The Solid State

Question 109

What is the co-ordination number of atoms:
(i) in a cubic close packed structure?
(ii) in a body centred cubic structure?


(i) 6:6
(ii) 8:8

Question 110

How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain.



The atomic mass of an unknown metal can be determined by knowing its density and the dimension of unit cell.

Let ‘a’ be the edge length of a cubic unit cell and it contains ‘Z’ atoms of a substance of atomic mass ‘M’.

From the above expression for density, d we can calculate the atomic mass, M of metal as other parameters are known.
Question 111

'Stability of a crystal is reflected in the magnitude of its melting points. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the inter molecular forces between these molecules?



Stability of a crystal is reflected in the magnitude of its melting points because higher the melting point, greater is the intermolecular force and greater the intermolecular force greater is the stability. And hence, a substance with higher melting point would be more stable.

The melting points of the given substances are as follows:

Solid water - 273 K

Ethyl alcohol – 158.8 K

Diethyl ether – 156.85 K

Methane – 89.34 K

As we can see the melting point of solid water is highest and melting point of methane is lowest among the given substance. This says that intermolecular force in solid water is strongest and the intermolecular force in methane is weakest.

Question 112

How will you distinguish between the following pairs of terms:
Hexagonal close-packing and cubic close-packing?


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