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

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.

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)

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

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

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).

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

. 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

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

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

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

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

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?