Chapter 9: Models of Chemical Bonding

1. Lewis Dot Symbols

2. Ionic Bonding

3. Metallic Bonding

4. Covalent Bonding

5. Polar Bonds

6. Electronegativity

• Lewis symbols

::C and Na :Examples

Practice

• Give Lewis dot symbols for:

magnesium

nitrogen

fluorine

argon

boron

Oxide ion

Sodium ion

Types of Bonding and PropertiesIonic

Metallic

Covalent

Ionic Bonding• Ionic compounds form

• The attraction

• Forming ions takes energy

Metallic Bonding• Outer electrons

• Large numbers of metal atoms

• The electrons are ‘delocalized’

Covalent Bonds

Network Covalent Solids

http://www.hull.ac.uk/chemistry/intro_inorganic/images/diamond.jpg

Polar Covalent Bonds

• When two identical atoms form a covalent bond

• When different kinds of atoms combine,

(a)

(b)

• Electronegativity

• In general, electronegativity increases

http://www.chem.ufl.edu/~itl/2045/change/C9F16.GIF

• The difference in electronegativity

• There is no sharp dividing line between ionic and covalent bonding:

• A bond is mostly ionic when

• The degree of polarity, or ionic character, varies continuously with the electronegativity difference

Each atom in a bond has a partial charge of about +0.5 or –0.5 units when the electronegativity difference is 1.7.

Electronegativity diff. Type of bonding

>1.7 Mostly ionic

0.4-1.7 Polar covalent

<0.4 Mostly covalent

0 Nonpolar covalent

Practice Questions• Identify the following bonds as

A. Mostly ionic

B. Polar Covalent

C. Mostly Covalent

D. Nonpolar Covalent

C-Cl

Re-H

Li-Cl

P-HSn-BrSr-O

• Lewis symbols can be used to represent the covalent or electron pair bond

HH H H :

Formulas drawn with Lewis symbols are called Lewis formulas or Lewis structures

• The term structural formula

• Many molecules obey the octet rule:

• The number of bonds an atom (second-row) forms is determined by:

Number of Bonds

C Has 4 e- Needs 4 e- Forms 4 bonds

N

O

F

B

• single bond

• Double and triple bonds

• The bond order

• A single bond has bond order of• a double bond a bond order of • a triple bond a bond order of • Bond length depends on bond order:

Drawing Lewis Structures• The least electronegative atom is usually in the

middle. (Or the atom which can form the most bonds.)

• Count total number of valence electrons in molecule/ion.

• Place them around the atoms to satisfy the octet rule:

Phosphorus Trichloride

Carbon Dioxide

• Not all structures obey the octet rule

Sulfur tetrafluoride

Bromine pentafluoride

• The preferred Lewis structure is the one that best fits the experimental data

The structure of sulfuric acid in the vapor state. There are two different sulfur-oxygen bond lengths. The preferred Lewis structure needs different bond orders for these atoms.

• Structure I

• Structure II

II Structure I Structure

HO

:O:

:O:

SOH HO

:O:

:O:

SOH||

||

|

|

• formal charge

• The formal charge on an atom is calculated

• Consider the sulfur atoms in the two structures for sulfuric acid:

Structure I: formal charge on S = Structure II: formal charge on S =

When several Lewis structures are possible,

II Structure I Structure

HO

:O:

:O:

SOH HO

:O:

:O:

SOH||

||

|

|

Iodate ion

Nitrate ion

Formal charge on O = Formal charge on I =

Formal charge on O = Formal charge on I =

• Some molecules and ions are not well represented by a single Lewis structure

• Consider the case of the formate ion, HCO2-

Experiment gives

These are called resonance structures

Draw the resonance forms for nitrate.

Draw the resonance forms for dinitrogen monoxide.

Draw resonance forms for thiosulfate.