Lewis Structures: Representations of Molecular Compounds, Slides of Chemistry

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Lewis Structures

Representations of

Molecular Compounds

Drawing Lewis Structures

1. Sum the number of valance electrons.
2. Add electrons for anions (negative ions).
3. Subtract electrons for cations (positive ions).
4. Write the symbols for the atoms; what elements are connected to what other atoms and connect then with a single line (single bond).
5. The central atom is usually the atom first in a formula and is usually the least electronegative. 1. Example SO 4 2 - , SF 4
6. Chemical formulas are often written in order.
   1. Example CH 3

COOH

7. Formulas for ions are written in [ ].

Examples

1. phosphorus trichloride

2. dichloromethane: CH

2

Cl

2

3. hydrocyanic acid: HCN

4. water

5. ammonia

6. ethane: C

2

H

6

7. ethene: C

2

H

4

8. ethyne: C

2

H

2

9. carbon dioxide

Formal charge

 In some cases several Lewis structures can be drawn for the same molecule or ion.  In these cases we use formal charge to help us evaluate with structure is the best.

1. All unshared electrons are assigned to the atom on which they reside.
2. All shared electrons are divided equally between the atoms in the bond.
3. Formal charge is the number of valance electrons minus the number of electron assigned.
4. Choose the structure that give the formal charges closest to zero.
5. Choose the structure that has the negative charge on the most electronegative atom.

Resonance

1. When 2 or more alternate structures can be drawn for the same molecule or ion.
2. The actual structure is “between” the two drawn structures.
3. structure #1 ↔ structure #
4. sulfate
5. hypochlorite
6. chlorite
7. chlorate
8. perchlorate
9. benzene