Chapter 9

� Valence electron

� Covalent bonding

� Single, double and triple bonds

� Bonding electrons and Non-bonding electrons (lone pair of electrons)

�Bond energy and bond length

�Electronegativity

Keywords:

Covalent Bonding

Chapter 9

Ionic Versus Covalent Bonding

� Ionic compounds are formed when electrons are

transferred from one atom to another

�The transfer of electrons forms ions

�Each ion is isoelectronic with a noble gas

�Electrostatic force (ionic bond) holds atoms together

� Covalent bonding involves sharing of electrons to achieve noble gas configurations for the atoms

involved

�Covalent bond = attractive force resulting from atoms attracted to a shared pair of electrons

Chapter 9

Covalent Bonding� First, some background

�The octet rule: The atoms tend to gain, lose or share electrons to have eight electrons in the valence shells (electronic configuration of noble gas, stable configuration)

�Carbon (group IVA) would need to gain or lose 4 electrons to achieve noble gas configuration in an ionic compound

�Diatomic molecules, like O2, Cl2, I2 do not transfer electrons to form ionic bonds

�Cl- ⇔ Cl+ ???? Does this look OK?

Chapter 9

Covalent Bonding – H2

� Another type of bonding is at work here

� G. N. Lewis (1916) theorized that noble gas configurations could be attained by “electron sharing”

For H2(a) Two noninteracting H atoms,

each with one electron in its

1s orbital

(b) As the two H atoms approach,

the spins pair and the s

orbitals merge into a molecular

orbital, electrons are shared

(c) Fully formed molecular orbital

(d) Each H atom is “satisfied”

Chapter 9

Formation of Covalent BondChapter 9

Distribution of electron density in H2

�Electron density is highest around and

between the nuclei.

Chapter 9

Covalent Bonding – Cl2

� Each Cl atom has a 3p orbital that is singly occupied

� Bonding pairs

� Lone pairs

� Check the octets!

3p ↑↓ ↑↓ ↑

3s ↑↓

2p ↑↓ ↑↓ ↑↓

2s ↑↓

Cl 1s ↑↓

C l..

..: . Cl

..

.. :Cl..

.. :.

+ Cl..

..: ..

Chapter 9

Multiple Covalent Bonds

� Each Cl atom in Cl2 achieves its octet with a single Cl-Clbond

� What about N?

� How can nitrogen satisfy its octet?

� Which bond is stronger, that in Cl2 or in N2?

N..

.. . N

..

.. . N

..

.. . N

..

.. .+

Cl..

.. :Cl..

..: .. Cl..

.. :Cl..

..:or

Chapter 9

Types of Bond and Bond Order

Types of bond Bond order

Single (Cl2, HCl) 1

Double (O2) 2

Triple (N2) 3

� Bonding pairs and lone pairs

Cl..

.. :Cl..

..: ..

Bonding pairs

Lone pairs

Chapter 9

Bond energy and bond length

� Bond formation: exothermic process, -E

� Bond breakage: endothermic process, +E

� Bond length: distance between the nuclei between two bonded atoms

Chapter 9

Electronegativity and Bond Polarity

� Electronegativity

A measure of the ability of an atom in a molecule to draw electrons to itself.

-An atom with large negative electron affinity and large ionization energy has large electronegativity.

Example: fluorine, oxygen

Chapter 9

i) Both men are equally strongii) Both men are equally weak

iii) One is stronger than the other

No one will win

The stronger will win

Electronegativity and Bond Polarity

Chapter 9

Electronegativity Scale

Decreases

Increases

Chapter 9

Electronegativity and Atomic Size

�Electronegativity is inversely related to atomic size.

Chapter 9

Electronegativity and bond polarity

> 2.0

0.5 -2.0

< 0.5

Chapter 9

�Bond polarity increase as the value of ∆χ increase

�∆χ : difference in electronegativity values between thebonded atoms

∆χH2 0.0 non-polar covalent (∆χ = 0)

HF 1.8 polar covalent (0 <∆χ <2.0)

LF 3.0 ionic (∆χ >2.0)

�Bond polarity order for hydrogen halides:

HF (∆χ = 1.8) > HCl (∆χ = 1.0) > HBr (∆χ = 0.8) > HI (∆χ = 0.5)

Bond Polarity and Electronegativity

Chapter 9

Bond Polarity

� Use electronegativity values to classify the bonding

in the following molecules (ionic, polar covalent, or nonpolar covalent)

� KF

� NO

� SiCl4

� K2O

Chapter 9

Naming Covalent Molecules

� General rules

� Make sure the compound qualifies as a covalent molecule (and not ionic)

� Give the name of the least electronegative element first

� Give the stem name of the more electronegative element, ending with “ide”

� Indicate the number of each type of atom by the prefixes, mono, di, tri, tetra, penta, etc.

� PCl5 phosphorus pentachloride

� SO2 sulfur dioxide

� N2O dinitrogen monoxide

� CO carbon monoxide

� CO2 carbon dioxide

Chapter 9

Naming Covalent Molecules

� CCl4

� NO

� SeF4

� S2O7

� CaCl2

� N2O4

Chapter 9

Naming Covalent Molecules

� Diphosphorus pentoxide

� Sulfur trioxide

� Silicon tetrachloride

� Trihydrogen nitride

� Dihydrogen monoxide