Question

Which of the following statement (s) is (are) correct ?

(a) The coordinate number of each type of ion in CsCl crystal is 8

(b) A metal that crystallizes in bcc structure has a coordination number of 12

(c) A unit cell of an ionic crystal shares some of its ions with other unit cells.

(d) The length of the unit cell in NaCl is 552 pm (r_Na⁺ = 95 pm,r_cl^- = 181 pm)

Answer 1

Correct option (a),(c),(d)

Explanation:

The crystal of CsCl has body centred cubic unit cell. Hence, each ion in this structure has coordination number of eight.

In case of NaCl crystal, two interpenetrating face centred cubic (fcc) lattices are present in these one composed entirely of Na⁺ ions and the other of Cl^- ions. Each Na⁺ ion is located half way between two Cl^- ions and vice-versa. A unit cell of NaCl crystal has Cl^- ions at the corners as well as at the face centres and Na⁺ ions are located in octahedral voids. On each edge of cubic unit cell, there are two Cl^- ions and one Na⁺ ion.

Hence,   a = 2(r_Na⁺ + r_cl^-)

= 2(95 pm + 181 pm) = 552 pm