Question

1. The coordination number of a metal atom in a close-packed structure is?

2. The coordination number of a metal atom in a BCC structure is?

3. The coordination number of an atom in the diamond or zinc blende structure is?

4. Metallic hydrogen is a proposed high pressure state of hydrogen with exciting electrical properties. Calculations have been used to propose that this material will have a low coordination number. Of the three cubic structures we have discussed, body centered cubic is the most reasonable.

True or False?

Answer 1

Coordination number is the number of neighboring atoms that are bonded to that atom.

1. In cubic-closest-packed structure, a sphere is in contact with 6 spheres in its same plane, with 3 spheres in its above plane and with 3 spheres in its below plane. Thus the coordination number of a metal in this structure is 12.

In hexagonal-closest-packed structure, a sphere is in contact with 12 other spheres which are in same, above and below planes to the sphere. Thus the coordination number of a metal in this structure is 12.

2. In a BCC structure, each atom is bonded to 8 atoms. Thus the coordination number of metal in BCC unit cell is 8.

3. In diamond, strong covalent bonds exist between the carbon atoms. Each carbon is connected to four other carbon atoms. So, the coordination number of an atom in the diamond is 4. In zinc blende structure (ZnS), each sulfide ion is connected to four zinc ions. So, the coordination number of an atom in the zinc blende is 4.

4. The structure with least CN is 4, that is of diamond or ZnS. So, the statement is false.