1. Lewis Model Bonding models and ionic bonds
By John Fleming

2. Lewis Model
Named after GN Lewis - shows electrons as dots around the symbol
Predicts how elements will bond
Draw dots on the four compass points of the chemical symbol, put one on each side before going back to put a second on each side
For cations you put the charge at the top left side
For anions you enclose the symbol and electrons in brackets before putting the charge

3. Types of chemical bonds - review
Ionic bond - between metal and non-metal
Covalent bond - between two nonmetals
Metallic bond - between two metals

4. Valence Electrons - Octet Rule
Every elements wants to be stable - needs full valence shell
Elements ionize to get to closest full valence shell
Metals lose electrons to get to Noble Gas config from previous period
Non-metals (generally) gain electrons to reach the noble gas config farther down their group
Noble gases are uninterested in bonding
It’s called the octet rule because atoms generally are looking to have 8 electrons in their outermost shell, unless they’re small enough that their outermost shell would have 2 electrons

5. Lattice Energy & the Born-Haber Cycle
Lattice energy - energy associated with the formation of a crystalline lattice of alternating cations and anions.
lattice energy can be calculated with the Born-Haber cycle.
The Born-Haber cycle is a “hypothetical series of steps that represents the formation of an ionic compound from its constituent elements.”
Born
Haber

6. Finding Lattice Energy of NaCl - from the Textbook
Finding the lattice energy requires using known ΔHo values.
Na(s) Na(g) ΔHostep 1 = 108 kJ
1/2 Cl(g) Cl2(g) ΔHostep 2 = 122 kJ
Na(g) Na+(g) ΔHostep 3 = 496 kJ
Cl(g) + e- Cl-(g) ΔHostep 4 = -349 kJ
Na+(g)+ Cl-(g) NaCl(s) ΔHostep 5 = ΔHolattice = ?

7. Finding Lattice Energy (cont.)
Using Hess’s law, we know that the overall enthalpy of formation is equal to the sum of the steps in the Born-Haber cycle.
ΔHof = Hostep 1 + ΔHostep 2 +ΔHostep 3 +ΔHostep 4 +ΔHostep 5
ΔHof = Hostep 1 + ΔHostep 2 +ΔHostep 3 +ΔHostep 4 +ΔHolattice
ΔHolattice = -411 kJ -(108 kJ kJ kJ kJ)
ΔHolattice = -788 kJ

8. Practice Problem
Draw a Lewis structure for Sodium, Phosphorus and Oxygen.

9. Chemistry Joke
What did Na+ say to Cl-?
I’ve got my ion you.