4.5 4.5 Physical Properties of Giant Physical Properties of Giant

Ionic CompoundsIonic Compounds

Giant ionic solids form as the result of electron transfer from a metal to a non-metal atom leading to an electrostatic attraction between the positive and negative ions – the IONIC BOND.

Intramolecular

Electrostatic attraction between the oppositely charged ions are very STRONG

Physical properties of giant ionic Physical properties of giant ionic compoundscompounds

High Melting and Boiling pointThe strong intramolecular electrostatic

attractions between the positive and negative ions, means that a large amount of heat energy is needs to be absorbed to overcome (break) the electrostatic attraction.

Electrical Conductivity

To conduct electricity an ionic compound must have mobile ions.

Solid ionic compounds don’t conduct electricity, because there are no free ions to carry a current, they are locked into the lattice.

When an ionic compound is heated above its meltingpoint it becomes molten (pure liquid). The ions are separated from the lattice and are free to move and conduct electricity.

NaCl (s) + heat NaCl (l)

Ionic compounds will be soluble in water if the polarwater molecule and can form a dipole-ion interaction that is strong enough to overcome the intramolecular electrostatic attraction between the ions in the ionic lattice. When this happens the ions are separated from its lattice and are free to move through the solution and conduct a current.

NaCl (s) + H2O (l) Na+ (aq) + Cl- (aq)

SolubilityIonic solids will be soluble in water if the

attraction between water and the ions is strong enough to overcome the electrostatic attraction between the ions in the lattice.

VolatilityIonic compounds are not volatile. This means

they don’t turn into a vapor (gas) at low temperatures because the strong intramolecular electrostatic attractions between the ions are too strong to be broken at low temperatures.

Brittle

When a stress is applied to an ionic solid it shifts the ion layers slightly.

Ions of the same charge (+ and + or – and -) are brought side-by-side and repel causing the crystal to shatter!

Compound Ions Mpt (oC) Bpt (oC)

NaCl Na+ , Cl- 801 1465

MgO Mg2+ , O2- 2800 3600

RbI Rb+ , I- 647 1300

Comparing the physical properties of melting point and boiling point

Differences in m.pt and b.pt of NaCl,

MgO andRbI are the result of:

1. The charge on the ions. The greater the charge on the ions the stronger the

electrostaticattraction between them. The 2+ and 2- ions in MgO attract each other more strongly than the 1+ and 1- ions in NaCl and RbI, giving MgO a higher mpt and bpt.

Na+

Cl-

Mg2+

O2-

2. Radius of the ions Smaller ions can get closer together and so the

electrostatic attraction between them will be greater, decreasing the radius.

ionic radius of Rb+ > Na+ > Mg2+

ionic radius of I- > Cl- > O2-

The electrostatic attraction between bigger ions is weaker, thus less heat energy needs to be absorbed to separate them, decreasing the mpt and bpt

mpt RbI NaCl MgO

Effects of impurities Effects of impurities on the mpt and bpt on the mpt and bpt of ionic solidsof ionic solids

Impurities in an ionic solid prevent the ordering necessary to form a strong crystal lattice. This weakens the electrostatic attraction between the ions lowering the melting point of the solid.

Therefore ionic solids with impurities have lower melting points than in the pure substance. This is why salt is placed on icy roads in the winter. Pure water melts at 0C but water (ice) with salt added as an impurity freezes at about -20C causing the ice to melt, even when the air temperature is below freezing (0C).