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Why is glass considered a super cooled liquid?

Glass is an amorphous solid. Just like liquids, amorphous solids also have a tendency to flow, though very slowly. Therefore, these are called pseudo solids or supercooled liquids.

1.26     Explain the following with suitable examples:

             (v)12-16 and 13-15 group compounds

(v)12-16 and 13-15 group compounds 12-16 group compounds: Compounds formed between elements of group 12 and elements of group 16 are called 12-16 group compounds. Example: ZnS 13-15 group compounds: Compounds formed between elements of group 13 and elements of group 15 are called 13-15 group compounds. Example: GaAs

1.26     Explain the following with suitable examples:

                    (iv) Antiferromagnetism

 (iv) Antiferromagnetism :  Substances like MnO showing antiferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other's magnetic moment as shown in the figure.

1.26    Explain the following with suitable examples:

                    (iii)    Ferrimagnetism

(iii)    Ferrimagnetism :  When the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers than ferrimagnetism is observed. Refer to the given figure. They are weakly attracted by a magnetic field as compared to ferromagnetic substances. Example:. These substances also lose ferrimagnetism on heating and become paramagnetic.

1.26     Explain the following with suitable examples:

              (ii) Paramagnetism

(ii) Paramagnetism :  The substances that are weakly attracted by a magnetic field are called paramagnetic substances. They are magnetised in a magnetic field in the same direction. They lose their magnetism in the absence of a magnetic field. Paramagnetism is due to the presence of one or more unpaired electrons which are attracted by the magnetic field. Example:.

1.26     Explain the following with suitable examples:

                    (i) Ferromagnetism

(i) Ferromagnetism Substances that are attracted very strongly by a magnetic field are called ferromagnetic substances. Example: iron, cobalt, nickel, gadolinium and CrO2. Besides strong attractions, these substances can be permanently magnetised. The metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an...

1.25     If NaCl is doped with 10–3 mol % of SrCl2, what is the concentration of cation vacancies?

 

NaCl is doped with 10–3 mol % of  Concentration in % so that take a total of 100 mol of solution. Moles of NaCl = 100 - moles of  Moles of  is very less , so we can neglect them. Moles of NaCl =100 1 mole of NaCl is dipped with =   mol of . So cation vacanties per mole of NaCl = mol   particles  So cation vacancies per mol of NaCl =                                                         ...

1.25     Aluminium crystallises in a cubic close-packed structure. Its metallic radius is 125 pm.
                    (ii) How many unit cells are there in 1.00 cm3 of aluminium?

For cubic close-packed structure, we have  Given :                          the volume of one unit cell                                                                                    Total unit cells in 1.00 cm3                                                                                                  

1.24 Aluminium crystallises in a cubic close-packed structure. Its metallic radius is 125 pm.
(i) What is the length of the side of the unit cell?

For  cubic close-packed structure, we have  Given :                         

1.23     Explain the following terms with suitable examples

                    (iv) F-centres

When any negative ion is absent from lattice site then result the crystal now has an excess of cations. To maintain electrical neutrality the vacant anionic site is occupied by an electron as shown in the figure. The anionic sites occupied by unpaired electrons are called F-centres. The F- centre is responsible for most interstitial properties of the compound. This defect can be observed in  NaCl.

1.23     Explain the following terms with suitable examples:

                    (iii) Interstitials

Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site as shown in the figure, the crystal is said to have the interstitial defect. This defect increases the density of the substance. Interstitial defects can be shown by non-ionic solids.  The figure representing the interstitial defect is as shown :

1.23     Explain the following terms with suitable examples

                (ii)Frenkel defect

(ii)Frenkel defect  : Frenkel defect is shown be ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site as shown in the figure . It creates a vacancy defect at its original site and an interstitial defect at its new site. Frenkel defect is also called a dislocation defect. Frenkel defect does not affect the density of the solid. Frenkel defect...

1.23    Explain the following terms with suitable examples:
            (i) Schottky defect

 (i) Schottky defect The Schottky defect is basically a vacancy defect in ionic solids. To maintain electrical neutrality, the number of missing cations and anions are equal as shown in the figure. Schottky defect decreases the density of the substance. The number of such defects in ionic solids is quite significant. For example, in NaCl, there are approximately 106 Schottky pairs per cm3 at...

1.22     In terms of band theory, what is the difference
                (ii) between a conductor and a semiconductor?

Conductor: A conductor may conduct electricity through the movement of electrons or ions. The conductivity of metals depends upon the number of valence electrons available per atom. The atomic orbitals of metal atoms form molecular orbitals which are so close in energy to each other as to form a band. If this band is partially filled or it overlaps with a higher energy unoccupied conduction...

1.22  In terms of band theory, what is the difference
     (i) between a conductor and an insulator

Conductor: A conductor may conduct electricity through the movement of electrons or ions. The conductivity of metals depends upon the number of valence electrons available per atom. The atomic orbitals of metal atoms form molecular orbitals which are so close in energy to each other as to form a band. If this band is partially filled or it overlaps with a higher energy unoccupied conduction...

1.21     Gold (atomic radius = 0.144 nm) crystallises in a face-centred unit cell. What is the length of a side of the cell?

Atomic radius = 0.144 nm  for  a face-centred unit cell the length of a side of the cell is 0.407.

1.20    Classify each of the following as being either ap-type or a n-type semiconductor: 

               (ii) Si doped with B.

Si belongs to group 14 and B belongs to group 13 so it is a p-type semiconductor.

1.20   Classify each of the following as being either a p-type or a n-type semiconductor:

               (i) Ge doped with In.

We know that Ge is a group 14 element (having 4 valence electrons) and In is a group 13 element  (having 3 valence electrons). So this is a p-type semiconductor.

1.19   Ferric oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.

Let the number of oxide ions be x. Then, the number of octahedral voids = number of oxide ions = x. According to question, number of ferric ions                                                                    Ratio of ferric ion: oxide ion = 2:3 So the required formula of the compound is Fe2O3.

1.18    Non-stoichiometric cuprous oxide, Cu_{2}O can be prepared in a 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 semiconductor?    

In the given compound Cu2O, the charge on Cu is +1. But the charge on copper in normal compounds is +2. So Cu+2 will try to replace Cu+1 in atmospheric conditions. This will lead to a generation of positive charge holes which are the cause of conductivity in p-type semiconductors. So given compound can act as p-type semiconductors.
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