+2 UNIT 2 PAGE- 4
IMPURITY DEFECTS : SEMI-CONDUCTORS
Certain defects in crystals arise from the presence of chemical impurities. These are known as impurity defects. One important application of these defects lies in the use of germanium and silicon crystals as semiconductors in transistors. Germanium and silicon belong to Group 14 of the periodic table. These elements , in pure state, have very low electrical conductivity. However, on adding even traces of an element belonging to Group 13 or 15 , the electrical conductivity is greatly enhanced. This may be explained as under.
Suppose , in the first instance , a group 15 element , like arsenic, is added to germanium crystal. As germanium atom is substituted by an atom of arsenic, four electrons in arsenic form covalent bonds with surrounding germanium atoms but the fifth electron remains free. In this way , an extra electron, over and above the number required for forming four covalent bonds, gets introduced into the crystal. This extra electron can serve to conduct electricity, i.e., it behaves like conductor-electron as in metals. Thus , germanium containing traces of arsenic (known as arsenic-doped germanium) begins to exhibit fairly high electrical conductivity. This type of conduction is known as extrinsic conduction. It is much greater than the intrinsic conduction. Since, in this type of conduction, current is carried by excess electrons in the normal way, it is n-type semi-conduction.
Now, suppose a Group 13 element, like indium , having only 3 electrons in the outer shell , is added in small traces, say, pure germanium. The atoms of indium, evidently are not able to complete tetrahedral covalent structures because they have one electron short of the requirement. Hence, some of the sites normally occupied by electrons will be left empty. This give rise to electron-vacancies.
The electron-vacant sites are known as ‘positive holes’ because the net charge at these sites is positive. When electric field is applied, adjacent electrons move into the positive holes and in this way other electron-vacancies ( or positive holes) are formed. The migration of positive holes thus continues and current is carried thereby throughout the crystal.
Thus , doping of germanium with traces of indium increases the electrical conductivity of germanium crystal. Since the current , in the present case is carried by positive holes, this type of conduction is p-type semi-conduction.
Introducing impurity defect in ionic solids
In case of ionic solids, the impurities are introduced by adding impurity of ions. If the impurity ions are in a different valence state from that of host ions, vacancies are created. For example, if molten sodium chloride , containing a little SrCl2 as impurity is allowed to cool , in the crystals of NaCl formed, at some lattice sites Na+ ions are removed to maintain electrical neutrality. One of of these lattice sites is occupied by Sr2+ ion and the other remains vacant. These vacancies result in the higher electrical conductivity of the solid.
Effect of temperature on conductivity of semi-conductors
In semi-conductors electrons are bound rather tightly to local centres at room temperature. When temperature is raised the electrons are freed and are now able to move through the crystal. The higher the temperature, the greater is the number of electrons freed. Due to greater number of free electrons the conductivity increases even though lattice vibrations offer more resistance at higher temperature.
Applications of Semi-conductors
The combination of n and p-type of semi-conductors (known as n-p junctions) , finds interesting applications in the manufacture of transistors. This device can conduct electric current more easily in one particular direction than in the reverse direction and therefore can be used as a rectifier for changing alternating current into direct current.
PROPERTIES OF SOLIDS
There is a close relationship between the properties of solid and its structure and composition. Some of these properties are :
i) Electrical Properties
ii) Magnetic properties.
iii) Dielectric properties.
i) Electrical properties ( Electrical conductivity )
Based on their electrical conductivity, solids can be broadly classified into three types.
a. Metals
b. Semiconductors
c. Insulators
Metals are good conductors and have conductivities in the order 107(m)1. At the other extreme are solids with very low conductivities , ranging between 1010 and 1020 (m)1, these are electrical insulators. The solids with intermediate conductivities generally from 106 and 104 (m)1 are termed semiconductors.
In most of the solids and many insulating solids, conduction is through electron movement under an electric field. However, in some ionic solids, conduction is by ions. The magnitude of the electrical conductivity strongly depends on the number of electrons available to participate in the conduction process. In metals conductivity strongly depends on the number of valence electrons available per atom. The atomic orbitals form molecular orbitals , which are close in energy to each other as to form a band. If conduction band is not completely full or it lies very close to a higher unoccupied band , then electrons can flow easily under an electric field thereby showing conductivity.
Distinction among metals, insulators and semiconductors
In each case, an unshared area represents a conduction band
In case of semiconductors, the gap between valence band and conduction band is small and therefore some of the electrons may jump from valence band to conduction band and some conductivity is observed. Electrical conductivity of semiconductors increases with temperature. This is due to the fact that with increase in temperature, large number of electrons from the valence band can jump to the conduction band. Pure substances that exhibit conducting behaviour like that of silicon and germanium are called intrinsic semiconductors.
For practical purposes, the conductivity of pure silicon and germanium is too low at room temperature. In order to increase the number of electrons (or number of holes which are created after the electrons leave their positions), the pure substances are carefully doped with impurities. Doping means introduction of small amounts of impurities like phosphorus, arsenic or boron into the pure crystal. Conductivity of silicon increases dramatically by doping it with certain other elements. In pure crystalline silicon at room temperature, four valence electrons of each atom are used for forming four normal covalent bonds . When a silicon crystal is doped with a group – 15 elements such as P, As, Sb or Bi, the structure of the crystal lattice is left unchanged, but an occasional atom with five valence electrons occupies a site that would normally be occupied by a silicon atom. The foreign atom(i.e., dopant) uses four of its electron in covalent bonding just as if it were a silicon atom, but because the fifth electron is not needed for bonding it becomes delocalized and is thus free to contribute to electrical conduction. Silicon that has been doped with a group-15 element is called an n-type semiconductor, ‘n’ standing for negative since electrons are responsible for semiconducting behaviour (Fig 45).
Doping a silicon crystal with a group-13 element , such as B, Al, Ga or In produces a silicon crystal structure in which an occasional dopant atom is present with only three valence electrons. The place where the fourth valence electron is missing is called electron vacancy or a hole. Such a hole can move through the crystal like a positive charge giving rise to electrical conductivity. Direction of motion of the holes in an electric field is opposite to that of the electrons. Group-13 doped crystals of silicon are called a p-type semiconductors since holes (positive in charge) appear to be responsible for the semiconducting properties. n-type and p-type semiconductors are shown in Fig 46.
Various combinations of n-type and p-type semiconductors are used to make electronic components, for example a diode is a combination of p- and n-type semiconductors and is used as a rectifier. Transistors which are pnp or npn ‘sandwich’ semiconductor are used to detect or amplify radio or audio signals. The solar cell is essentially an efficient photo diode used for converting radiant (light) energy into electrical energy.
Germanium and silicon are group-14 elements and have therefore , a characteristic valence of four and form four bonds as in diamond. A large variety of solid state materials have been prepared by the combination of elements of group-13 and 15 or 12 and 16 to stimulate average valence of four as in Ge or Si. Typical of group 13-15 compounds are InSb, AlP and GaAs . Galium arsenide (GaAs) semiconductors have very fast responses and have revolutionised the design of semiconductor devices . ZnS, CdS. CdSe and HgTe are examples of group 12-16 compounds. In these compounds, the bonds are not perfectly covalent and the ionicity depends on the electronegativities of the two elements.
The transition metal oxides show marked differences in electrical properties. TiO, CrO2 and ReO3 behave like metals. Rhenium oxide, ReO3 is like metallic copper in its conductivity and appearance. Certain other oxides like VO, VO2, VO3 , TiO3 show metallic or insulating properties depending on temperature.
The following TABLE gives the electrical properties of some oxides of transition metals to illustrate the wide variations found in such materials.
TABLE
------------------------------------------------------------------------------
TiO(M) VO(M) MnO(I) FeO(I)
Ti2O3(M- I) V2O3(M- I) Cr2O3(I) Mn2O3(I) Fe2O3(I)
TiO2(I) VO2(M- I) CrO2 (M) MnO2(I)
V2O5(I )
CoI (I) NiO(I) CuO(I)
-------------------------------------------------------------------------
M = metal ; I = Insulator ; M- I = shows a transition from metal to insulator behaviour.
It is particularly interesting that the monoxide, all of which possess the NaCl structure, show marked differences in electrical properties. Some are metallic while others are semi-conductors or insulators. Thus, the oxide , ReO3 is like metallic copper in its electrical conductivity and appearance.
2. MAGNETIC PROPERTIES
The macroscopic(observable) magnetic properties of materials are due to the magnetic moments associated with individual electron. Each electron in an atom has magnetic moment which originates from two sources :
(i) Orbital motion around the nucleus.
(ii) Spin of electron around its own axis.
A moving electron may be regarded as a small current loop generating a small magnetic moment along its axis of rotation as shown in Fig 47 (a) . The magnetic moment which originates from electron spin is directed along the spin axis. The spin moments are generally shown by up and down direction as shown in Fig (b). Thus , each electron in an atom may be regarded as a small magnet having permanent orbital and spin magnetic moments . The fundamental magnetic moment is the Bohr magneton B which is equal to 9.27 x 1024 A m2. For each electron in an atom , the spin moment is B depending upon two possibilities of the spin. The contribution of orbital magnetic moment is equal to mℓ B where mℓ is the magnetic quantum number of the electron.
Fig 47 (a) An orbital electron (b) A spinning electron
The magnetic properties of solids are also related to the electronic structures. Materials can be classified into different types depending upon their behaviour towards magnetic fields.
(i) Diamagnetic materials : Materials, which are weakly repelled by magnetic fields are called diamagnetic materials. Diamagnetism arises when all the electrons are paired. In other words, diamagnetic substances contain only filled orbitals, e.g., alkali metal halides, TiO2, C6H6 etc.
ii) Paramagnetic materials : Materials which are weakly attracted by magnetic fields are called paramagnetic materials and the property thus exhibited is called paramagnetism. However, such substances lose their magnetism in the absence of a magnetic field. In such materials there are permanent magnetic dipoles due to the presence of atoms, ions or molecules with unpaired electrons.
Example : O2, NO, CuO, Ti2O3, VO2 etc.
iii) Ferromagnetic materials : Materials which are strongly attracted by magnetic fields are called ferromagnetic materials and the property thus exhibited is called ferromagnetism. Such substances show permanent magnetism even after the magnetic field is removed. Only three elements (Fe, Co, Ni) show ferromagnetism at room temperature. The ferromagnetic character of these elements can be explained on the basis of their electronic configurations.
Fe( Z = 26) : 1s2 2s22p63s23p64s23d6
Co( Z = 27) : 1s2 2s22p63s23p64s23d7
Ni( Z = 28) : 1s2 2s22p63s23p64s23d8
From these configurations it is clear that the dipositive ions which exist in the metal lattice of these elements contain unpaired electrons. However, the magnetisation is so large and so persistent that it cannot be explained on the basis of number of unpaired electrons alone. The explanation is in these metals there are domains of magnetisation. Domains are regions of a millions or so ions, all of which co-operatively direct their individual magnetic effects in the same way. In an un-magnetised piece of metal these domains point randomly in all directions in such a way that , the sum of the magnetic effect cancels. When placed in a magnetic field, the domains are turned so that all point in the same direction giving rise to a large magnetic effect. If the metal is now removed from the field, it remains permanently magnetised unless the domain orientation is disorganised, as by heating.
The conditions for the formation of domains are satisfied only in case of these metals. The conditions are that the ions contain unpaired electrons and that the distance between ions be just exactly right in order that the interaction for lining up all the ions to form a domain may be effective. Manganese metal has most of the properties needed to be ferromagnetic, but the ions of the metal are too close. Addition of copper to manganese increases this average spacing, and the resulting alloy is ferromagnetic. Examples of ferromagnetic materials are Fe, Co, Ni, CuO, CrO2 etc. These are very important in technology. For example, CrO2 is used as the magnetic material in the magnetic recording tapes.
The phenomenon of ferromagnetism depends on temperature. Ferromagnetic material, if heated above a particular temperature becomes paramagnetic. This temperature is called curie point. For example, 1023 K , 1373 K and 623 K are curie points for iron, cobalt and nickel respectively.
(iv) Antiferromagnetic materials
Materials which are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but actually have zero net magnetic moment are called antiferromagnetic materials. Antiferromagnetism is due to alignment of magnetic moments in a compensatory way (i.e., equal number of magnetic moments in opposite directions) as in Fig 48 b. e.g., V2O3, Cr2O3, MnO, Mn2O3, FeO and Co3O4
(v) Ferrimagnetic materials
Materials which are expected to possess large magnetism on the basis of unpaired electrons but actually possesses small net magnetic moment are called ferrimagnetic materials. In these materials , the magnetic moments are aligned in parallel and anti-parallell directions in unequal numbers such that there is net magnetic moment as in Fig 48 c. e.g., Fe3O4, MgFe2O4, ZnFe2O4 and CuFe2O4. The magnetic properties of some typical transition metal oxides are given in the TABLE.
Magnetic properties of typical transition metal oxides
------------------------------------------------------------------------
TiO(p) VO(p) MnO(af) FeO (af)
Ti2O3(p) V2O3(af) Cr2O3(af) Mn2O3(af) Fe2O3(af)
TiO2(d) VO2(p) CrO2(f) MnO2(af) CoO (af)
V2O5 (p) Fe3O4 (fe)
CoI(af) NiO(af) CuO(p)
------------------------------------------------------------------------
p = paramagnetic ; af = anti-ferromagnetic.
fe = ferrimagnetic ; f = ferromagnetic
d = diamagnetic .
3. DIELECTRIC PROPERTIES
The electrons in insulators are closely bound to individual atoms or ions and therefore they do not generally migrate under applied electric field. However, due to shift in charges, dipoles are created which result into polarisation. The alignment of dipoles created by shift in charges may occur in the following ways :
i) The dipoles may align themselves in an ordered manner such that there is some resultant dipole moment in the crystals.
ii) They may align in such a way that the dipole moments cancel each other and resultant dipole moment is zero .
iii) There may be no dipoles in the crystals but only ions are present.
Depending upon the alignment of the dipoles the crystals have very interesting properties such as :
1. Piezoelectricity(pressure electricity) : Crystals in which dipoles align themselves in an ordered manner under the influence of an applied electric field exhibit piezoelectricity. When such a crystal is deformed by mechanical stress, electricity is produced due to the asymmetric displacement of ions or conversely if an electric field is applied to the crystal, there will be atomic displacement causing mechanical strain.
Piezoelectric crystals are utilised in transducers, devices that convert electrical energy into mechanical strains, or vice versa. Familiar applications that employ piezoelectrics include phonograph pickups, microphones, ultrasonic generators, strain gauges and sonar detectors. Piezoelectric materials include titanates of barium and lead, lead zirconate (PbZrO3), ammonium dihydrogen phosphate (NH4H2PO4) and quartz. Some of piezoelectric crystals, when heated produce small electric potential or pyroelectricity.
2. Ferroelectricity : This is a category of piezoelectric crystals in which the dipoles are permeantly aligned up in the absence of electric field and the direction of polarisation can be changed by applying an electric field. This phenomenon is known as ferroelectricity. The common examples are barium titinate BaTiO3, sodium potassium tartrate (Rochelle salt), sodium nitrate, and potassium dihydrogen phosphate KH2PO4 are typical ferroelectric substances. Such substances are used as mechanical-electrical transducers in various devices like gas lighters, pick up for record players. Devices (mikes and speakers) used to convert sound energy into electric energy and vice versa can also be made out of such crystals. The occurrence of ferroelectricity undoubtedly depends upon the structure and only certain geometric arrangement in crystal can exhibit this phenomenon.
3. Pyroelectricity : Some of the polar crystals when heated produce small electrical current. This phenomenon is called pyroelectricity due to symmetric variation in interatomic distances.
4. Anti-ferroelectricity : In some crystals there is no net dipole moment in spite of the presence of small dipoles and therefore they do not exhibit ferroelectric character. This is because of the fact that the dipoles in the alternate polyhedra point up and down giving net zero dipole moment. Such crystals are called anti-ferroelectric. The common example is lead zirconate PbZrO3.
SUPER CONDUCTIVITY
Electrical resistance of metals and substances showing metallic conductance (TiO, VO etc.) decreases with decrease in temperature and in certain cases becomes almost zero near the absolute zero. Materials in this state are said to possess super conductivity.
Electric current in a superconductor is without any heat loss, unlike current in a typical conductor . Once a current has been started in a super conducting circuit , it continues to flow indefinitely. Another strange property of a super conductor is its perfect diamagnetism. This means that a superconductor completely repels magnetic lines of force. In fact a small magnet can be made to suspend in mid air over a super conducting material.
The phenomenon of superconductivity was first discovered by Kammerlingh Onnes in 1913 when he found that mercury becomes superconductor at 4 K. Most metals have transition temperature between 2 K 5 K. The highest temperature at which superconductivity was known till recently was 23 K for alloys of niobium (e.g., Nb3Ge). Some organic compounds also become super conductors at temperatures below 5 K. Research is going on to find materials which behave as superconductors at higher temperatures. Since 1987 many complex metal oxides have been found to possess superconductivity at some what higher temperatures. Some of them are listed below.
Material Critical temperature
YBa2Cu3O7 90 K
Bi2Ca2Sr2Cu3O10 105 K
Ti2Ca2Ba2Cu3O10 125 K
The maximum temperature at which a superconducting material (or super conductor) exhibits superconductivity is called critical temperature (Tc)
Super conducting materials have great technological potential. These can be used in building powerful electromagnets, in power transmission and for levigation transportation (trains which move in air without touching rails). Larger super conducting magnets are now used in medical magnetic resonance imaging.
AMORPHOUS SOLIDS
All the solids are not crystalline. There are solids which lack the repeating ordered arrangements of atoms or ions that characterises crystalline solids. Instead they have a random, disordered arrangements of atoms. The ordinary 'glass' which is a metal silicate is an amorphous solid. Many plastics form amorphous solids. SiO2 which crystallises as quartz has SiO4 tetrahedra connected to one another such that oxygen atom of each SiO4 terahedra is shared with another Si atom. If SiO2 is melted and the melt is cooled , it forms a glass which is amorphous (Fig 50) . In this state the SiO4 tetrahedra are randomly joined. In fact any given material can be made amorphous or glassy by rapidly cooling the melt or freezing the vapour. Thus many complex materials including metallic alloys have been made in the glassy form.
Fig 50 Two dimensional representation of crystalline SiO2 and amorphous SiO2
(a) In crystalline SiO2, the SiO4 tetrahedra have a regular arrangement.
(b) In amorphous SiO2 (silica glass) , the SiO4 tetrahedra have an irregular , random arrangement.
Properties
i) Although amorphous solids do not possess the long range order of crystals, they do have some local or short range order just as liquids do.
ii) Amorphous solids do not have a sharp melting point. That is why glasses soften over a temperature range and can be blown into various shapes.
iii) When amorphous solids are heated (or annealed) they become crystalline at some temperature, but left to themselves under ordinary conditions they remain amorphous. This is the reason why some objects from ancient civilisations may have become milky in appearance because of some crystallisation.
Applications
Amorphous solids find many applications because of their unique properties, the obvious one being of the inorganic glasses, in construction, houseware, laboratory-ware, etc. Amorphous silica is likely to be the best material for converting sunlight into electricity (photovoltaic)
Distinction between Crystalline and Amorphous solids.
Crystalline substances Amorphous solids
1.The internal
arrangement of
particles is regular. 1.The internal arrangement of
particles is irregular.
2.There is regularity in the
external form when
crystals are formed. 2.There is no regularity in the
external form, when
amorphous solids are
formed.
3.There is a sharp melting
point. 3.There is no sharp melting
point.
4.There is a characteristic
heat of fusion. 4.There is no characteristic
heat of fusion.
5. Crystalline substances
are regarded as true
solids formed by the
process of
crystallisation. 5. Amorphous substances
are regarded as super
cooled liquids or as
intermediate between
solids and liquids.
Problems
01. Classify each of the following solids as ionic, metallic , molecular , network(covalent) or amorphous.
a. Tetraphosphorus decoxide (P4O10)
b. Graphite
c. Brass
d. Ammonium phosphate (NH4)3PO4
e. SiC f. Rb g. I2 h. LiBr i. P4
j. Si k. plastic
02. (a) What is meant by the term ‘coordination number’ ?
(b) What is the coordination of atoms :
(i) in a cubic close packed structutre.
(ii) in a body centred cubic structure
03. How can you determine the atomic of an unknown metal if you its density and dimension of its unit cell ? Explain your answer.
04. ‘Stability of acrystal is reflected in the magnitude of its elting point’. Comment. Collect melting point of water, ethyl alcohol, diethyl ether and methane from the data book. What can you say about the intermolecular forces between these molecules.
05. How will you distinguish between the following pairs of terms :
(a) Hexagonal close packing and cubic close packing.
(b) Crystal lattice and unit cell
(c) Tetrahedral void and octahedral void
06. How many lattice points are there in one unit cell of each of the following lattice ?
(a) face centred cubic
(b) face centred tetragonal
(c) body centred cubic
07. Explain :
(a) The basic similarities and differences between metallic and ionic crystals.
(b) Unit cell is not simply a cube of four sodium ions and four chloride ions
(c) Can a cube consisting of Na+ and Cl ions at alternate corners serve as a satisfactory unit cell for the sodium chloride lattice ?
08. Calculate the efficiency of packing in the case of a metal crystal for :
(a) simple cubic (b) body centred cubic
(c) face centred cubic
(with the assumption that atoms are touching each other)
09. Silver crystallizes in FCC lattice. If edge length of the cell is 4.077 x 108 cm and density 10.5 g cm3 , calculate the atomic mass of silver.
10. A cubic solid is made of two elements P and Q atoms . Q atoms are at the corners of the cube and P atom at the body centre. What is the formula of the compound ? What are co-ordination number of P and Q ?
11. Niobium crystallizes in bcc structure. If density is 8.55 g cm3 , calculate atomic radius of niobium .
12. If the radius of the octahedral void is r and radius of the atoms in close packing is R, derive the relation between r and R.
13. Copper crystallizes into a fcc lattice with edge length 3.61 x 108 cm. Show that the calculated density is in agreement with its measured value of 8.92 g cm3 .
14. Formula mass of sodium chloride is 55.45 g mol1 and density of its pure form is 2.167 g cm3. The average distance between adjacent sodium and chloride ions in the crystal is 2.814 x 108 cm. Calculate Avogadro’s constant.
15. Analysis shows that nickel oxide has the formula Ni0.98O1.00. What fractions of nickelexist as Ni2+ and Ni3+ ions ?
16. The compound CuCl has the ZnS(cubic) structure. Its density is 3.4 g cm-3. What is the length of the edge of the unit cell ?
17. If the radius of bromide ion is 0.182 nm, how large a cation can fit in each of the tetrahedral hole ?
18. The first order diffraction of X-rays from a certain set of crystal planes occur at an angle of 11.8 from the planes. If the planes are 0.281 nm apart, what is the wave length of X-rays ?
19. What is a semiconductor ? Describe the two main types of semiconductors and contrast their conduction mechanisms.
20. Non-stiochiometric 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 is a p-type semiconductor ?
21. Ferric oxide crystallizes in a hexagonal packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of ferric oxide.
22. Classify each of the following as being either a p-type or an n-type semiconductor :
(i) Ge doped with In
(ii) B doped with Si
23. Thallium chloride crystallizes in either a simple cubic lattice or a face centred lattice of Cl ions with Tl+ in the holes. If the density of the solid is 9.0 g cm3 and edge of the unit cell is 3.85 x 108 cm , what is the unit cell geometry ?
24. Gold (atomic radius = 0.144 nm) crystallizes in a face centred unit cell. What is the length of the side of the unit cell ?
25. In terms of band theory, what is the difference between (i) a conductor and an insulator (ii) between a conductor and a semiconductor ?
26. Explain the following terms with suitable examples :
(i) Schottky defect (ii) Frenkel defect
(iii) Interstitials (iv) F-centres
27. MgO hasd the structuire of NaCl and TℓCl has the structure of CsCl. What are co-ordination numbers of the ions in MgO and TℓCl ?
28. Explain the ionic radius ratio and its significance in the case of ionic crystals. Calculate theratio for alkali metal bromides on the basis of the data given below and predict the form of the crystal structure in each case.
Ion Ionic radius(pm)
Li+ 74
Na+ 102
K+ 138
Rb+ 14
Cs+ 170
Br 195
29. Aluminium crystallizes in a cubic close-packed structure. Its metallic radius is 125 pm.
(a) What is the length of the side of the unit cell ?
(b) How many unit cells are there in 1.00 cm3 of aluminium ?
30. If NaCl is doped with 10-3 mole % SrCl2, what is the concentration of cation vacancies ?
31. KF has NaCl structure. What is the distance between K+ and F in KF, if the density is 2.48 g cm3 ?
32. Calculate the value of Avogadro constant from the following data :
Density of NaCl = 2.165 g cm3 .
Distance between Na+ and Cl in NaCl = 281 pm.
33. Explain the following with suitable examples :
(a) ferromagnetism (b) paramagnetism
(c) ferrimagnetism (d) piezoelectric effect
(e) antifluorite structure
(f) 12-16 and 13-15 compounds
34. Define the term ‘amorphous’. Give a few examples of amorphous solids.
35. What makes glass different from a solid such as quartz ? Under what conditions could quartz be converted into glass ?