Ionic bonds and main
group chemistry
Learning objectives
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Write Lewis dot structures of atoms and ions
Describe physical basis underlying octet rule
Predict ionic charges using periodic table
Define lattice energy
Apply Born-Haber diagrams to calculations of
lattice energy
Three types of bonding
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Ionic
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Covalent
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Metal + nonmetal
Electron transfer
Nonmetal + nonmetal
Electron sharing
Metallic
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Metal + metal
Electron “pooling”
It’s all about Coulomb’s law
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Like charges attract
E inversely proportional
to r
E proportional to q x q
1 q1q2
E
4o r
Towards the noble gas configuration
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Noble gases are unreactive – they have filled
shells
Shells of reactive elements are unfilled
Achieve noble gas configuration by gaining or
losing electrons
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Metals lose electrons – form positive ions
Nonmetals gain electrons – form negative ions
Lewis dot model
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The nucleus and all of the core electrons are represented by the
element symbol
The valence electrons are represented by dots – one for each
Number of dots in Lewis model is equal to group number (in
1 – 8 system)
The Octet Rule
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All elements strive to
become a noble gas, at
least as far as the
electrons are concerned.
Filling the outer shell –
8 electrons
Achieve this by adding
electrons
Or taking them away
Predicting ion charges
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s and p block elements are easy:
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charge = group number for cations
charge = -(8 – group number) for anions
Group 1A
Group 2A
Group 3A
Group 5A
Group 6A
Group 7A
N3-
O2-
F-
H+
Li+
Be2+
Na+
Mg2+
Al3+
P3-
S2-
Cl-
K+
Ca2+
Ga3+
As3-
Se2-
Br-
Rb+
Sr2+
In3+
Te2-
I-
Cs+
Ba2+
Tl3+
The Octet Rule
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Main-group elements undergo reactions which
leave them with eight valence electrons
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Group 1 [NG](ns1) – e → M+ [NG]+
Group 2 [NG](ns2) – 2e → M2+ [NG]2+
Group 6 [NG](ns2np4) + 2e → X2- [NG]2Group 7 [NG](ns2np5) + e → X- [NG]-
Works very well for second row (Li – F)
Many violations in heavier p-block elements
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(Pb2+ not Pb4+, Tl+ not Tl3+, Sb3+ not Sb5+ or Sb3-)
Less predictable for transition
metals
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Occurrence of variable ionic charge
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4s electrons are lost first and then the 3d
Maximum oxidation states in first half correspond to loss of all
electrons (4s + 3d)
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Ti4+, V5+, Cr6+, Mn7+
Doesn’t continue beyond half-filled shell – 3d electron energy
decreases (more tightly held) across period
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Cr2+, Cr3+, Cr4+, Cr6+ etc.
No Fe8+ etc.
Desirable configurations tend to coincide with empty, halffilled or filled 3d orbitals
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Fe2+ ([Ar]3d6) is readily oxidized to Fe3+ ([Ar]3d5)
Ionization energy
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Energy required to remove an electron from a
neutral gaseous atom
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Always positive
Follows periodic trend
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Increases across period
Decreases down group
Removal of electrons from filled or half-filled shells is not as favourable
[He]2s22p3
[He]2s22p4
[He]2s2
[He]2s22p1
Higher ionization energies
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Depend on group number
Much harder to remove electrons from a filled shell
Stepwise trend below illustrates this
Completely
Partially filled –
valence
electrons
filled – core
electrons
Electron affinity
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Energy released on adding an electron to a neutral gaseous
atom
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Values are either
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negative – energy released, meaning negative ion formation is favourable
Or zero – meaning can’t be measured and negative ions are not formed
Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N)
It is easier to add an electron to Na (3s1) than to Mg (3s2)
Ionic bonding
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Reaction between elements that form positive and
negative ions
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Metals (positive ions) and nonmetals (negative ions)
Neutral Na + Cl → ionic Na+Cl
[Ne]3s1 + [Ne]3s23p5 = [Ne]+ + [Ar]-
Stability of the ionic lattice
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Forming ions does not give energy
payout:
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Ei(Na) = 496 kJ/mol
Ea(Cl) = -349 kJ/mol
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Formation of lattice from gaseous
ions releases energy to compensate
M+(g) + X-(g) → MX(s) +
energy
Lattice energy is energy released on
bringing ions from gas phase into
lattice (negative value)
Or: lattice energy is energy required
to separate lattice into gas phase ions
(positive value)
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Net energy investment (+150 kJ/mol)
Lattice energies follow simple trends
d
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Depends on coulombic attraction between ions
-U = κz1z2/d (κ = 8.99x109 JmC-2)
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As ionic charge increases, U increases (U  z1z2)
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U(NaF) < U(CaO)
As ion size decreases, U increases (U  1/d)
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U(LiCl) > U(NaCl) > U(KCl)
Cation
F-
Cl-
Br-
I-
O2-
Li+
1036
853
807
757
2925
Na+
923
787
747
704
2695
K+
821
715
682
649
2360
Be2+
3505
3020
2914
2800
4443
Mg2+
2957
2524
2440
2327
3791
Ca2+
2630
2258
2176
2074
3401
Al3+
5215
5492
5361
5218
15,916
Born-Haber cycle for calculating
energy
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Lattice energy difficult
to measure directly
Can be estimated very
well by models
Can be obtained using
other experimentally
determined quantities
and conservation of
energy
Drawing the Born-Haber cycle