Chemical Bonding • Compounds are formed from chemically bound atoms or ions. • Bonding involves only the valence electrons.
Mar 26, 2015
Chemical Bonding
• Compounds are formed from chemically bound atoms or ions.
• Bonding involves only the valence electrons.
Chemical Bonding
• Ionic Compounds
– Ionic Radii
– Lattice Energy
• Molecular Compounds
– Covalent bonds
– Bond Order
– Bond Strength
– Lewis Structures
Lewis Symbols
Lewis symbols show the valence electrons as dots arranged around the atomic symbol.
hydrogen:
sodium:
chlorine:
Na
H
Cl
The Octet Rule
Atoms tend to gain, lose, or share electrons until they have eight valence electrons.
Ionic and Molecular Compounds• Formation of sodium chloride:
• Formation of hydrogen chloride:
A metal and a nonmetal transfer electrons to form an ionic compound. Two nonmetals share electrons to form a molecular compound.
Na + Na+ [ ]Cl
Cl
H + Cl
Cl
H
Ionic CompoundsIonic compounds consist of a lattice of positive and negative ions.
NaCl:
Ionic Bonds
An ionic bond is simply the electrostatic attraction between opposite charges.
Ions with charges Q1 and Q2:
The potential energy is given by:
d
Q1
Q2
dQQ
E 21
Ionic Sizes
The Lattice Energy
• The lattice energy is the enthalpy change required to separate one mole of an ionic compound into its ions:
NaCl(s) Na+(g) + Cl(g) H = 788 kJ
• Lattice energy increases with increasing ionic charges and decreasing ionic sizes.
KCl(s) K+(g) + Cl(g) H = 701 kJ
MgCl2(s) Mg+2(g) + 2 Cl(g) H = 3795 kJ
The Born-Haber CycleThe Lattice energy can be calculated with Hess’s law and the following steps:
Hf(NaCl,s)
Hf(Na,g)
Hf(Cl,g)
I1(Na)E(Cl)
Latticeenergy
H
NaCl(s)
Na(s) + ½Cl2(g)
Na+(g) + Cl-(g)
Na(g) + Cl(g)
Na(g) + ½Cl2(g)
Na+(g) + e- + Cl(g)
Estimating Lattice EnergyArrange with increasing lattice energy:
KCl
NaF
MgO
KBr
NaCl 788 kJ
671 kJ
3795 kJ
910 kJ
701 kJ
d
K+ Cl
K+ Br
d
dQQ
E 21
Molecular CompoundsThe simplest molecule is H2:
The pair of shared electrons constitutes a covalent bond.
Increased electron density draws nuclei together
Lewis Structures• Covalent bonding in a molecule is repre-
sented by a Lewis structure.
• A valid Lewis structure should have an octet for each atom except hydrogen.
H H+
ClCl
H2: or H H
Cl2: Cl
Cl
+
or Cl Cl
H H
Bondingelectrons
Nonbonding electrons
Lewis StructuresDraw Lewis structures for:
HF:
H2O:
NH3:
CH4:
H F
or H F
H O H
or H O H
H N HH
or H N H
H
H C HH
H
or H C H
H
H
Double and Triple Bonds• Atoms can share four electrons to form a
double bond or six electrons to form a triple bond.
• The number of electron pairs is thebond order.
O2:
N2:
=O O
N N
Electronegativity• Polarity refers to a separation of positive and
negative charge. In a nonpolar bond, the bonding electrons are shared equally:
HCl:
• In a polar bond, electrons are shared unequally because of the difference in Zeff.
H2,Cl2:
Electronegativity• Electronegativity refers to the ability of an
atom in a molecule to attract shared electrons.
• The Pauling scale of electro-
negativity:
QuickTime Movie
Bond PolarityA polar bond can be pictured using partial charges:
= 0.9
ElectronegativityDifference Bond Type
0 - 0.5 Nonpolar
0.5 - 2.0 Polar
2.0 Ionic
2.1 3.0
+
H Cl
Drawing Lewis Structures• Sum the valence electrons from all atoms.
Add one for each negative charge and subtract one for each positive charge.
• Draw a skeleton structure with atoms attached by single bonds.
• Complete the octets of atoms bound to the central atom.
• Place extra electrons on the central atom.
• If the central atom doesn’t have an octet, try forming multiple bonds.
Drawing Lewis Structures
COCl2 24 ve’s
HOCl 14 ve’s
ClO3 26 ve’s
CH3OH 14 ve’s
Cl C Cl
O
H O Cl
O Cl O
O
H C O H
H
H