Chemical BondingChapter 8 Sections 1 & 2

• A chemical bond is: a force of attraction between any two atoms in a compound.

• Bonding between atoms occurs because it creates a more stable arrangement for the atoms.

Lewis Symbols – Dot Diagrams

• Convenient way to show the valence electrons

Three types of bonding• Metallic bonding – results from the

attraction between metal atoms and the surrounding sea of electrons

• Ionic bonding – results from the electrical attraction between large numbers of cations and anions

• Covalent bonding – results from the sharing of electron pairs between two atoms

Ionic Bonding• Many atoms transfer electrons

and other atoms accept electrons, creating cations (positive metal ions) and anions (negative nonmetal ions).

• The resulting ions are attracted to each other by electrostatic force.

Ionic bonding between Na and Cl

Sodium atom Chlorine atom 1s2 2s2 2p6 3s1 1s2 2s2 2p6 3s2 3p5 1s2 2s2 2p6 3s1 1s2 2s2 2p6 3s2 3p5 Sodium ion Na1+ Chlorine ion Cl1- 1s2 2s2 2p6 1s2 2s2 2p6 3s2 3p6

Covalent bonding

• In many cases electrons do not completely transfer from one atom to another.

• The electrons between atoms are shared.

Covalent bonding between H2

• Hydrogen’s electron configuration is 1s1

• Because both H atoms need 1 more electron to become isoelectronic with He, it is unlikely that either will give up an electron.

Covalent bonding between H2

↑

1s

↓ 1s

They share the two electrons.H· + H · H : H

Types of Covalent Bonds• When electrons are shared equally the

bond is called a NONPOLAR covalent bond. (i.e. H2)

• Sometimes the electrons between two atoms are NOT shared equally. The bond created is called a POLAR covalent bond.

. . . .

H· + ·Cl: H:Cl:

. . . .

Polar Covalent Bonding

• An example of this would be HCl.

HCl moleculeHydrogen atom

[Ne] ↑↓ ↑↓ ↑↓ ↓ 3s 3p

↑ 1s

Chlorine atom

How to classify bond types• Electronegativity – measure of the ability

of an atom to attract electrons in a chemical bond

• Each element on the periodic table is assigned an electronegativity value (see page 353) that ranges from 0.7 to 4.0.

• The difference in the electronegativity determines the bonding type (ionic, polar covalent, or nonpolar covalent).

If the electronegativity difference is:

1.7 and higher ionic

0.3 to 1.7 polar covalent

0.0 to 0.3 nonpolar covalent

What if I get an electronegativity difference that is 0.3 or 1.7?

• These cut-off numbers are guidelines.

• It is a gradual change not stair-step.

Ionic Character

• As the electronegativity difference increases, the ionic character increases as well!

Practice ProblemsWhat type of bond will occur between

iodine and the following elements: cesium, iron, and sulfur?

Bonding between I and:

Electronegativity difference

Bond Type

Cesium

Iron

Sulfur

Answers to problems

Bonding between I and:

Electronegativity difference

Bond Type

Cesium 2.5 – 0.7 = 1.8 Ionic

Iron 2.5 – 1.8 = 0.7 Polar covalent

Sulfur 2.5 – 2.5 = 0.0 Nonpolar covalent

Determine the type of bond between the following pairs.

Bonding between

Electronegativity difference

Bond type

Li & Cl

S & O

Ca & Br

P & H

Si & Cl

S & Br

AnswersBonding between

Electronegativity difference

Bond type

Li & Cl 3.0 – 1.0 = 2.0 Ionic

S & O 3.5 – 2.5 = 1.0 Polar covalent

Ca & Br 2.8 – 1.0 = 1.8 Ionic

P & H 2.1 – 2.1 = 0.0 Nonpolar covalent

Si & Cl 3.0 – 1.8 = 1.2 Polar covalent

S & Br 2.8 – 2.5 = 0.3 Nonpolar covalent