1. Formal Charges

2. # valence e- - # covalent bonds - # e- in lone pairs
Sometimes molecules can be drawn in more ways than one… which is the best way?
What are formal charges?
Look at the number of valence electrons an atom has
Subtract # of covalent bonds it is touching
Subtract # of e- in lone pair
# valence e- - # covalent bonds - # e- in lone pairs
THE MOST STABLE MOLECULE IS ONE WHERE THE FORMAL CHARGES ARE THE LOWEST!

3. Which HNO2 is more stable?

4. Sulfuric Acid – Which one is correct?
TOO MANY FORMAL CHARGES!
FORMAL CHARGE ON EACH ATOM IS ZERO!

5. Charged Molecules
You need to add or subtract electrons, depending on the charge
Be sure to add square brackets, and include the charge!
Ex. NH4+
the + means you need to take one electron away
5 e- (from nitrogen) + 4 e- (from hydrogens) -1 e- (from the + charge) = 8 e- to use

6. Resonance Structures
Sometimes molecules have fractional bonds as a result of electron movement among bonds
To accurately represent the molecule, you need to show all possibilities!
Each of these structures is called a RESONANCE STRUCTURE
The real molecule is all of these structures!
Each oxygen atom has 1.3 bonds
Which is why you must draw all 3 structures!

7. Try to draw the Lewis Structure for HPO32-

8. Bond Energy The measure of BOND STRENGTH
The stronger the bond, the HIGHER the bond energy
It is expressed as heat required to break 1 mole of molecules

9. It takes energy to BREAK bonds (endothermic)
Energy is released when bonds are FORMED (exothermic)

10. breaking required making released Example:
The reaction turning 1 molecule of H2(g) into two H(g) atoms involves bond _______________. Energy is ______________.
The reaction turning two oxygen atom into a molecule of O2 involves bond _______________. Energy is ______________.
breaking
required
making
released

12. c) Compare the bond energies of single, double, and triple bonds
c) Compare the bond energies of single, double, and triple bonds. Explain the data.

13. c) Compare the bond energies of single, double, and triple bonds
c) Compare the bond energies of single, double, and triple bonds. Explain the data.
Triple > double > single
More e- shared = more attractive forces between 2 nuclei

14. d) Compare the bond energies of C-N, C-O, C-F. Explain the data.

15. d) Compare the bond energies of C-N, C-O, C-F. Explain the data.
C-F > C-O > C-N

16. What do you notice about atomic radius?
C–N
C–O
C–F

17. d) Compare the bond energies of C-N, C-O, C-F. Explain the data.
C-F > C-O > C-N
Bond strength increases with increasing electronegativity and decreasing radius

18. Bond length: the distance between 2 nuclei of a molecule
The shorter the bond length, the stronger the bond, the higher the bond energy
Example: How do the bond lengths of single, double, and triple bonds compare?
C C
C C
C C
0.120 nm
0.134 nm
0.154 nm

19. LiF LiCl LiBr Electronegativity: F > Cl > Br
Example: Rank the following compounds in order of increasing melting point (increasing bond strength).
LiF LiCl LiBr
Electronegativity: F > Cl > Br
bond strength LiF > LiCl > LiBr
melting point LiF > LiCl > LiBr
845°C
605°C
552°C

20. Example: Which of the following compounds has the higher bond energy?
NaCl MgO
MgO is being held together by 2+ and 2- charge as opposed to 1+ and 1- in NaCl
801°C
2852°C

21. For ionic compounds, two things must be considered in order to determine bond strength. They are (in order of importance):
The charge: the ___________ the charge on each ion in the ionic compound, the ______________ the ionic bond, the ___________ the bond energy.
Electronegativity: the higher the electronegativity of the non-metal, the _______________ the attraction, the ______________ the bond energy.
higher
stronger
higher
higher
higher

22. How do you know if a bond is strong?
Covalent Compounds
Number of bonds: more bonds = strong bond
Atomic radii: short bond length = strong bond
Electronegativity: strong electronegativity = strong bond
Ionic Compounds
Charge: high charge = strong bond
Electronegativity of non-metal: high electronegativity = strong bond