Q: 4. (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?
(i) The number of nearest neighbours of any constituent particle present in the crystal lattice is called its coordination number.
(ii) The coordination number of atoms
(A)in a cubic close-packed structure is 12, and
(B) in a body-centred cubic structure is 8
Q: 5. How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain.
By knowing the density of an unknown metal and the dimension of its unit cell, the atomic mass of the metal can be determined.
Let 'a' be the edge length of a unit cell of a crystal, 'd' be the density of the metal, 'm' be the mass of one atom of the metal and 'z' be the number of atoms in the unit cell.
Now, density of the unit cell
[Since mass of the unit cell = Number of atoms in the unit cell × mass of one atom][Volume of the unit cell = (Edge length of the cubic unit cell) 3]
From equation (i), we have:
Now, mass of one atom of metal (m)
If the edge lengths are different (say a, b and c), then equation (ii) becomes:
Thus, using equation (iii) and (iv), the atomic mass of an unknown metal can be determined.