Question: What makes a glass
different from a solid such as quartz? Under what conditions could quartz be
converted into glass?
Answer The arrangement of the constituent particles makes
glass different from quartz. In glass, the constituent particles have short
range order, but in quartz, the constituent particles have both long range and
short range orders. Quartz can be converted into glass by heating and then
cooling it rapidly.
What is meant by the term 'coordination number'?
(ii) What is the coordination number of atoms:
(a) in a cubic
(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.
coordination number of atoms
(a) in a cubic close-packed structure is 12,
(b) in a body-centred cubic structure is 8
Question : If the radius of the octachedral void is r and radius of the
atoms in close packing is R, derive relation between r and R.
A sphere with centre O, is fitted into the octahedral void as shown in the
above figure. It can be observed from the figure that Δ POQ is right-angled
POQ = 90
Now, applying Pythagoras theorem, we can write:
Explain the following terms with suitable examples: (i) Schottky
Answer (i) Schottky defect: Schottky defect is basically a vacancy
defect shown by ionic solids. In this defect, an equal number of cations and
anions are missing to maintain electrical neutrality. It decreases the density
of a substance. Significant number of Schottky defects is present in ionic
solids. For example, in NaCl, there are approximately 106��
at room temperature. Ionic substances
containing similarsized cations and anions show this type of defect. For
example: NaCl, KCl, CsCl, AgBr, etc.