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
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