Question

Explain the structure of NaCl.

Draw a neat diagram for sodium chloride structure and describe it accordingly.

Discuss the structure of sodium chloride.

Answer 1

Sodium chloride possesses a cubic close-packing (ccp) or a face-centred cubic (fcc) arrangement, as shown in Fig.

The salient features of the structure of sodium chloride are as follows:

1. In sodium chloride, Cl⁻ ions make a lattice that is cube-shaped and tightly packed, and Na⁺ ions fill the eight-sided gaps. There are Cl⁻ ions in the sides and face centers of the cubic unit cell, and Na⁺ ions in the body centre and edge centers. It has a face-centered cubic shape, which is also known as cubic close-packing (ccp).
2. In the case of NaCl, it has been found that the radius ratio `(r_(Na^(+)))/(r_(Cl^(-)))`= 0.525.
   A Na⁺ ion can only fit into an octahedral void in the ccp arrangement of Cl⁻ ions with a radius ratio of 0.414. However, the situation at NaCl is not perfect. Sodium chloride has an enlarged face-centred cubic lattice with slightly open arrangements for bigger Na⁺ ions.
3. Each Na⁺ ion is coordinated by six Cl⁻ ions, while each Cl⁻ ion is coordinated by six Na⁺ ions. Consequently, the coordination number for each Na⁺ ion and each Cl⁻ ion is 6, resulting in a 6 : 6 coordination structure. The edge length of the fcc unit cell for NaCl is approximately 564 pm.
4. There are six Cl⁻ ions that coordinate each Na⁺ ion, and six Na⁺ ions that coordinate each Cl⁻ ion (Fig. A 6 : 6 coordination structure is thus produced, with each Na⁺ ion and each Cl⁻ ion having a coordination number of 6.
   Number of Cl⁻ ions = `8 xx 1/8 + 6 xx 1/2 = 4`
   In the unit cell, twelve Na⁺ ions are present at the edge-centres and one at the body-centre. Therefore,
   Number of Na⁺ ions = `12 xx 1/4 + 1 xx 1 = 4`
   Thus, a unit cell of sodium chloride possesses 4 NaCl units. The structure of sodium chloride is also referred to as rock salt structure.