1. Ionic structures
L.O. To be able to describe the energy changes involved in forming ionic compounds.

2. What was the question?
It is much easier for metals to lose 1 or 2 electrons from their outershell than to gain 5,6 or 7 to form a stable noble gas electron arrangement. By losing electrons metals become positively charged.
It is much easier for non-metals to gain 1 or 2 electrons than to lose 5,6 or 7 to form a noble gas configuration. By gaining electrons non-metals become negatively charged.

3. Write definitions of electron affinity and lattice energy (p71)
Energy changes and ionic bonding
Is the formation of an ionic bond exothermic or endothermic?
Define the first ionisation energy.
Write definitions of electron affinity and lattice energy (p71)

4. Important equations Lattice energy Enthalpy of formation
Na+ (g) + Cl- (g) NaCl (s) ΔHθlattice [NaCl(s)] = -787kJmol-1
Enthalpy of formation
Na (s) + 1/2Cl2(g) NaCl (s) ΔHθf [NaCl(s)] = -411kJmol-1
Draw out the Born Haber cycle in the middle of p72. Write out the worked example for Na2O

5. Testing the ionic model
L.O. To be able to apply data to determine the degree of ionic character in a bond
Keywords: Ionic, polarising, polarisable

6. Polarisation of the ionic bond
Compare the experimental lattice energy with the theoretical lattice energy.
Theoretical lattice energy comes from laws of electrostatics.
The electrostatic force F, depends on 2 factors:
The size of the charge on the ions
2. The distance between the 2 charges (big effect as it is the square of the distance used).

7. Which ions are polarising and which are polarisable?
Polarisation of the ionic bond
Polarisation: The distortion of the electron cloud in a molecule or ion by a nearby charge.
The greater the difference between the experimental and the theoretical values, the further towards covalent bonding the compound is.
Which ions are polarising and which are polarisable?
Diagrams on p75 + definitions of polarisable and polarising

8. . What are the effects of anion polarisation?
The electron density between :
(a) unpolarised ions is ZERO
(b) covalently bonded atoms is HIGH + - . + -
If an anion is polarised
electron density between ions INCREASES
 compound less ionic,
and more covalent in properties

9. If polarised : (a) melting point lower than expected
(b) conductivity of metal lower than expected
(c) solubility in covalent solvents greater than expected
NB If polarisation is EXTREME, ionic bonding becomes covalent!
eg C compounds always covalent –
“C4+” would be TOO polarising to ever exist near an anion
because of its very high charge and very small radius

10. e.g.1 ACROSS PERIOD 3 :
CHLORIDE
NaCl
MgCl2
AlCl3
SiCl4
MELTING POINT /K
1074
987
463
260
Na+ < Mg2+ << Al3+ <<<< Si4+ in polarising power
because charge increases
and cation radius decreases
 more polarisation of Cl- ion
 more covalent
 lower mpt
 NaCl & MgCl2 are ionic
but AlCl3 and SiCl4 are covalent

11. e.g.2 DOWN GROUP 2 :
CHLORIDE
BeCl2
MgCl2
CaCl2
SrCl2
MELTING POINT /K
678
987
1055
1148
Be2+ >>> Mg2+ > Ca2+ > Sr2+ in polarising power
because cation radius increases
(charge constant)
 less polarisation of Cl- ion
 less covalent
 higher mpt
 BeCl2 is covalent
but others are ionic

12. e.g.3 DOWN GROUP 7 :
CHLORIDE
LiF
LiCl
LiBr
LiI
MELTING POINT /K
1118
878
823
722
F- < Cl- < Br- < I- in anion polarisability
because anion radius increases
(charge constant)
 more polarisation by Li+ ion
 more covalent character
 decreasing melting points

13. AgCl KI Which of the following has the most IONIC character?
Compound
Experimental lattice energy /kJmol-1
Theoretical lattice energy /kJmol-1
NaI
-705
-687
AgCl
-905
-833 KI
-651
-636
AgCl KI
Which of the following has the most COVALENT character?