Question

(i) What is meant by the term 'coordination number'? (ii) What is the coordination number of atoms: (a) in a cubic close-packed structure? (b) in a body-centred cubic structure?

Answer 1

Hey there!

----------

Answer :-

→ Coordination number : It is the number of atoms surrounding a central metal atom or ion in a crystal lattice.

(i) Cubic close-packed structure : In a ccp structure, there is 1 octahedral void in the centre of the body and 12 octahedral voids are on 12 edges of the cube or we can say, each sphere is surrounded by 12 spheres. The coordination number is 12.

(ii) Body-centred cubic structure : In a bcp structure, it has lattice points not only at the center of the cube but at also corner of the cube. Therefore, bcp is surrounded by 8 spheres at the corner of the cube and the central sphere is at body centre. The coordination number of central sphere is 8.