CHEM 6A Lecture Notes - Lecture 23: Vsepr Theory, Bond Energy, Ionic Bonding
3 Jun 2018
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Exceptions to Octet Rule
1. Free radicals (odd number of electrons)
a. E.g. NO2
b. #VE = 5
2. Electron deficient compounds
a. Group 13 atoms (B and Al) can have < 8 e-
b. E.g. BF3 (see ex 7-11 on p. 217)
3. Expanded valence shell compounds
a. Atoms in 3rd period or higher can have >8 e-
b. E.g. PCl5 #VE = 5 + 7(5) = 40 e- and octet = 48 e- meaning SE = 8 e- for 4 bonds
- Guidelines fail
Ex. 5: HClO2 chlorous acid
â—Ź VE = 1 + 7 + 6(2) = 20e-
Textbook p221-222
HW Q 12, 15, 19
â—Ź 19: HNO2: also optimize FC
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H first in formula = acid
Oxyacid = contains oxygen
â—Ź In both cases, H is attached to O (HW 219)
Only exception H3PO3 chlorous acid
Valence and Bond Pattern
â—Ź 8 = (# valence electrons)
= # bonds the arom wants to form usually
â—Ź C forms 4 bonds
â—Ź N forms 3 bonds
â—Ź O forms 2 bonds
â—Ź Halogens and hydrogen (h) form 1 bond
â—Ź H is always a terminal atom
â—Ź Exception: B and Al form 3 bonds and have no lone pair electrons
Bond Polarity
● Difference in electronegativity () between bonded atoms indicates bonding
character:
1. Large  (>2.0): ionic bond
a. E.g. KF:  = 3.98 - 0.82 = 3.16 K+F-
2. Intermediate : polar covalent bond
a. E.g. HCl:  = 3.16 - 2.1 = 1.1
3. Small  (<0.4): pure covalent bond/nonpolar bond
a. E.g. diatomic elements ( = 0);
i. Carbon-carbon bonds ( = 0)
ii. C-H bonds ( = 0.45)
Special note:
â—Ź BeCl2
● Be: = 1.57
find more resources at oneclass.com
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CHEM 6A Full Course Notes
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Document Summary
Exceptions to octet rule: free radicals (odd number of electrons, e. g. No2: #ve = 5, electron deficient compounds, group 13 atoms (b and al) can have < 8 e, e. g. Bf3 (see ex 7-11 on p. 217: expanded valence shell compounds, atoms in 3rd period or higher can have >8 e, e. g. Pcl5 #ve = 5 + 7(5) = 40 e- and octet = 48 e- meaning se = 8 e- for 4 bonds. In both cases, h is attached to o (hw 219) = # bonds the arom wants to form usually. Halogens and hydrogen (h) form 1 bond. Exception: b and al form 3 bonds and have no lone pair electrons. Difference in electronegativity () between bonded atoms indicates bonding: large (>2. 0): ionic bond, e. g. Kf: = 3. 98 - 0. 82 = 3. 16: intermediate : polar covalent bond, e. g.
