1. DO NOW Pick up notes and Electron Dot handout
Do the Electron Dot handout NOW. You have ten minutes.

2. ELECTRON DOT DIAGRAMS

3. Determining the Structure of Molecules
MOLECULAR GEOMETRY
Determining the Structure of Molecules

4. Molecular Structures Molecular Formula BeH2
Electron Dot Structure H:Be:H
Structural Formula H – Be – H
Ball and Stick
Molecular Model

5. MOLECULAR GEOMETRY
Structural formulas, such as NH3, provide information about bonding only.
It does not provide direct information about the shape of the bond or the shape of the molecule.
The repulsion between charge clouds in the outer levels of atoms determines the arrangement of the orbitals. The orbital arrangement determines the shape of the molecules.

6. VSEPR VSEPR - Valence Shell Electron Pair Repulsion theory
is based on the number of regions of high electron density around a central atom.
can be used to predict structures of molecules or ions by minimizing the electrostatic repulsion between the regions of high electron density.
can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.
does fail in some cases.

7. VSEPR
In small molecules, electron pairs will spread as far apart as possible to minimize repulsive forces.
Two electron pairs = 180 apart
Three electron pairs = 120 apart
Four electron pairs = 109.5 apart

8. PAIRS OF ELECTRONS
In bonding, electrons can be shared with another atom or paired up within the same atom.
Shared pair = a pair of electrons, one coming from each atom participating in the bond.
Lone or Unshared pair = a pair of electrons in the same energy level of the same atom.

9. SHAPES WE WILL LEARN
Linear
Trigonal Planar
Tetrahedral
Pyramidal
Bent

10. LINEAR Atoms are connected in a straight line.
One or two bonded pairs of electrons
180 bond angles
Examples: HCl CO2

11. TRIGONAL PLANAR Atoms are connected in a flat equilateral triangle.
Three bonded pairs of electrons.
120 bond angle.
Example: BCl3

12. TETRAHEDRAL Atoms are connected in a shape with four surfaces.
Four bonded pairs of electrons
109.5 bond angles.
Example: CH4

13. PYRAMIDAL Atoms are in the shape of a pyramid.
Similar to the tetrahedral but only has three bonded pairs of electrons, not four; has one unshared/lone pair of electrons.
Less than 109.5 bond angle due to unshared pair of electrons.
Example: NH3

14. BENT
Atoms are close to the shape of a tetrahedral, but the two unshared pairs of atoms exert a greater repulsive force than the two pairs in the bonds.
Two shared pairs of electrons and two unshared pairs of electrons
105 bond angle
Example: H2O

15. OTHER SHAPES

16. VSEPR TABLE

17. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
USE ELECTRON DOT DIAGRAMS
1. Look at the chemical formula. Figure out the location of the atoms
Hydrogen is always on the outside
The least electronegative atom is the central atom (most “electropositive”).
2. Draw the Electron Dot Diagram for each atom. Count up the total amount of valence electrons for all the atoms involved.

18. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
3. Determine the number of bonding pairs of electrons by dividing the total # of electrons by two. Remember that each atom has to be bonded at least one other atom.
4. Arrange a skeletal diagram of the molecule by placing the other atoms around the central atom. Place a bonding pair of electrons (2) between the central atom and each of the terminal atoms.

19. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
5. Subtract the number of bonded pairs of electrons for the central atom from the electron total. If there are any electrons left over, these electrons are lone pairs to be placed around the central atom.

20. HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
6. If the terminal (outside) atoms do not have a full octet, place lone pairs of electrons around them. The rest go around the central atom.
7. If the central atom does not have four pairs of electrons around it (and it had a least four valence electrons to begin with), try converting some of the lone pairs to double or triple bonds. (Carbon, nitrogen, oxygen, and sulfur like to form multiple bonds)

21. EXCEPTIONS There are exceptions to the OCTET rule:
Atoms with less than an Octet:
Hydrogen – only 2 valence electrons
Group 2A – only 4 valence electrons
Group 3A – only 6 valence electrons
Atoms with more than an Octet:
Sulfur and phosphorus – 10+ valence electrons
Krypton, xenon, iodine, and others with “d” orbitals will accept more than 8.

22. PRACTICE
CF4 BBr3

23. PRACTICE
MgI2 CO2

24. PRACTICE
Fill in the following chart and predict the molecular shape for the following substances:
Molecule
E- Dot diagram
# of shared/
# of lone/
Electron Dot
Shape of
Bond Type
Formula
all elements
bonded e-
unshared e-
Structure
(e-negativity)

25. TO DO Molecular Geometry – fill in the first column: H2O SiCl4 NH3 Cl2
GaF3