Sodium Chloride Structure

SODIUM CHLORIDE STRUCTURE

Ionic solids are made up of cation (+ ion) and anions (- ion). Generally, the cations (+ ve) are smaller in size to anions.

Sodium chloride and many other ionic crystals crystallize in rock salt structure which is also known as sodium chloride structure.

Structure composition

NaCl crystal has FCC structure with Na⁺ ion (sodium) and

Cl^- ion (chlorine) is shown in fig. 1.27 (a). Fig. 1.27 (b) shows a unit cell of NaCl lattice.

Fig. 1.27 (a) NaCl structure. An example of close packing of spheres of different radii. The large ions of chlorine form a close three layer packing in which empty space is occupied by the smaller ions of sodium.

The Cl^- ions are situated at the corners as well as at the centres of the faces of the cube. Na⁺ ions are situated exactly at the middle of the axial length of the unit cell along each axis.

Thus, NaCl crystal can be thought of as composed of two FCC Na⁺  and Cl^- sub lattices.

Fig. 1.27 (b) Model of unit cell of NaCl structure

When one of Cl^- ions has the origin at (0, 0, 0) then one of the nearest Na⁺ ion has the origin at

(1/2, 0, 0)

Let us discuss the important parameters of the NaCl crystal.

1. Number of atoms per unit cell

In NaCl structure, it has two types of ions namely, Na⁺ and Cl^-. Let us find the number of sodium ions and chlorine ions separately.

(a) Number of Na⁺ ions per unit cell

Na⁺ ion is located at the mid point of the axial length. There are '12' such mid point Na⁺ ions. Each sodium ion is shared by 4 adjacent unit cells. (Fig. 1.28)

Share of one unit cell ie.,

number of mid point Na⁺ ions per unit cell = (1/4) × 12 = 3 ions

One sodium ion is located at the centre of the unit cell.

Therefore, the number of sodium ions per unit cell = 3+1 = 4 ions.

(b) Number of CI ions per unit cell

Here, there are two types of Cl^- ion namely (a) corner Cl^- ions and (b) face centered Cl^- ions as shown in fig. 1.28

There are 8 chlorine ions in the corners and they are shared by 8 adjacent unit cells.

Number of corner Cl^- ions per unit cell = (1/8) x 8 = 1 ion

Each face centered Cl^- ion is shared by 2 adjoining unit cell. There are 6 face centered Cl^- ions.

Number of face centered Cl^- ion per unit cell = (1/2) x 6 = 3 ions

Therefore, total number of chlorine ions per unit cell = 1+3 = 4 ions.

Thus, there are 4 Na⁺ and 4 Cl^- ions per unit cell. i.e., total number of sodium and chlorine ions per unit cell is 8.

2. Co-ordination number

Each Na⁺ has 6 Cl^- ions as nearest neighbours and similarly each Cl^- ion has 6 Na⁺ ions as nearest neighbours. Hence, the coordination number of NaCl for opposite kind of ions is 6.

3. Nearest neighbouring distance

Nearest neighbouring distance is a/2