Chapter 9
Chapter 9
Ionic Versus Covalent Bonding
Covalent Bonding
Keywords:
Valence electron
Covalent bonding
Single, double and triple bonds
Bonding electrons and Non-bonding
electrons (lone pair of electrons)
Bond energy and bond length
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
Electronegativity
Chapter 9
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?
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
Chapter 9
Formation of Covalent Bond
Distribution of electron density in H2
Electron density is highest around and
between the nuclei.
Chapter 9
Chapter 9
Covalent Bonding – Cl2
Each Cl atom has a 3p orbital that
is singly occupied
3p
3s
↑↓ ↑↓ ↑
↑↓
2p
2s
↑↓ ↑↓ ↑↓
↑↓
1s
↑↓
Bonding pairs
Lone pairs
Check the octets!
..
..
.
+
.
:C
Cl
.. :
.. l
Cl
Multiple Covalent Bonds
Each Cl atom in Cl2 achieves its octet with a single Cl-Cl
.. ..
.. ..
bond
: Cl .. Cl: or : Cl Cl:
..
..
..
What about N?
..
..
. N. + . N.
.
.
.. ..
. N. . N.
. .
How can nitrogen satisfy its octet?
.. ..
.. .. :
: Cl
.. Cl
Which bond is stronger, that in Cl2 or in N2?
..
Chapter 9
Chapter 9
Bond energy and bond length
Types of Bond and Bond Order
Types of bond
Bond order
Single (Cl2, HCl)
Double (O2)
Triple (N2)
1
2
3
Bonding pairs and lone pairs
Bond formation: exothermic process, -E
Bond breakage: endothermic process, +E
Bond length: distance between the nuclei between two bonded atoms
Bonding pairs
.. ..
.. .. :
: Cl
.. Cl
Lone pairs
Chapter 9
Chapter 9
Electronegativity and Bond Polarity
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
i) Both men are equally strong
ii) Both men are equally weak
iii) One is stronger than the other
No one will win
The stronger will win
Chapter 9
Chapter 9
Electronegativity Scale
Electronegativity and Atomic Size
s
Increase
ses
Decrea
Electronegativity is inversely related to atomic size.
Chapter 9
Chapter 9
Electronegativity and bond polarity
Bond Polarity and Electronegativity
Bond polarity increase as the value of ∆χ increase
> 2.0
0.5 -2.0
< 0.5
∆χ : difference in electronegativity values between the
bonded atoms
∆χ
H2
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)
Chapter 9
Chapter 9
Bond Polarity
Use electronegativity values to classify the bonding
in the following molecules (ionic, polar covalent, or
nonpolar covalent)
KF
Naming Covalent Molecules
NO
SiCl4
K2O
Chapter 9
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
Naming Covalent Molecules
CCl4
Diphosphorus pentoxide
NO
Sulfur trioxide
SeF4
Silicon tetrachloride
S2O7
Trihydrogen nitride
CaCl2
Dihydrogen monoxide
N2O4