Why does the electrical conductivity of semiconductors increase with rise in temperature?
The energy gap between valence band and conduction band is small. At room temperature, they do not conduct electricity but when temperature is raised large number of electron from valence band get sufficient energy to jump to conduction band. This is known as thermodynamic conduction in intrinsic semiconductors. Thus, they become more conducting as the temperature increases.
Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
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 semiconductor?
Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body- centred cubic and (iii) hexagonal close-packed lattice?
Classify each of the following solids as ionic, metallic, modular, network (covalent) or amorphous:
(i) Tetra phosphorus decoxide (P4O10) (ii) Ammonium phosphate, (NH4)3P04 (iii) SiC (iv) I2 (v) P4 (vii) Graphite (viii), Brass (ix) Rb (x) LiBr (xi) Si
The value of magnetic moment is zero in the case of antiferromagnetic substances because the domains …
(a) get oriented in the direction of the applied magnetic field
(b) get oriented opposite to the direction of the applied magnetic field
(c) are oppositely oriented with respect to each other without the application of magnetic field
(d) cancel out each other's magnetic moment
A cubic solid is made of two elements P and Q. Atoms Q are at the corners of the cube and P at the body centre. What is the formula of the compound ? What is the co-ordination number of P and Q?
Cations are present in the interstitial sites in
(a) Frenkel defect (b) Schottky defect
(c) vacancy defect (d) metal deficiency defect .
A perfect crystal of silicon (fig) is doped with some elements as given in the options. Which of these options show n-type semiconductors?
What is the two-dimensional coordination number of a molecule in a square close-packed layer?
What is the coordination number in a square close packed structure in two dimensions? (a) 2 (b) 3 (c) 4 (d) 6
In spite of long range order in the arrangement of particles why are the crystals usually not perfect?
In a compound, nitrogen atoms (N) make cubic close packed lattice and metal atoms (M) occupy one-third of the tetrahedral voids present. Determine the formula of the compound formed by M and N?
A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?
Niobium crystallises in a body centred cubic structure. If density is 8.55 g cm-3, calculate atomic radius of niobium, using its atomic mass 93u.
A compound forms hexagonal close-packed. structure. What is the total number of voids in 0. 5 mol of it? How many of these are tetrahedral voids?
Copper crystallises into a fee lattice with edge length 3.61 x 10-8 cm. Show that the calculated density is in agreement with its measured value of 8.92 gcm-3.
Which of the following statements are correct?
(a) Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic
(b) Ferrimagnetic’substances do not lose ferrimagnetism on heating and remain ferrimagnetic
(c) Antiferromagnetic substances have domain structure similar to ferromagnetic substances and their magnetic moments are not cancelled by each other
(d) In ferromagnetic substances, all the domains get oriented in the direction of magnetic field and remain as such even after removing magnetic field.
What types of stoichiometric defects are shown by (C.B.S.E. Delhi 2013)
(i) ZnS
(ii) AgBr?
Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
‘Stability of a crystal is reflected in the magnitude of its melting points’. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?
How many lattice points are there is one unit cell of each of the following lattices?
(i) Face centred cubic (if) Face centred tetragonal (iii) Body centred cubic
Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, Teflon, potassium nitrate, cellophane, polyvinyl chloride, fibreglass, copper
Solid A is a very hard electrical insulator in. solid as well as in molten state and melts at extremely high temperature. What type of solid is it?
What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?
Which of the following statement is not true about the hexagonal close packing?
(a) The coordination number is 12
(b) It has 74% packing efficiency
(c) Tetrahedral voids of the second layer are covered by the spheres of the third layer
(d) In this arrangement, spheres of the fourth layer are exactly aligned with those of the first layer.
Ferric oxide crystallises in a hexagonal dose- packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.
Classify .the following solids in different categories based on the nature of intermolecular forces operating in them: Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon, silicon carbide
Distinguish between
(i) Hexagonal and monoclinic unit cells
(ii) Face-centred and end-centred unit cells.
Explain how much portion of an atom located at
(i)corner and (ii)body centre of a cubic unit cell is part of its neighbouring unit cell.
What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?
Calculate the efficiency of packing in case of a metal crystal for (i) simple cubic, (ii) body centred cubic, and (iii) face centred cubic (with the assumptions that atoms are touching each other).
Which of the following is not a characteristic of a crystalline solid?
(a) Definite and characteristic heat of fusion
(b) Isotropic nature
(c) A regular periodically repeated pattern of arrangement of constituent particles in the entire crystal
(d) A true solid
In which of the following structure coordination number for cations and anions in the packed structure will be same?
(a) Cl– ions form fee lattice and Na+ ions occupy all octahedral voids of the unit cell.
(b) Ca2+ ions form fee lattice and F- ions occupy all the eight tetrahedral voids of the unit cell
(c) O2- ions form fee lattice and Na+ ions occupy all the eight tetrahedral voids of the unit cell
(d) S2- ions form fee lattice and Zn2+ ions go into alternate tetrahedral voids of the unit cell.
Under the influence of electric field, which of the following statements are true about the movement of electrons and holes in a p-type semiconductor?
(a) Electron will move towards the positively charged plate through electron holes
(b) Holes will appear to be moving towards the negatively charged plate
(c) Both electrons and holes appear to move towards the positively charged plate
(d) Movement of electrons is not related to the movement of holes
Assertion (A): Graphite is a good conductor of electricity, however, diamond belongs to the category of insulators.
Reason (R): Graphite is soft in nature on the other hand diamond is very hard and brittle.
Assertion (A): The packing efficiency is maximum for the fee structure. Reason (R): The coordination number is 12 in fee structures.
