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Ionic bonds and main
group chemistry
Learning objectives
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
Ionic
Metal + nonmetal
Electron transfer
Covalent
Nonmetal + nonmetal
Electron sharing
Metallic
Metal + metal
Electron “pooling”
It’s all about Coulomb’s law
Like charges attract
E inversely proportional
to r
E proportional to q x q
r
qqE
o
21
4
1
Towards the noble gas configuration
Noble gases are unreactive – they have filled
shells
Shells of reactive elements are unfilled
Achieve noble gas configuration by gaining or
losing electrons
Metals lose electrons – form positive ions
Nonmetals gain electrons – form negative ions
Lewis dot model
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
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
s and p block elements are easy:
charge = group number for cations
charge = -(8 – group number) for anions
Group 1A Group 2A Group 3A Group 5A Group 6A Group 7A
H+
Li+ Be2+ N3- O2- F-
Na+ Mg2+ Al3+ P3- S2- Cl-
K+ Ca2+ Ga3+ As3- Se2- Br-
Rb+ Sr2+ In3+ Te2- I-
Cs+ Ba2+ Tl3+
The Octet Rule
Main-group elements undergo reactions which leave them with eight valence electrons
Group 1 [NG](ns1) – e → M+ [NG]+
Group 2 [NG](ns2) – 2e → M2+ [NG]2+
Group 6 [NG](ns2np4) + 2e → X2- [NG]2-
Group 7 [NG](ns2np5) + e → X- [NG]-
Works very well for second row (Li – F)
Many violations in heavier p-block elements
(Pb2+ not Pb4+, Tl+ not Tl3+, Sb3+ not Sb5+ or Sb3-)
Less predictable for transition
metals Occurrence of variable ionic charge
Cr2+, Cr3+, Cr4+, Cr6+ etc.
4s electrons are lost first and then the 3d
Maximum oxidation states in first half correspond to loss of all electrons (4s + 3d)
Ti4+, V5+, Cr6+, Mn7+
Doesn’t continue beyond half-filled shell – 3d electron energy decreases (more tightly held) across period
No Fe8+ etc.
Desirable configurations tend to coincide with empty, half-filled or filled 3d orbitals
Fe2+ ([Ar]3d6) is readily oxidized to Fe3+ ([Ar]3d5)
Ionization energy Energy required to remove an electron from a
neutral gaseous atom Always positive
Follows periodic trend Increases across period
Decreases down group
Removal of electrons from filled or half-filled shells is not as favourable
[He]2s2
[He]2s22p3
[He]2s22p4
[He]2s22p1
Higher ionization energies
Depend on group number
Much harder to remove electrons from a filled shell
Stepwise trend below illustrates this
Partially filled –
valence
electrons
Completely
filled – core
electrons
Electron affinity
Energy released on adding an electron to a neutral gaseous atom
Values are either 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
Reaction between elements that form positive and negative ions
Metals (positive ions) and nonmetals (negative ions)
Neutral Na + Cl → ionic Na+Cl-
[Ne]3s1 + [Ne]3s23p5 = [Ne]+ + [Ar]-
Stability of the ionic lattice
Forming ions does not give energy payout:
Ei(Na) = 496 kJ/mol
Ea(Cl) = -349 kJ/mol
Net energy investment (+150 kJ/mol)
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)
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
Lattice energies follow simple trends
Depends on coulombic attraction between ions
-U = κz1z2/d (κ = 8.99x109 JmC-2) As ionic charge increases, U increases (U z1z2)
U(NaF) < U(CaO)
As ion size decreases, U increases (U 1/d) U(LiCl) > U(NaCl) > U(KCl)
d
Born-Haber cycle for calculating
energy
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
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