1. Chapter 8 Section 3
Molecular Structures

2. Structural Formulas
Molecular formulas tell you the type and number of each atom in a molecule.
Example: C2H5OH
There a different models that can be used to represent a molecule.

3. Structural Formulas
One of the most useful is the structural formula, which uses letter symbols and bonds to show the relative positions of atoms.
You can predict the structural formula of a molecule by drawing the Lewis structure of it.

4. Rules for drawing Lewis Structures
1. Predict the central atom – usually the atom in the molecule closest to the left side of the periodic table.
2. Determine the number of electrons available for bonding – This is equal to the sum of the valence electrons for all atoms present.
3. Bond each atom – use two electrons to form a single bond between each atom present.
4. Distribute remaining electrons.

5. Rules for drawing Lewis Structures
5. Check that the octet rule is satisfied for all atoms (except hydrogen)
If not, begin making double and triple bonds until the octet rule is obeyed by all atoms (except hydrogen).

6. Lewis Structure Example
NH3
Predict central atom – N
Sum electrons – = 8
Bond each atom using a pair of electrons
Distribute remaining electrons
Check octet rule

7. Lewis Structure Example
You try…H2O

8. Lewis Structure Example
CO2
Predict central atom – C
Sum up electrons – = 16
Bond each atom
Distribute remaining electrons
Check for octet rule…is it satisfied?

9. Lewis Structure example
CO2 continued
The octet rule was not satisfied for each element, so we now start over using a double bond instead of a single bond and repeat the process

10. Lewis Structure Example
You try CO

11. Lewis Structures for Polyatomic Ions
Only difference is that these molecules have charges.
If the charge is negative, add that number to the total number of valence electrons.
If the charge is positive, subtract that number from the total number of valence electrons.

12. Polyatomic Example
PO4-3

13. Polyatomic Example
You try NH4+

14. Exceptions to the Octet Rule
Suboctets: A few elements can form bonds and have less than an octet. An example is Boron
BH3
Expanded Octets: Usually can occur in elements found in periods 3 or higher because of its d energy levels. These have more than an octet. Example: Phosphorous
PCl5

15. Chapter 8 Section 4
Molecular Shapes

16. VSEPR Model
The shape of a molecule determines many of its physical and chemical properties.
The molecular geometry or shape of a molecule can be determined once a Lewis structure is drawn.
The used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion Model or VSEPR Model

17. VSEPR Model
This model is based on the arrangement that minimizes the repulsion of shared and unshared electron pairs around the central atom.
The electron pairs in a molecule repel each other and this force causes the atoms in the molecule to be positioned at fixed angles relative to one another.
The angle formed by two terminal atoms and the central atom is the bond angle.

18. Molecular Shapes Molecular Shape Total Electron Pairs Shared Pairs
Lone Pairs
Bond Angles
Linear 2
180
Trigonal Planar 3
120
Tetrahedral 4
109.5
Trigonal Pyramidal 1
107.3
Bent
104.5
Trigonal Bipyramidal 5
90/120
Octahedral 6 90

19. Molecular Shapes
Linear Example: BeCl2
Trigonal Planar Example: AlCl3

20. Molecular Shapes Tetrahedral Example: CH4
Trigonal Pyramidal Example: PH3

21. Molecular Shapes
Bent Example: H2O
Trigonal Bipyramidal Example: NbBr3

22. Molecular Shapes
Octahedral Example: SF6