Section 8.3 Molecular Structures

• List the basic steps used to draw Lewis structures.

ionic bond: the electrostatic force that holds oppositely charged particles together in an ionic compound

• Explain why resonance occurs, and identify resonance structures.

• Identify three exceptions to the octet rule, and name molecules in which these exceptions occur.

Section 8.3 Molecular Structures (cont.)

structural formula

resonance

coordinate covalent bond

Structural formulas show the relative positions of atoms within a molecule.

Section 8-1

Why do atoms bond?

• Atoms gain stability when they share electrons and form covalent bonds.

• Lower energy states make an atom more stable.

• Gaining or losing electrons makes atoms more stable by forming ions with noble-gas electron configurations. (octet)

• Sharing valence electrons with other atoms also results in noble-gas electron configurations.

Section 8-1

Why do atoms bond? (cont.)

• Atoms in non-ionic compounds share electrons.

• Water is formed when Hydrogen and Oxygen atoms share electrons.

• The chemical bond that results from sharing electrons is a covalent bond.

• A molecule is formed when two or more atoms bond.

• Shared electrons are considered to be part of the outer energy level of all the atoms that share it.

• This type of bonding usually occurs between elements near each other on the periodic table.

• Most of this type of bonding is done with nonmentals.

• The chemical bond that results from sharing electrons is a covalent bond

• 3 types of Covalent bonds– Single Covalent bond = sharing 2 electrons– Double Covalent bond = Sharing 4 eletrons– Triple Covalent Bond = Sharing 6 electrons

•Atoms do not share more than 6 electrons between them (quadruple bond does not exist)

Section 8-1

Why do atoms bond? (cont.)

• The most stable arrangement of atoms exists at the point of maximum net attraction, where the atoms bond covalently and form a molecule.

• F: 1s22s2 2p5

• Shared are covalently bonded…loan pairs are unbonded.

Structural Formulas• In a Lewis structure dots or a line are

used to symbolize a single covalent bond.

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

• Electron Dot Diagrams– show valence e- as dots– distribute dots like arrows

in an orbital diagram– 4 sides = 1 s-orbital, 3 p-orbitals– EX: oxygen

2s 2pO

X

Structural Formulas

Drawing Lewis Diagrams

• Arrange atoms – least electronegative atom usually in the middle. (never hydrogen)

• Calculate the total number of valence e-.• Place one bonding pair (line) between the central

atom and all the terminal atoms• Place the remaining pairs of electrons around the

terminal atoms (three pairs for each) and any remaining pairs go on the central atom

• Check to be sure the central atom has four pairs of electrons, if not convert a pair of electrons from a terminal atom to make a double or triple bond

Drawing Lewis DiagramsDrawing Lewis Diagrams

CFCF441 C × 4e1 C × 4e-- = 4e = 4e--

4 F × 7e4 F × 7e-- = = 28e28e--

32e32e--

FF C F

F

- 8e- 8e--

24e24e--

Drawing Lewis DiagramsDrawing Lewis Diagrams

COCO221 C × 4e1 C × 4e-- = 4e = 4e--

2 O × 6e2 O × 6e-- = = 12e12e--

16e16e--

O C O - 4e- 4e--

12e12e--

Polyatomic IonsPolyatomic Ions

To find total # of valence eTo find total # of valence e--:: Add 1eAdd 1e-- for each negative charge. for each negative charge. Subtract 1eSubtract 1e-- for each positive charge. for each positive charge.

Place brackets around the ion and label the Place brackets around the ion and label the charge.charge.

Polyatomic IonsPolyatomic Ions

ClOClO44--

1 Cl × 7e1 Cl × 7e-- = 7e = 7e--

4 O × 6e4 O × 6e-- = = 24e24e--

31e31e--

OO Cl O

O

+ 1e+ 1e--

32e32e--

- 8e- 8e--

24e24e--

NHNH44++

1 N × 5e1 N × 5e-- = 5e = 5e--

4 H × 1e4 H × 1e-- = = 4e4e--

9e9e--

HH N H

H

- 1e- 1e--

8e8e--

- 8e- 8e--

0e0e--

Polyatomic IonsPolyatomic Ions

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 Nitrate.

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.

Resonance StructuresResonance Structures

OO S O

OO S O

OO S O

SO3

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.

Section 8-3

Exceptions to the Octet Rule (cont.)

• A few compounds form stable configurations with less than 8 electrons around the atom—a suboctet.

• BF3 is an example of a suboctet

• Draw a Lewis structure of BF3

Section 8-3

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 (row) 3 or higher have a d-orbital and can form more than four covalent bonds.

Hydrogen Hydrogen 2 valence e 2 valence e--

Groups 1,2,3 get 2,4,6 valence eGroups 1,2,3 get 2,4,6 valence e--

Expanded octet Expanded octet more than 8 valence e more than 8 valence e--, ,

central atom onlycentral atom only and must be in the and must be in the third third

row or lowerrow or lower Radicals Radicals odd # of valence e odd # of valence e--

ExceptionsExceptions::

Octet RuleOctet Rule

F B FFH O HN O

Very unstable!!

F F

F S F

F F

A. A

B. B

C. C

D. D

Section 8-3

A B C D

0% 0%0%0%

Section 8.3 Assessment

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

A. A

B. B

C. C

D. D

Section 8-3

Section 8.3 Assessment

A B C D

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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