Ch 6 Molecular Structure I Lewis Diagrams p

Ch. 6 – Molecular Structure I. Lewis Diagrams (p. 170 – 175) I II III

A. Octet Rule n Remember… · Most atoms form bonds in order to have 8 valence electrons.

A. Octet Rule n F F · Hydrogen 2 valence e F B F · Groups F 1, 2, 3 get 2, 4, 6 valence e S F H N O O H · Expanded octet more than 8 F Very unstable!! valence e (e. g. S, P, Xe) F F Exceptions: - - · Radicals odd # of valence e- -

B. Drawing Lewis Diagrams n Find total # of valence e-. n Arrange atoms - singular atom is usually in the middle. n Form bonds between atoms (2 e-). n Distribute remaining e- to give each atom an octet (recall exceptions). n If there aren’t enough e- to go around, form double or triple bonds.

B. Drawing Lewis Diagrams n CF 4 1 C × 4 e- = 4 e 4 F × 7 e- = 28 e 32 e- 8 e 24 e- F F C F F

B. Drawing Lewis Diagrams n Be. Cl 2 1 Be × 2 e- = 2 e 2 Cl × 7 e- = 14 e 16 e- 4 e 12 e- Cl Be Cl

B. Drawing Lewis Diagrams n CO 2 1 C × 4 e- = 4 e 2 O × 6 e- = 12 e 16 e- 4 e 12 e- O C O

C. Polyatomic Ions n To find total # of valence e-: · Add 1 e- for each negative charge. · Subtract 1 e- for each positive charge. n Place brackets around the ion and label the charge.

C. Polyatomic Ions n Cl. O 4 - 1 Cl × 7 e- = 7 e 4 O × 6 e- = 24 e 31 e+ 1 e 32 e- 8 e 24 e- O O Cl O O

C. Polyatomic Ions n NH 4+ 1 N × 5 e- = 5 e 4 H × 1 e- = 4 e 9 e- 1 e 8 e- 8 e 0 e- H H N H H

D. Resonance Structures Molecules that can’t be correctly represented by a single Lewis diagram. n Actual structure is an average of all the possibilities. n Show possible structures separated by a double-headed arrow. n

D. Resonance Structures n SO 3 O O S O