The Solid State

Short Answer Type

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Explain the following term with suitable examples:
'F-centres'.

F-centres – This is type of defect and called metal excess defect. These type of defects seen because of missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called F-centre and responsible for showing colour by the compound. example sodium chloride impart yellow colour beacuse of F centre .
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    Explain the following term with suitable examples:
    'Interstitials'.
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    How would you account for the following:
    Frenkel defects are not found in alkali metal halides.
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    Explain the following term with suitable examples:
    'Frenkel defect'
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    Explain the following terms with suitable examples:
    'Schottky defect'
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    Schottky defects generate an equal number of cation and anion vacancies while doping produces only cation vacancies and not anion vacancies. Why?
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    In terms of band theory, what is the difference between a conductor and a semi-conductor?

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    In terms of band theory, what is the difference between a conductor and an insulator?

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    Examine the illustration of a portion of the defective crystal given below and answer the following questions:
    (i) What are these types of vacancy defects called?
    (ii) How is the density of a crystal affected by these defects?
    (iii) Name one ionic compound which can show this type of defect in the crystalline state.
    (iv) How is the stoichiometry of the compound effected?


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    Long Answer Type

    The ions of NaF and MgO all have the same number of electrons, and the internuclear distance are about the same (235 pm and 215 pm). Why then are the melting points of NaF and MgO so different. (992°C and 2642°C)?
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