1. CHEMISTRY Matter and Change
Chapter 8: Covalent Bonding

2. SECTION8.3
Molecular Structures
List the basic steps used to draw Lewis structures.
Explain why resonance occurs, and identify resonance structures.
Identify three exceptions to the octet rule, and name molecules in which these exceptions occur.
ionic bond: the electrostatic force that holds oppositely charged particles together in an ionic compound

3. Molecular Structures structural formula resonance
SECTION8.3
Molecular Structures
structural formula
resonance
coordinate covalent bond
Structural formulas show the relative positions of atoms within a molecule.

4. Molecular Structures Structural Formulas
SECTION8.3
Molecular Structures
Structural Formulas
A structural formula uses letter symbols and bonds to show relative positions of atoms.

5. Structural Formulas (cont.)
SECTION8.3
Molecular Structures
Structural Formulas (cont.)
Drawing Lewis Structures
Predict the location of certain atoms, the atom that has the least attraction for shared electrons will be the central atom in the molecule (usually, the one closer to the left side of the periodic table). All other atoms become terminal atoms. Note: Hydrogen is always a terminal atom.
Determine the number of electrons available for bonding, the number of valence electrons.
Determine the number of bonding pairs, divide the number of electrons available for bonding by two.

6. Structural Formulas (cont.)
SECTION8.3
Molecular Structures
Structural Formulas (cont.)
Place the bonding pairs, place a single bond between the central atoms and each of the terminal atoms.
Determine the number of bonding pairs remaining, Subtract the number of bonding pairs in step 4 from the number of bonding pairs in step 3. Place lone pairs around terminal atoms, except hydrogen, to satisfy the octet rule. Any remaining pairs will be assigned to the central atom.
Determine whether the central atom satisfies the octet rule, If not, convert one or two of the lone pairs on the terminal atoms into a double bond or a triple bond between the terminal atom and the central atom. Remember: carbon, nitrogen, oxygen and sulfur often form double and triple bonds.

7. Structural Formulas (cont.)
SECTION8.3
Molecular Structures
Structural Formulas (cont.)
Atoms within a polyatomic ion are covalently bonded.
The procedure for drawing Lewis structures is similar to drawing them for covalent compounds.
Difference is, you need to determine the number of electrons available for bonding, find the number of electrons available in the atoms present and then subtract the ion charge if the ion is positive or add the ion charge if the ion is negative.

8. Molecular Structures Resonance Structures
SECTION8.3
Molecular Structures
Resonance Structures
Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.
This figure shows three correct ways to draw the structure for (NO3)-1.

9. Resonance Structures (cont.)
SECTION8.3
Molecular Structures
Resonance Structures (cont.)
Two or more correct Lewis structures that represent a single ion or molecule are resonance structures.
The molecule behaves as though it has only one structure.
The bond lengths are identical to each other and intermediate between single and double covalent bonds.

10. octet rule
 States that atoms of low (<20) atomic number tend to combine in such a way that they each have eight electrons in their valence shells

11. Exceptions to the Octet Rule
SECTION8.3
Molecular Structures
Exceptions to the Octet Rule
Some molecules do not obey the octet rule.
A small group of molecules might have an odd number of valence electrons.
NO2 has five valence electrons from nitrogen and 12 from oxygen and cannot form an exact number of electron pairs.

12. Exceptions to the Octet Rule (cont.)
SECTION8.3
Molecular Structures
Exceptions to the Octet Rule (cont.)
A few compounds form stable configurations with less than 8 electrons around the atom—a suboctet.
A coordinate covalent bond forms when one atom donates both of the electrons to be shared with an atom or ion that needs two electrons.

13. Exceptions to the Octet Rule (cont.)
SECTION8.3
Molecular Structures
Exceptions to the Octet Rule (cont.)
A third group of compounds has central atoms with more than eight valence electrons, called an expanded octet.
Elements in period 3 or higher have a d-orbital and can form more than four covalent bonds.

14. SECTION8.3
Section Check
What is it called when one or more correct Lewis structures can be drawn for a molecule?
A. suboctet
B. expanded octet
C. expanded structure
D. resonance

15. Where do atoms with expanded octets occur?
SECTION8.3
Section Check
Where do atoms with expanded octets occur?
A. transition metals
B. noble gases
C. elements in period 3 or higher
D. elements in group 3 or higher