Section 9.3

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

Resonance

structural formula resonance coordinate covalent bond Write a paragraph that includes all of the terms above.

Section 9-3 Section 9.3 Molecular Structures (cont.) Structural formulas show the relative positions of atoms within a molecule.

A structural formula uses letter symbols and bonds to show relative positions of atoms.

Drawing Lewis Structures Predict the location of certain atoms. Determine the number of electrons available for bonding. Determine the number of bonding pairs. Place the bonding pairs. Determine the number of bonding pairs remaining. Determine whether the central atom satisfies the octet rule.

Atoms within a polyatomic ion are covalently bonded.

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.

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.

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.

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.

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.

Molecular Structures Mastering Concepts: 272(79) Terms: 252 Practice Problems: 255(30-34), 256(35-38), 258(39-41) Cornell Notes: 9.3 Section Assessment: 258(42-46) Mastering Problems: 272(99-104)

Mastering Concepts: 272(79) 79. What must be known in order to draw the Lewis structure for a molecule? (9.3) the number of valence electrons for each atom

Molecular Structures Homework: Mastering Concepts: 272(79) Terms: 252 Practice Problems: 255(30-34), 256(35-38), 258(39-41) Homework: Cornell Notes: 9.3 Section Assessment: 258(42-46) Mastering Problems: 272(99-104)

Practice Problems: 255 (30-34) Draw a Lewis structure for each of the following: 30. NF3 31. CS2 32. BH3 33. ClO4- 34. NH4+

3) Electron pairs in bonds Compound 30. NF3 3) Electron pairs in bonds 26 – 6= 20 1) total valence electrons 5+ 3(7) = 26 4) Octet for outer atoms .. :F: N: :F: :F: .. .. 2) Skeletal Structure F N F F

1) total valence electrons 4+ 2(6) = 16 Compound 31. CS2 4) Octet for outer atoms .. .. :S C S: .. .. 1) total valence electrons 4+ 2(6) = 16 5) Remaining e– s on center atom 2) Skeletal Structure S C S 6) Create multiple bonds? 3) Electron pairs in bonds 16-4=12 Final structure

1) total valence electrons 3+ 3(1) = 6 Compound 32. BH3 4) Octet for outer atoms 1) total valence electrons 3+ 3(1) = 6 5) Remaining e– s on center atom 2) Skeletal Structure H B H H 6) Create multiple bonds? none 3) Electron pairs in bonds 6-6=0 Final structure

.. .. . . Cl .. Compound 33. ClO4- 4) Octet for outer atoms .. .. .. .. :O O: . . Cl .. .. .. . . .. 1) total valence electrons 7+4(6)+1 = 32 5) Remaining e– s on center atom 2) Skeletal Structure O O Cl 6) Create multiple bonds? 3) Electron pairs in bonds 32-8=24 Final structure

3) Electron pairs in bonds Compound 34. NH4+ 3) Electron pairs in bonds 8 – 8=0 1) total valence electrons 5+4 (1) -1= 8 4) Octet for outer atoms 2) Skeletal Structure H H N H

[ ] Resonance structures 05/10/99 For example, the bond lengths of CHO2– predicted by the Lewis structure are incorrect H C O – [ ] The double CO bond should be shorter, and possess a greater bond energy (due to the higher concentration of e–s in a double bond) Yet, experimentally, both bonds are the same The reason is due to “resonance”

[ ] [ ] [ ] Resonance structures H C O – – O 1 H C – O 2 H C O 05/10/99 H C O – [ ] [ ] – O 1 H C [ ] – O 2 H C O

[ ] [ ] [ ] Resonance structures H C O – – O 1 H C – O 2 H C O 05/10/99 A resonance structure can be drawn for any molecule in which a double bond can be formed from two or more identical choices Resonance structures can be drawn 2 ways… H C O – [ ] [ ] – O 1 H C [ ] – O 2 H C O

Molecular Structures Mastering Concepts: 272(79) Terms: 252 Practice Problems: 255(30-34), 256(35-38), 258(39-41) Cornell Notes: 9.3 Section Assessment: 258(42-46) Mastering Problems: 272(99-104)

Practice Problems: 256 (35-38) Draw the Lewis resonance structures for the following. 35. SO3 36. SO2 37. O3 38. NO2-

1) total valence electrons 6+ (3)6 = 24 Compound 35. SO3 4) Octet for outer atoms .. :O: S :O: :O: .. .. 1) total valence electrons 6+ (3)6 = 24 5) Remaining e– s on center atom 2) Skeletal Structure O O O 24 -6=18 6) Create multiple bonds? :O: :O: .. ..

.. Compound 36. SO2 4) Octet for outer atoms .. .. :O S O: .. .. 1) total valence electrons 6+ (2)6 = 18 5) Remaining e– s on center atom .. .. .. 2) Skeletal Structure O S O 18 -4=14 6) Create multiple bonds? ..

1) total valence electrons (3)6 = 18 5) Remaining e– s on center atom Compound 37. O3 4) Octet for outer atoms O :O: :O: .. .. 1) total valence electrons (3)6 = 18 5) Remaining e– s on center atom .. .. O .. :O O: 2) Skeletal Structure O O 18 -4=14 6) Create multiple bonds?

1) total valance electrons 5+(2)6+1 = 18 Compound 38. NO2- 4) Octet for outer atoms N :O: :O: .. .. 1) total valance electrons 5+(2)6+1 = 18 5) Remaining e– s on center atom .. .. N .. :O O: 2) Skeletal Structure O O 18 -4=14 6) Create multiple bonds?

Molecular Structures Mastering Concepts: 272(79) Terms: 252 Practice Problems: 255(30-34), 256(35-38), 258(39-41) Cornell Notes: 9.3 Section Assessment: 258(42-46) Mastering Problems: 272(99-104)

Practice Problems: 258 (39-41) Draw the correct Lewis structures for the following molecules, which contain expanded octet. 39. SF6 40. PCl5 41. ClF3

Practice Problems: 258 (39-41) Draw the correct Lewis structures for the following molecules, which contain expanded octet. 39. SF6

Practice Problems: 258 (39-41) Draw the correct Lewis structures for the following molecules, which contain expanded octet. 40. PCl5

Practice Problems: 258 (39-41) Draw the correct Lewis structures for the following molecules, which contain expanded octet. 41. ClF3

Practice Quiz 975 (4-7) Section 9-3