Giant Ionic

A giant ionic lattice is a highly regular arrangement of anions and cations - a giant, regularly repeating array. For the ionic compound sodium chloride this can be explored below.

Chloride ion, Cl-
  Sodium ion, Na+
Surround an Na+ ion by 6 Cl- ions
Surround a Cl- ion by 6 Na+ ions
Show Unit Cell 
Click through the items above...

The number of nearest neighbours around a given atom or ion is known as its co-ordination number. Both ions in sodium chloride have a co-ordination number of 6; it is 6:6 co-ordinate.

It is important to appreciate that each type of ion is immediately 'involved' with six of the other kind; there is no such separate entity as NaCl. Avoid referring to NaCl (and other ionic compounds) as molecules; they are ionic, not molecular. NaCl is an empirical formula.

In sodium chloride there are equal numbers of oppositely-charged particles (Na+ and Cl- ions) vibrating in fixed positions. But the crystal still holds together and sodium chloride has a high melting point of 801 °C. The electrostatic forces of attraction between oppositely-charged ions must exceed the repulsions between ions of the same charge. Essentially this is because ions of like charge are further apart than those of opposite charge. Both attraction and repulsion obey Coulomb's Law and decrease in proportion to the square of the distance apart. So, repulsion occurs at a larger distance.

An electrostatic attraction of the kind referred to above is called an ionic bond. An ionic bond is not uni-directional; it exists in all directions. In a crystal lattice, ionic bonding extends throughout the entire structure. This strong bonding confers certain properties upon ionic compounds. They are typically hard with high melting points and boiling points. They can conduct electricity when molten or when in aqueous solution, that is, when their ions are free to move about. When they do conduct electricity the salt becomes chemically changed by electrolysis. For example,

2NaCl(l)   ®   2Na(s)   +   Cl2(g)

Typically, ionic compounds are soluble in the solvent water with its polar H2O molecules. Water molecules are able to surround (solvate) both anions and cations.

However, some ionic compounds, for example silver chloride, AgCl, are only slightly (sparingly) soluble in water owing to their high endothermic lattice enthalpies.

The Unit Cell...

The Unit Cell is the simplest repeating unit in a crystal. It repeats over and over to make up a crystal of the salt. Looking at the unit cell above, an ion at the corner of the cube contributes 1/8 to that cube. Those at the centre of a face contribute 1/2 to that cube. An ion in the edge of a face contributes 1/4 and, of course, an ion at the centre belongs entirely to that cube. The unit cell of sodium chloride therefore contains four sodium ions [1 + (12 x 1/4) = 4] and four chloride ions [(6 x 1/2) + (8 x 1/8) = 4].

Notice also that there is a particle at the centre of each face of the the sodium chloride unit cell. For this reason the sodium chloride crystal lattice is a face-centred cubic (f.c.c.) lattice. It is easy to see where a Cl- ion is at the centre of a face, but look carefully and you can see that Na+ ions too occupy face-centred positions. You could think of the sodium chloride crystal as a lattice of Cl- ions with the smaller Na+ ions filling the holes between them. This idea is illustrated below, and you can add a further two layers of ions.

Add Na+ ions
Add Second Layer
Add Third Layer

Click through the items above...
A lattice of Cl- ions...

Now consider Caesium Chloride...

Cs+ ions are larger than Na+ ions, and unlike the sodium ions in sodium chloride they cannot fit in the spaces within the lattice of Cl- ions. In caesium chloride the chloride ions become organised in a more open cubic arrangement with the caesium ions occupying the holes between them. This idea is illustrated below.

Add layer of Cs+ ions
Add layer of Cl- ions
Add another layer of Cs+ ions
Add another layer of Cl- ions

Click through the items above...
A lattice of Cl- ions...

Each Cs+ ion is surrounded by eight Cl- ions and each Cl- ion by eight Cs+ ions. Therefore caesium chloride has 8:8 co-ordination. The unit cell of caesium chloride contains only one ion of each kind, for example, there is a single Cs+ ion in the middle and (8 x 1/8) = 1 Cl- ions at the corners.

Return to Chemical Structure