigonal Planar
Central atom is bonded to 3 other atoms AND the central atom has no unshared pairs of electrons.
Bond angle is 120°
Draw:

12. Shape: Tetrahedral Draw:
Central atom has 4 pairs of bonding electrons / 4 regions of electron density
It is like a pyramid with a flagpole on the top.
Bond angle is 109.5°
Draw:

13. Shape: Pyramidal Draw:
Central atom bonded to 3 other atoms and an unshared pair of valence electrons
Bond angle is 107°
Draw:

14. Shape: Bent Draw: 3 or more regions of electron density
Typically two regions of bonding electrons and two regions of nonbonding electrons
Bond angle is 105°
Draw:

15. Practice Draw the Lewis Structure and identify the shape: CCl4 BF3
BeCl2
H2S
PCl3

16. Answer CCl4, Tetrahedral BF3, Trigonal Planar BeCl2, Linear H2S, Bent
PCl3, Pyramidal

17. Molecule Polarity
To determine whether a molecule is polar or nonpolar, you need to look at:
Polarity of the bonds
Shape of the molecule
You already know how to determine the polarity of bonds….

18. Molecule Polarity Have polar bonds
If a molecule has only non-polar bonds, it is a non-polar molecule.
If a molecule contains polar bonds, it is not necessarily polar. If symmetry cancels out the polarity, the molecule is non-polar.
To be a polar molecule:
Have polar bonds
Have the polar bonds arranged in such a way that their polarity is not cancelled out
HF and H2O are both polar molecules.
CCl4 is non-polar

19. Determining Polar VS Nonpolar Molecule
Draw lewis structure/shape
Look at the electronegativity difference between each atom involved in a bond. Identify each bond as nonpolar or polar.
If all bonds are nonpolar, it is a nonpolar molecule.
If bonds are polar….
Draw arrows in the direction of highest E.N. on each bond.
If the arrows do not cancel each other out, it is polar. There is a positive and negative end.
If symmetry of the molecule cancels out the arrows, it is nonpolar.

20. Example: Cl2
Polar or nonpolar molecule?

21. Example: H2O
Polar or nonpolar molecule?

22. Example: CO2
Polar or nonpolar molecule?