Ch'7 Ionic Bonding

1
Ionic Bonding
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons 7.2 Energetics of Formation
of Ionic Compounds 7.3 Stoichiometry of Ionic
Compounds 7.4 Ionic Crystals 7.5 Ionic Radii
2
Introduction (SB p.186)
Sodium

• When sodium exposed in air, it becomes tarnished
  rapidly
• ? Reacts with oxygen in air
• ? Form a dull oxide layer on the metal surface

3
Introduction (SB p.186)
When sodium is placed in a bottle containing
chlorine gas ? Burns fiercely ? Gives a white
coating of sodium chloride
4
Introduction (SB p.186)
Noble gases

• Very stable
• Rarely participate in chemical reactions and form
  bonds with other elements
• ? Octet configuration

5
Introduction (SB p.186)
Formation of compounds

• Transfer or sharing of valence electron(s) takes
  place
• Atoms achieve the electronic configuration of the
  nearest noble gas in the Periodic Table
• Atoms are joined together by chemical bonds

6
Introduction (SB p.186)
Three types of chemical bonds
1. Ionic bond Electrostatic attraction between
positively charged particles and negatively
charged particles
7
Introduction (SB p.186)
Three types of chemical bonds
2. Covalent bond Electrostatic attraction
between nuclei and shared electrons
8
Introduction (SB p.186)
Three types of chemical bonds
3. Metallic bond Electrostatic attraction
between metallic cations and delocalized
electrons (electrons that have no fixed positions)
Let's Think 1
9
Formation of Ionic Bonds Donating and Accepting
Electrons
10
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Ionic Bonds

• Formed by a transfer of electrons from metallic
  atoms to non-metallic atoms
• e.g. Formation of sodium chloride
• Both the sodium ion and chloride ion attain the
  electronic configurations of noble gases which
  give rise to stability

11
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Formation of ionic bond between sodium atom and
chlorine atom
Cl
Na
Sodium atom, Na 1s22s22p63s1
Chlorine atom, Cl 1s22s22p63s23p5
12
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Formation of ionic bond between sodium atom and
chlorine atom
Cl
Na
Sodium ion, Na 1s22s22p6
Chloride ion, Cl- 1s22s22p63s23p6
13
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Ionic Bonds Donating and Accepting Electrons
14
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Ionic Bonds Donating and Accepting Electrons
15
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Ionic Bonds Donating and Accepting Electrons
Cationic radius(r)
Anionic radius(r-)
Internuclear distance
Internuclear distance r r-
16
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.187)
Ionic Bonds Donating and Accepting Electrons
Ionic bonds are the strong non-directional
electrostatic attraction between ions of opposite
charges.
17
7.1 Formation of Ionic Bonds Donating and
Accepting Electrons (SB p.188)
Electron transfer from a magnesium atom to two
chlorine atoms
Electron transfer from two lithium atoms to an
oxygen atom
18
Energetics of Formation of Ionic Compounds
19
Energetics of Formation of Ionic Compound
7.2 Energetics of Formation of Ionic Compounds
(SB p.189)
macroscopic level
Na(s) Cl2(g) ? NaCl(s)
microscopic level
20
Consider the formation of the ionic compound via
a serious of steps
7.2 Energetics of Formation of Ionic Compounds
(SB p.189)
1. The conversion of the elements to the gaseous
atoms (standard enthalpy change of atomization,
)
21
Consider the formation of the ionic compound via
a serious of steps
7.2 Energetics of Formation of Ionic Compounds
(SB p.189)
2. The conversion of the gaseous atoms to gaseous
ions (ionization enthalpy, and electron
affinity, )
22
Consider the formation of the ionic compound via
a serious of steps
7.2 Energetics of Formation of Ionic Compounds
(SB p.189)
3. The combination of the gaseous ions to form an
ionic crystal (lattice enthalpy, )
23
7.2 Energetics of Formation of Ionic Compounds
(SB p.189)
1. Standard Enthalpy Change of Formation (?H f)
ø
The enthalpy change when one mole of the ionic
compound is formed from its constituent elements
(in their standard states) under standard
conditions.
24
7.2 Energetics of Formation of Ionic Compounds
(SB p.190)
2. Standard Enthalpy Change of Atomization (?H
atom)
ø
The enthalpy change when one mole of gaseous
atoms is formed from an element in the standard
state under standard conditions.
Questions
Why are the changes endothermic?
What type of bond is broken in each case?
25
7.2 Energetics of Formation of Ionic Compounds
(SB p.190 191)
3. Ionization Enthalpy (?HI.E.)
The energy required to remove one mole of
electrons from one mole of atoms or ions in the
gaseous state.
Questions
Why are the changes endothermic?
26
7.2 Energetics of Formation of Ionic Compounds
(SB p.191)
4. Electron affinity (?HE.A.)
The enthalpy change when one mole of electrons is
added to one mole of atoms or ions in the gaseous
state.
Questions
Why may E.A. have -ve or ve values?
27
7.2 Energetics of Formation of Ionic Compounds
(SB p.192)
Electron affinities (in kJ mol1) of some
elements and ions
28
7.2 Energetics of Formation of Ionic Compounds
(SB p.192)
ø
5. Lattice enthalpy ( ?Hlattice)
The enthalpy change when one mole of an ionic
crystal is formed from its constituent ions in
the gaseous state under standard conditions.
29
7.2 Energetics of Formation of Ionic Compounds
(SB p.192)
L.E. can be calculated from the values of other
experimentally determined enthalpy changes by
constructing a Born-Haber cycle and applying
Hesss law
30
7.2 Energetics of Formation of Ionic Compounds
(SB p.193)
Born-Haber Cycle
A simplified enthalpy level diagram used to
calculate the lattice enthalpy of an ionic
compound.

• Two different routes to form an ionic compound
• Route 1 Direct single-step reaction of the
  elements to form the ionic compound
• Route 2 Consists of a number of steps. The
  enthalpy change of each step can be found from
  experiments, except the lattice enthalpy

31
7.2 Energetics of Formation of Ionic Compounds
(SB p.193)
Born-Haber Cycle for the formation of sodium
chloride
32
7.2 Energetics of Formation of Ionic Compounds
(SB p.194)

• Or draw enthalpy level diagram to represent the
  enthalpy changes in the Born-Haber cycle

Example 7-2
33
7.2 Energetics of Formation of Ionic Compounds
(SB p.196)
Lattice enthalpy
A measure of ionic bond strength which in turn
represents the strength of the ionic lattice.

• The higher (more negative) the lattice enthalpy
  of an ionic lattice? The higher is the ionic
  bond strength? The more stable is the ionic
  lattice

34
7.2 Energetics of Formation of Ionic Compounds
(SB p.196)
Factors affect lattice enthalpy
Let's Think 2

• Effect of ionic size
• ? The greater the ionic size? The lower (or
  less negative) is the lattice enthalpy
• Effect of ionic charge
• ? The greater the ionic charge? The higher
  (or more negative) is the lattice enthalpy

Check Point 7-2
35
Stoichiometry of Ionic Compounds
36
7.3 Stoichiometry of Ionic Compounds (SB p.197)
Stoichiometry
Stoichiometry of a compound is the simplest ratio
of the atoms bonded to form the compound.
How can the stoichiometry of an ionic compound be
determined?
37
7.3 Stoichiometry of Ionic Compounds (SB p.197
198)
A. In Terms of Electronic Configuration
magnesium chloride
Example
Mg (Group II) Cl (Group VII)
Elements involved
Mg2 Cl-
Ions formed
Ratio of ions
Chemical formula
Mg2(Cl-)2 or MgCl2
38
7.3 Stoichiometry of Ionic Compounds (SB p.198)
B. In Terms of Enthalpy Change of Formation

• The more negative the enthalpy change of
  formation of an ionic compound
• ? The greater is the driving force for its
  formation? The more stable the compound

Check Point 7-3
39
Ionic Crystals
40
7.4 Ionic Crystals (SB p.201)
Structure of Sodium Chloride
Unit cell of NaCl
Co-ordination number of Na 6
6 6 co-ordination
Co-ordination number of Cl- 6
41
7.4 Ionic Crystals (SB p.202)
Face-centred cubic lattice
42
7.4 Ionic Crystals (SB p.202)
A unit cell is the smallest basic portion of the
crystal lattice that, when repeatedly stacked
together at various directions, can reproduce the
entire crystal structure.
43
7.4 Ionic Crystals (SB p.202)
Structure of Caesium Chloride
Simple cubic lattice
Co-ordination number of Cs 8
8 8 co-ordination
Co-ordination number of Cl- 8
44
7.4 Ionic Crystals (SB p.203)
Structure of Calcium Fluoride
Face-centred cubic lattice
Co-ordination number of Ca 8
8 4 co-ordination
Co-ordination number of F- 4
45
7.4 Ionic Crystals (SB p.203)
Some simple ionic structures
Example 7-4
Check Point 7-4
46
Ionic Radii
47
7.5 Ionic Radii (SB p.205)
X-ray and electron diffraction technique
Photographic plate
48
7.5 Ionic Radii (SB p.205)
Electron density plot for sodium chloride crystal
49
7.5 Ionic Radii (SB p.206)
A. Cations

• Smaller radius than the corresponding atom
• Reasons
• 1. The number of electron shells decreases
• 2. No. of protons gt No. of electrons (p/e
  ratio increases) The nuclear attraction is more
  effective to cause a contraction in the
  electron cloud

50
7.5 Ionic Radii (SB p.206)
Size of ion vs size of atom
Comparing relative atomic radii of some elements
with the ionic radii of the corresponding ions
51
7.5 Ionic Radii (SB p.206)
B. Anions

• Larger radius than the corresponding atom
• Reasons
• 1. Repulsion between newly added electron(s)
  with other electrons2. No. of protons lt No. of
  electrons (p/e ratio decreases) The nuclear
  attraction is less effective and there is an
  expansion of the electron cloud

52
7.5 Ionic Radii (SB p.206)
C. Isoelectronic Ions

• They have the same number of electrons
• Sizes decrease along the isoelectronic series
• 1. H gt Li gt Be2 gt B3 (isoelectronic to He)
• 2. N3 gt O2 gt F gt Na gt Mg2 gt Al3
  (isoelectronic to Ne)
• 3. P3 gt S2 gt Cl gt K gt Ca2 (isoelectronic
  to Ar)

53
7.5 Ionic Radii (SB p.206)
C. Isoelectronic Ions

• Reason
• They have the same number of electrons. An
  increase in the number of protons implies an
  increase in the p/e ratio which leads to a
  contraction of the electron cloud

54
7.5 Ionic Radii (SB p.206)
isoelectronic ions
Why ionic radius decreases along the
isoelectronic series?
Check Point 7-5
Example 7-5
55
The END
56
Introduction (SB p.186)
Let's Think 1
Why do two atoms bond together? How does covalent
bond strength compare with ionic bond strength?
The two atoms tend to achieve an octet
configuration which brings stability.
Answer
57
7.2 Energetics of Formation of Ionic Compounds
(SB p.195)
Example 7-2
Given the following data ?H (kJ mol1) First
electron affinity of oxygen 142 Second electron
affinity of oxygen 844 Standard enthalpy change
of atomization of oxygen 248 Standard enthalpy
change of atomization ofaluminium 314 Standard
enthalpy change of formation ofaluminium
oxide 1669
58
7.2 Energetics of Formation of Ionic Compounds
(SB p.195)
Answer
Example 7-2
?H (kJ mol1) First ionization enthalpy of
aluminium 577 Second ionization enthalpy of
aluminium 1820 Third ionization enthalpy of
aluminium 2740 (a) (i) Construct a labelled
Born-Haber cycle for the formation of
aluminium oxide. (Hint Assume that aluminium
oxide is a purely ionic compound.) (ii) State
the law in which the enthalpy cycle in (i) is
based on. (b) Calculate the lattice enthalpy
of aluminium oxide.
59
7.2 Energetics of Formation of Ionic Compounds
(SB p.195)
Example 7-2
60
7.2 Energetics of Formation of Ionic Compounds
(SB p.195)
Example 7-2
61
7.2 Energetics of Formation of Ionic Compounds
(SB p.196)
Let's Think 2
What are the forces that hold atoms together in
molecules and ions in ionic compounds?
Electrostatic attractions between oppositely
charged particles
Answer
62
7.2 Energetics of Formation of Ionic Compounds
(SB p.197)
Check Point 7-2
(a) Draw a Born-Haber cycle for the formation of
magnesium oxide.
Answer
63
7.2 Energetics of Formation of Ionic Compounds
(SB p.197)
Check Point 7-2
(b) Calculate the lattice enthalpy of magnesium
oxide by means of the Born-Haber cycle drawn in
(a). Given ?Hatom Mg(s) 150 kJ
mol1 ?HI.E. Mg(g) 736 kJ mol1 ?HI.E.
Mg(g) 1 450 kJ mol1 ?Hatom O2(g)
248 kJ mol1 ?HE.A. O(g) 142 kJ
mol1 ?HE.A. O(g) 844 kJ mol1 ?Hf
MgO(s) 602 kJ mol1
ø
ø
Answer
ø
64
7.2 Energetics of Formation of Ionic Compounds
(SB p.197)
Check Point 7-2
65
7.3 Stoichiometry of Ionic Compounds (SB p.201)
Check Point 7-3
Give two properties of ions that will affect the
value of the lattice enthalpy of an ionic
compound.
Answer
The charges and sizes of ions will affect the
value of the lattice enthalpy. The smaller the
sizes and the higher the charges of ions, the
higher (i.e. more negative) is the lattice
enthalpy.
66
7.4 Ionic Crystals (SB p.204)
Example 7-4
Write down the formula of the compound that
possesses the lattice structure shown on the
right
Answer
67
7.4 Ionic Crystals (SB p.205)
Check Point 7-4
The diagram on the right shows a unit cell of
titanium oxide. What is the coordination number
of (a) titanium and (b) oxygen?
Answer
(a) The coordination number of titanium is 6 as
there are six oxide ions surrounding each
titanium ion. (b) The coordination number of
oxygen is 3.
68
7.5 Ionic Radii (SB p.208)
Example 7-5
The following table gives the atomic and ionic
radii of some Group IIA elements.
69
7.5 Ionic Radii (SB p.208)
Example 7-5
Explain briefly the following (a) The ionic
radius is smaller than the atomic radius in each
element. (b) The ratio of ionic radius to atomic
radius of Be is the lowest. (c) The ionic radius
of Ca is smaller than that of K(0.133 nm).
Answer
70
7.5 Ionic Radii (SB p.208)
Example 7-5
(a) One reason is that the cation has one
electron shell less than the corresponding atom.
Another reason is that in the cation, the number
of protons is greater than the number of
electrons. The electron cloud of the cation
therefore experiences a greater nuclear
attraction. Hence, the ionic radius is smaller
than the atomic radius in each element. (b) In
the other cations, although there are more
protons in the nucleus, the outer most shell
electrons are further away from the nucleus, and
electrons in the inner shells exhibit a screening
effect. Be has the smallest atomic size. In Be2
ion, the electrons experience the greatest
nuclear attraction. Therefore, the contraction in
size of the electron cloud is the greatest when
Be2 ion is formed, and the ratio of ionic radius
to atomic radius of Be is the lowest.
71
7.5 Ionic Radii (SB p.208)
Example 7-5
(c) The electronic configurations of both K and
Ca2 ions are 1s22s22p63s23p6. Hence they have
the same number and arrangement of electrons.
However, Ca2 ion is doubly charged while K ion
is singly charged, so the outermost shell
electrons of Ca2 ion experience a greater
nuclear attraction. Hence, the ionic radius of
Ca2 ion is smaller than that of K ion.
72
7.5 Ionic Radii (SB p.208)
Check Point 7-5
Arrange the following atoms or ions in an
ascending order of their sizes (a) Be, Ca, Sr,
Ba, Ra, Mg (b) Si, Ge, Sn, Pb, C (c) F, Cl,
Br, I (d) Mg2, Na, Al3, O2, F, N3
(a) Be lt Mg lt Ca lt Sr lt Ba lt Ra (b) C lt Si lt Ge lt
Sn lt Pb (c) F lt Cl lt Br lt I (d) Al3 lt Mg2 lt
Na lt F lt O2 lt N3
Answer