IONIC COMPOUNDS

Formation of Ionic Bonds and their Properties

IONIC SOLIDSExist in the solid state in a crystalline form.

Commonly referred to as a salt.

Lattice is a regularly repeating three-dimensional array of atoms, molecules, or ions

Ionic vs Molecular Compounds In a solid ionic compound, each ion is surrounded by ions of the opposite charge, but is not associated with any particular ion.

There are no molecules in an ionic solid. The formula of an ionic compound refers to the formula of the formula unit, the lowest ratio of the positive and negative ions that make up the compound.

What are ionic compounds made of?Ionic compounds are formed as a result of the electrostatic attraction between oppositely charged ions – a cation and an anion.

Monoatomic Ions e.g. Na+, Ca+2, O-2, or Cl-

Polyatomic Ions e.g. NH4+1, MnO4

-, SO3-2, or PO4

-3

Since the number of positive charge equals the number of negative charges, the net compound is neutral.

LATTICE ENERGYA measure of the strength of bonds in that ionic compound

Energy change or enthalpy of formation

For NaCl, the final step to create NaCl (s) would be:

Na+ (g) + Cl− (g) → NaCl (s)

This process is exothermic.

Why Exothermic?

Coulomb's Law

Formation of lithium fluoride (LiF)Li (s) Li (g)

Li (g) Li+ (g) + e-

½ F2(g) F (g)

F (g) + e- F- (g)

Li+ (g) + F- (g) LiF (s)_______________________

Li (s) + ½ F2(g) LiF (s)

Lattice Energy of Alkali MetalsF- Cl- Br- I-

Li+ 1036 853 807 757

Na+ 923 780 747 704

K+ 821 715 682 649

Rb+ 785 689 660 630

Cs+ 740 659 631 604

The values are energy released when the ionic crystal is formed. The units are kJ/mol.

BE ABLE TO EXPLAIN THE TREND!

Impact of Charge

The stronger the charge on an ion the stronger the attractive force that will result in an ionic lattice. Therefore, 1+/- ions form compounds with lower lattice enthalpies than 2+/- ions. Moreover, the Mg+2 ion is smaller than the Na+1 ion.

CompoundNaClNa2OMgCl2

MgO

Lattice Energy780

247825263791

EXPLANATION?

Let's Make Sodium!

Impact of Ionic Radius

In general, the larger the ionic radius the lower the lattice enthalpy as the force of attraction between the ions is smaller and the packing of the ions is less efficient.

NaCl 780 NaF 918KCl 711 NaCl 780RbCl 685 NaBr 742CsCl 661 NaI 705

EXPLANATION?

Would you expect the lattice energy of CsI be relatively large or relative small?

CHECK FOR UNDERSTANDING

For each pair, indicate which bond will have a higher lattice energy. Justify your answer

a. NaCl or LiF

b. MgO or SrO

c. CaO or ScN

788 v 1030 kJ/mol

3795 v 3217 kJ/mol

3414 v 7547 kJ/mol

CHECK FOR UNDERSTANDING

Two ions with charges of +2 and -2 are separated by a distance of 1.0 angstroms (0.1 nm). By what factor is the potential energy changed when those ions are moved to 4.0 angstroms away?

Charles Coulomb

By what factor is the potential energy changed when distance is kept constant but the charge of the ions becomes +1 and -1?

ELECTROLYSIS

ALUMINIUM VIDEO

How Is It Made?

Let's Make Sodium Again!