1. IIIIII Lewis Diagrams (p. 170 – 175) Ch. 6 – Molecular Structure

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

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

4. B. Rules for Lewis Structures n 1. Ensure that chemical formula is the correct one. n 2. Determine the total number of valence electrons for each element. n 3. Determine the central atom. General rule is the atom with the lowest electronegativity. Hydrogen is never central. If carbon is present, it will be central almost every time. If an atom is a single atom, then it will be central (Ex. ClO4-; Cl central atom). n 4. Draw a skeletal structure with the central atom and all the terminal atoms attached to it. Connect the terminal atoms to the central atom with a dash. Each dash represents two electrons. Subtract the bonding electrons from the total.

5. B. Rules for Lewis Structures n 5. Begin placing lone pair electrons on the terminal atoms to satisfy the octet rule for all of the terminal atoms. When the octet rule is satisfied for all terminal atoms, any remaining electrons are placed on the central atom. n 6. If all of the valence electrons are in place and the octet rule is not satisfied for all of the atoms, begin to search for double and triple bonds. ONLY SIX ATOMS CAN FORM MULTIPLE BONDS: C, N, O, Si, S, P.

6. B. Rules for Lewis Structures Exceptions to the Octet Rule n 1. Hydrogen can accommodate only two electrons. (Hydrogen is never the central atom.) n 2. Boron and Beryllium can have less than eight electrons. (They can be satisfied with six electrons, but the reasons are unknown.) n 3. Elements in the 3rd period and beyond can accommodate more than 8 electrons (expanded valence).

7. 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 arent enough e - to go around, form double or triple bonds.

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

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

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

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

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

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

14. D. Resonance Structures n Molecules that cant 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.

15. D. Resonance Structures O O S O O O S O O O S O n SO 3