Covalent Bonding, Lewis Dot Structures
1. Covalent bond energies- is an average of the of individual bond dissociation energies. The
amount of energy needed to break a bond.
2. Bond Length--distance between the nuclei of
two bonded atoms
Bond
C-O
C=O C≡O
Bond order
1
2
3
Bond length (pm)
143
122
113
a. Multiple bonds have higher bond energies.
b. Multiple bonds have shorter bond lengths. (stronger bonds hold atoms tighter)
2. Calculation of ΔH from bond energy
D = bond energy per mole (always a positive number)
 = Sum of
 H o reaction =  D (bonds broken) -  D(bonds made)
Exercise 5
∆H from Bond Energies
Using the bond energies listed in Table 8.4, calculate ∆H for the reaction of methane with
chlorine and fluorine to give Freon-12 (CF2Cl2).
CH4(g) + 2Cl2(g) + 2F2(g) → CF2Cl2(g) + 2HF(g) + 2HCl(g)
∆H = -1194 kJ
3. Localized Electron Bonding Model- a way to visualize covalent bonding. Relax, it’s the
stuff you learned in Chem I.
a. lone pair – electrons that are localized on an atom and not shared.
b. bonding pair- electrons that are found in the space between the atoms and are shared
c. Three parts to the model.
1. Description of the valence electrons using Lewis Dot structures.
2. Prediction of the geometry of the molecule using VSEPR charts
3. Description of the type of atomic orbitals used by the atoms to share electrons.
4. Lewis Dot Structures – based on obtaining a duet for Hydrogen and an octet for the others.
A. Predict the location of the atoms
a. Hydrogen is always at the END
b. The least electronegative atom is the CENTRAL ATOM. CARBON is always
the central atom
B. Find the total number of electrons available for bonding. This is the total number of valence
electrons.
C Determine the number of bonding pair by dividing the number of electrons by two.
D. Place one bonding pair (single bond) between the central atom and the terminal atom.
E. Dot the remaining electrons around the atoms.
Practice These:
F2
CCl4
H2S
F2O
NH3
ClO4-
CH3I
NH4+
5. Exceptions to the Octet Rule.
A. Boron- will bond with only 6 electrons around it. It behaves as electron deficient and hunts
electron rich molecules such as NH3.
B. Berylium will also bond with less than 8 electrons.
B. Period 3 or high atoms- exceed the octet rule.
Exercise 7
Lewis Structures for Molecules That Violate the Octet Rule I
Write the Lewis structure for PCl5.
See diagram – pg
383
Exercise 8
Lewis Structures for Molecules That Violate the Octet Rule II
Write the Lewis structure for each molecule or ion.
a. ClF3
b. XeO3
c. RnCl2
d. BeCl2
e. ICl4-
See diagram – pg 384