121.Give reasons for Non-stoichiometric sodium chloride is a yellow solid.
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122.What is a semi-conductor? Describe the two main types of semi-conductors and contrast their conduction mechanism.
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123.A cubic solid is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body centre. What is the formula of the compound? What are the coordination numbers of P and Q?
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124.What is the difference between Schottky defects and Frenkel defects?
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125.If the radius of the octahedral void is r and radius of the atoms in close packing is R, derive relation between r and R.
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126.Pure silicon is an insulator. Silicon doped with phosphorus is a semi-conductor. Silicon doped with gallium is also a semiconductor. What is the difference between the two doped silicon semi-conductor?
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127.What are paramagnetic and ferromagnetic substances. Account for the paramagnetic character of transition metal compounds. How does the paramagnetic character of the bivalent ions of the first transition metal series varies from Ti (Z = 22) to Cu (Z = 29)?
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128.Non-stoichiometric cuprous oxide, Cu2O can be prepared in laboratory. In this oxide, copper to oxygen ratio is slightly less than 2 :1. Can you account for the fact that this substance is a p-type semi-conductor ?
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129.Ferric oxide crystallizes in a hexagonal close packed array of oxide ions with two out of every three tetrahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.
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130.Classify each of the following as being either a p-type or an n-type semi conducor: (i) Ge doped with In (ii) B doped with Si.
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