Structure of Sodium Chloride or Structure of Rock Salt:
Sodium chloride crystal consists of Na+ and Cl- ions. Each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions. Thus, in sodium chloride, NA+ AS WELL AS Cl- ions have the coordination number of 6. Therefore, sodium chloride has 6:6 coordination.
Sodium chloride has a face centred cubic (fcc) structure. Na+ ions occupy the corners and face centres of a cube. The Cl – ions are present at the body and edge centres.One unit cell of sodium chloride consists of 14 Na+ ions and 13 Cl- ions or vice versa. These ions correspond to 4 units of Na+Cl-, i.e., one unit cell of sodium chloride contains four Na+Cl- units ( 4 Na+ and 4 Cl- ions), per unit cell.
In sodium chloride structure , each Na+ ion has the following neighbours:
- Six Cl- at a distance of r, where r is the distance between Na+ and Cl-.
- Twelve Na+ at a distance of √2 r (as the next nearest neighbour)
- Eight Cl- at a distance of √3 r 9as the third nearest neighbour)
- Six Na+ at a distance of √4 r or 2r (as the fourth nearest neighbour).
AgCl, AgBr, Kcl, RbCl and the oxides and sulphides of alkaline earth metals (expect beryllium sulphide ) have sodium chloride structure.
Structure of Cesium Chloride Structure:
Cesium ion is bigger than Na+. The cation – anion radius ratio for CsCl is higher than that for Nacl. As a result, Cs+ ions has a co – ordination number of 8.The structure of CsCl is a body – centred cubic structure. A unit cell of CsCl consists of a Cs+ at the body – centre and surrounded by eight Cl- (or one Cl- at the centre and surrounded by eight Cs+). Each corner ion contributes 1/8 to a unit cell, and the body – centered ion belongs to the unit cell.
Therefore, one unit cell of cesium chloride consists of one cs+ ion and one Cl- ion, Cl- and Cs+ i.e., one unit cell of Cesium chloride consists of one molecule of cesium chloride.