UNIT : - SOLID STATE CLASS :- 12
Chapter : – 1 (Exercise 1.2)
Q – 1.1 Describe the term ‘amorphous’. Give a few examples of amorphous solids.
Ans. Amorphous solids are those solids in which the
constituent particles may have a short range order but do not have a long range
order. They have irregular shapes and are isotropic in nature. They do not
undergo a clean cleavage. They do not have sharp melting points or definite
heats of fusion.
Example. Glass, rubber and plastics.
Q – 1.2 What makes a glass different from a solid such as
quartz ? Under what conditions could quartz be converted into glass ?
Ans. Glass is an amorphous solid in which the constituent
(SiO4 tetrahendra) have only a short range order and there is
no long range order. In quartz, the constituent particles (SiO4 tetrahendra)
have short range as well as long range order. On melting quartz and then
cooling it rapidly, it is converted into glass.
Q – 1.3 Classify each of the following solids as
ionic, metallic, molecular, network (covalent) or amorphous : (i)
Tetraphosphorus decoxide (P4O10) (ii)
Ammonium phosphate, (NH4)3PO4
(iii) SiC (iv) I2 (v)
P4 (vi) Plastics (vii)
Graphite (viii) Brass (ix) Rb (x) LiBr (xi)
Si
Ans. Ionic = (NH4)3PO4 and
LiBr ; Metallic = Brass, Rb ; Molecular =P4O10, I2,
P4, ; Network (covalent) = Graphite, SiC, Si ; Amorphous =
Plastics.
Q – 1.4 What is meant by the term ‘coordination
number’? What is the coordination number of atoms (a) in a cubic close
packed structure ? (b) in a body-centred cubic structure ?
Ans. The coordination number of a constituent particle (
atom, ion or molecule) in a crystal is the number of constituent particles
which are the immediate neighbours of that particles in the crystals. In ionic
crystals coordination number of an ion in the crystal is the number of
oppositely charged ions surrounding that particular ion.
(a) 12 (b) 8
Q – 1.5 How can you determine the atomic mass of an
unknown metal if you know its density and the dimensions of its unit cell ?
Explain your answer.
Q – 1.6 (a) ‘Stability of a crystal is reflected in the magnitude of its melting point’. Comment .
(b) The melting points of some compounds are given below :
Water = 273 K, Ethyl alcohol = 155.7 K, Diethyl ether = 156.8 K, Methane = 90.5
K.
What can you say about the intermolecular forces between these molecules ?
Ans. (a) Higher the melting point, greater are the forces
holding the constituent particles
together and hence grater is the stability.
(b) The intermolecular forces in water and ethyl alcohol are mainly the
hydrogen bonding. Higher melting point of water then alcohol shows that
hydrogen bounding in ethyl alcohol molecules is not as strong as in water
molecules . Diethyl ether is a polar molecule. The intermolecular forces
present in them are dipole-dipole attraction. Methane is a non-polar molecule.
The only forces present in them are the weak van der wall’s forces (London
dispersion forces).
Q – 1.7 How well you distinguish between the following
pairs of terms
(i) Hexagonal close packing and cubic close packing
(ii) Crystal lattice and unit cell
(iii) Tetrahedral void and octahedral void.
Q – 1.8. How many lattice points are there in one unit cell of each of the following lattices ?
Q – 1.9. Explain : (a) The basis of similarities and differences between metallic and ionic crystals. (b) Ionic crystals are hard and brittle.
Ans. (a) Similarities. Both ionic and
metallic crystals have electronic forces of attraction. In ionic crystal, these
are between the oppositely charged ions. In metals, these are among the valence
electrons and the kernels. Thar is why both have high melting point.
In both cases, the bond is non – directional.
Differences. In ionic crystals, the ions are not free to move. Hence,
they cannot conduct electricity in the solid state. They can do so only in the
molten state or in aqueous solution. In metals, the valence electrons are free
to flow. Hence, they can conduct electricity in the solid state.
Ionic bond is strong due to electrostatic forces of attraction. Metallic bond
may be weak or strong depending upon the number of valence electrons and the
size of the kernels.
(b) Ionic crystals are hard because there are strong electrostatic forces of
attraction among the oppositely charged ions. They are brittle because ionic is
non-directional.
Q – 1.10. Calculate the efficiency of packing in case of
a metal crystal for
(i) simple cubic (ii) body centred
cubic (iii) face centred cubic
(with the assumption that atoms are touching each other )
Q – 1.11. Silver crystallises in fcc lattice. If the edge
length of the cell is 4.077 × 10-8 cm and density is 10.5 g cm-3 ,
calculate the atomic mass of silver.
Q – 1.12. A cubic solid is made up of two elements P and
Q . Atoms Q are present at the corners of the cube and atoms P at the body
centre. What is the formula of the compound ? What are the coordination numbers
of P and Q ?
Q – 1.13. Niobium crystallizes in a body centred cubic
structure. If density is 8.55 g cm-3 , calculate atomic radius
of niobium , given that is atomic mass is 93 u.
Q – 1.14. If the radius of the octahedral void is r and
the radius of the atoms in the close packing is R, derive relationship between
r and R.
Ans. A sphere fitting into the octahedral void is
shown by a small circle ( Fig ). A sphere above and a sphere below this
small sphere have not been shown in the fig .
Obviously, ABC is a right angled tringle. Applying Pythagoras theorem,
Q – 1.15. Copper crystallizes into a fcc lattice with
edge length 3.61 × 10-8 cm. Show that the calculated
density is in agreement with its measured value if 8.92 g cm-3 .
Q – 1.16. The composition of a sample of wustite id Fe0.93O1.00.
What percentage of the iron is present in the form of Fe (III) ?
Q – 1.17. What is a ‘semiconductor’ ? describe the two
main types of semiconductors and contrast their conduction mechanisms.
Ans. Substances whose conductance lies in between that of
metals (conductors) and insulators are called semiconductors.
(i) n-type semiconductors These delocalized electrons increase the conductivity
of silicon or germanium . as the increase in conductivity is due to negatively
charged electrons , the silicon or germanium crystal doped with electron rich
impurities are called.
(ii) An electron from the neighbouring atom can jump to fill up this electron hole but then an electron hole is created at the site from where electron has jumped. As it continues, the electron holes will move in a direction opposite to that of the flow of electrons. Hence, silicon and germanium doped with electron-deficit impurities are called p-type semiconductors.
Q – 1.18 . Non-stoichiometric cuprous oxide, Cu2O
can be prepared in the laboratory. In this oxide, copper to oxygen ratio is
slightly less then 2 : 1 . Can you account for the fact that this substance is
a p-type semiconductor.
Ans. The ratio less then 2 : 1 in Cu2O shows
that some cuprous (Cu+) ions have been replaced by cupric (Cu2+)
ions. To maintain electrical neutrality, every two Cu+ ions
will be replaced by one Cu2+ ion thereby creating a hole. As
conduction will be due to presence of these positive holes, hence it is a
p-type semiconductor.
Q – 1.19. Ferric oxide crystallizes 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.
Q – 1.20. Classify each of the following as being either a p-type or n-type semiconductor :
Ge doped with
in
B doped with Si.
Ans. (i) Ge is group 14 elements and in is Group 13
element. Hence, an electron deficit hole is created and therefore, it is
p-type.
(ii) B is group 13 element and Si is group 14 element, there will be a free
electron, Hence, it is n-type.
Q – 1.21. Gold ( atomic radius = 0.144 nm ) crystallises
in a face centred unit cell. What is the length of the side of the cell ?
Q – 1.22. In terms of band theory, what is the difference
between a conductor and an insulator ? between a conductor and a
semiconductor ?
Ans. (i) The energy gap between the valence band
and conduction band in an insulator is very large whereas in a conductor, the
energy gap is very small or there is overlapping between valence band.
(ii) In a conductor, there is very small energy gap or
there is overlapping between valence band and conduction band but in a
semiconductor, there is always a small energy gap between them.
Q – 1.23. Explain the following terms with suitable
examples :
(i) Schottky defect (ii) Frenkel
defect (iii) Interstitials
Ans. (i) Schottky
defect if in an ionic crystal of the type A+B– ,
equal number of cations and anions are missing from their lattice sites so that
the electrical neutrality is maintained, it is called schottky defect.
(ii) Frenkel defect if an ion is missing from its lattice site (causing a vacancy or a hole there) and it occupies the interstitial site, electrical neutrality as well as the stoichiometry of the compound are maintained. This type of defect is called frenkle defect.
(iii) Interstitials
defect when some extra constituent particle are present in the interstitial
site, the crystal is said to have interstitial defect.
Q – 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 ?
(ii) How many unit cells are there in 1-00 cm3 of
aluminium ?
Q – 1.25. If NaCl is doped with 10-3 mol
% SrCl2, what is the concentration of cation vacancies ?
Q – 1.26. Explain the following with suitable examples :
(i) ferromagnetism (ii)
paramagnetism (iii)
ferrimagnetism (iv) anti –
ferromagnetism
Ans. (i) ferromagnetism which show permanets
magnetism even in the absence of the magnetic field
are called ferromagnetism .
(ii) paramagnetism which are attached by the external magnetic field are called
paramagnetism .
(iii) ferrimagnetism which are expected to possess large magnetism on the basis
of the magnetic
moments of the domains but actually have small net magnetic moment are called
ferrimagnetism.
(iv) Anti – ferromagnetism which are expected to possess paramagnetism or
ferromagnetism on the
basis of magnetic moments of the domains but actually they possess zero net
magnetic moment
are called anti – ferromagnetism.
