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Chapter 8:
Bonding:Genera
l Concept
s
Cartoon courtesy of NearingZero.net
Chemical Bonding Forces that hold groups of atoms together and make them function as a unit.
Bond EnergyEnergy required to break a bond
Bond Polarity and Dipole Moments
Dipolar Molecules1. Molecules with a somewhat negative end and a somewhat positive end (a dipole moment)2. Molecules with preferential orientation in an electric field
+ + + + + + + +
- - - - - - - -3. All diatomic molecules with a polar covalent bond are dipolar
Bond Polarity and Dipole Moments
Molecules with Polar Bonds but no Dipole Moment1. Linear, radial or tetrahedral symmetry of charge distribution
a. CO2 - linearb. CCl4 – tetrahedral
See table 8.2
Ionic Bonding
Ionic bond: the electrostatic force that holds ions together in an ionic compound.
Examples of Ionic Compounds (aka Salts):NaClBaCl2
Ionic Bonding Electrons are transferred
Electronegativity differences are
generally greater than 1.7 The formation of ionic bonds is
always exothermic!
Determination of Ionic
Character
Compounds are ionic if they conduct electricity in their molten state
Electronegativity difference is not the final determination of ionic character
Properties of Ionic Compounds
Structure: Crystalline solids
Melting point:
Generally high
Boiling Point:
Generally high
Electrical Conductivity:
Excellent conductors, molten and aqueous
Solubility in water:
Generally soluble
Coulomb’s Law
r
QQnmJxE 2119 )1031.2(
“The energy of interaction between a pair of ions
• E is in Joules• r is the distance between the center of the ions
• Q1 and Q2 are the charges of the ions• A negative quantity indicates attraction • A positive quantity indicates repulsion
Coulomb’s Law
• Example: In solid NaCl the distance between the centers of the ions is 2.76 Å (0.276 nm) Calculate the ionic energy per pair of ions:
Sodium Chloride Crystal Lattice
Ionic compounds form solid crystals at ordinary temperatures.
Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions.
All salts are ionic compounds and form crystals.
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Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.
Electronegativity - F is highest
X (g) + e- X-
(g)
13
Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
Classification of bonds by difference in electronegativity
Difference Bond Type
0 Covalent
2 Ionic
0 < and <2 Polar Covalent
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Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H2S; and the NN bond in H2NNH2.
Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3Ionic
H – 2.1 S – 2.5 2.5 – 2.1 = 0.4Polar Covalent
N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent
Formation of Ionic compounds
• Stable compounds are formed when nonmetallic elements take electrons from metals.
• Atoms usually have a noble gas configuration
Formation of Ionic compounds
In general: • When a binary ionic compound is
formed – the nonmetal has noble gas
configuration– The valence orbitals of the
representative metal is emptied
• The term ionic compounds refers to the solid state of the compound
• A collection of positive and negative ions arranged to minimize repulsions and maximize attractions
Predicting formulas of ionic compounds
Predicting formulas of ionic compounds
• Large electronegativity differences between atoms mean electrons will be transferred
Predicting formulas of ionic compounds
• Hydrogen typically behaves as a nonmetal
• The number of electrons transferred depends on how many each atom needs to gain or lose to achieve noble gas notation
Predicting formulas of ionic compounds• EXCEPTIONS:
– Tin forms Sn2+ and Sn4+
– Lead forms Pb2+ and Pb4+
– Bismuth forms Bi3+ and Bi5+
– Thallium forms Tl+ and Tl3+
Energy and Binary Ionic Compounds
• Factors that influence stability and structure
• Ionic compounds form because together they have lower energy than the original elements
Lattice Energy
• The energy released when an ionic solid is formed from its ions
• LE is negative (exothermic)• Used as a step to calculate
energy of formation
MX(s)(g)X(g)M
Lattice energy increases as Q increases and/or as r decreases.
CompoundLattice Energy (kJ/mol)
MgF2
MgO
LiFLiCl
29573938
1036853
Q: +2,-1Q: +2,-2
r F- < r Cl-
Q+ and Q- is the charge on the cation and anionr is the distance between the ions
E is the potential energy
k is a constant based on the compound
r
QQkE
Calculating Energy of formation Hf
• If we know the steps in the process then we can apply Hess’s law
• Because energy is a state function
• Break the reaction up into steps• Add them up
Estimate Hf for Sodium Chloride
Na(s) + ½ Cl2(g) NaCl(s)Lattice Energy -786 kJ/mol
Ionization Energy for Na 495 kJ/mol
Electron Affinity for Cl -349 kJ/mol
Bond energy of Cl2 239 kJ/mol
Enthalpy of sublimation for Na
109 kJ/mol
Na(s) Na(g) + 109 kJNa(g) Na+(g) + e- + 495 kJ
½ Cl2(g) Cl(g) + ½(239 kJ)Cl(g) + e- Cl-(g) - 349 kJ
Na+(g) + Cl-(g) NaCl(s) -786 kJ Na(s) + ½ Cl2(g) NaCl(s) -412
kJ/mol
The energy diagram for the formation of MgO and NaCl.
The Lattice energy to combine Mg2+ and O2- is much more negative than the energy needed for the process that produces Mg2+ and O2- ions.