229. Assign reasons the following:
(i) Phosphorus doped silicon is a semi-conductor.
(ii) Schottky defect lowers the density of a solid.
(iii) Some of the very old glass objects appear slightly milky instead of being transparent.

(i) When silicon is doped with phosphorus four valence electrons of phosphorus are involved in bond formation with the neighbouring silicon atoms, while the fifth valence electron is left free to conduct electricity. This type of conduction which arises due to the availability of excess electrons is called n-type conduction.

(ii) Schottky defect arises when equal number of cations and anions are missing from their lattice sites. As the number of ions decreases due to this defect In order to maintain electrical neutrality, the number of missing cations and anions are equal, the mass decreases, but the volume remains the same. As a result, the density of the solid decreases.

(iii) Some of the very old glass objects appear slightly milky instead of being transparentbecause of some crystallisation at that point.