How many sodium ions and chloride ions are present in a unit cell of sodium chloride crystal?  

Solution

(i) For a crystal of sodium the type if lattice in which it crystallises is Face Centered Cubic (fcc). (ii) The co-ordination number of each sodium ion and chloride ion in the crystal lattice is  6:6. (iii) The number of sodium ions and chloride ions present in a unit cell is 4 each. (iv) The structural arrangement of the sodium chloride crystal is cubic close packing with Cl− ions at corners and face centres and Na+ ions at edge centres and body centre, With respect to Na+ ion it is octahedral.