Unit 1 the Solid State

UNIT 1: THE SOLID STATE UNIT 1: THE SOLID SOLID STATE STATE

SYNOPSIS • General characteriscs of solid state; • Disnguish between amorphous and crystalline solids; • Classify crystalline solids on the basis of the nature of binding forces; • Dene crystal lace and unit cell; • Explain close packing of parcles; • Dierent types of voids and close packed structures; • Calculate the packing eciency of dierent types of cubic unit cells; • Correlate the density of a substance with its unit cell properes; • Imperfecons in solids and their eect on properes; • Electrical and magnec properes of solids and their structure.

General Characteristics of Solid State The following are the characterisc properes of the solid state: (i) They have denite mass, volume and shape. (ii) Intermolecular distances are short. (iii) Intermolecular forces are strong. (iv) Their constuent parcles (atoms, molecules or ions) have xed posions and can only oscillate about their mean posions. (v) They are incompressible and rigid.

Amorphous and Crystalline Solids Solids can be classied as crystalline or amorphous. A crystalline solid has orderly arrangement of constuent parcles with denite geometrical shape. The regular paern of arrangement of parcles repeats itself periodically over the enre crystal. Sodium chloride and quartz are the examples of crystalline solids. An amorphous solid (Greek amorphos = no form) consists of parcles of irregular shape. The arrangement arrangement of constuent parcles (atoms, molecules or ions) in such a solid has only short range order. In such an arrangement, a regular and periodically repeang paern is observed over short distances only. Such porons are scaered and in between the arrangement is disordered. The structures of quartz (crystalline) and quartz glass (amorphous) are shown in Fig. 1.1 (a) and (b) respecvely. respecvely. The structure of amorphous solids is similar to that of liquids. Glass, rubber and plascs are examples of amorphous solids.

Like liquids, amorphous solids can ow, though very slowly, hence called pseudo solids or super cooled liquids. Glass panes xed to windows or doors of old buildings are invariably found to be slightly thicker at the boom than at the top as the glass ows down very slowly and makes the boom poron slightly thicker. Crystalline solids are anisotropic, that is, their physical properes like electrical resistance or refracve index are dierent when measured along dierent direcons. This is due to dierent arrangement of parcles in dierent direcons. This is illustrated in Fig. 1.2. Whereas amorphous solids are isotropic, because there is no long range order in them and arrangement is irregular along all the direcons. Therefore, value of any physical property would be same along any direcon. These dierences are summarised in Table 1.1.

1

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UNIT 1: THE SOLID STATE

Amorphous solids like glass, rubber and plascs nd many uses in our daily lives. Amorphous silicon is a best photovoltaic material used for conversion of sunlight into electricity.

Classification of Crystalline Solids Based on intermolecular forces, crystalline solids are classied into four types. 1. Molecular Solids: These are crystalline solids having polar or non-polar molecules at the lattice points, which are held by either van-der van- der Waal’s forces or dipole-dipole dipole-dipole interactions or hydrogen bonds. Eg: Ice, Solid NH3, Solid CO2 (dry ice), benzoic acid, glucose etc. (i) Non polar Molecular Solids have have the atoms or molecules are are held by weak dispersion dispersion forces or London forces. Eg I 2 (ii) Polar Molecular Solids: Solids: The molecules of substances substances like HCl, HCl, SO2, etc. are formed by polar covalent bonds. The molecules in such solids are held together by relavely stronger dipoledipole-dipole interacons. Solid SO2 and solid NH3 are some examples of such solids. (iii) Hydrogen Bonded Molecular Molecular Solids: The molecules of such solids contain polar polar covalent bonds between H and F, O or N atoms. Strong hydrogen bonding binds molecules of such solids like H2O (ice). 2. Ionic Solids: These contain caons and anions at the lace points. Eg: NaCl, ZnS, KNO 3, MgO, CsCl etc. 3. Metallic Solids: These are crystalline solids having posively charged metal ions at the lace points immersed in a sea of electrons. Eg: Metals like Na, Cu, Zn, Fe, Al and alloys etc. 4. Covalent or Network Solids: These are crystalline solids having covalently bonded nonnon metallic atoms at the lace points. They are also called giant molecules. Eg: Diamond, Graphite, Silicon carbide (SiC), Quartz (SiO 2), BN, Iodine etc.










Crystal Lattices and Unit Cells A regular three dimensional arrangement of points in space is called a crystal lace. There are 14 possible three dimensional laces. These are called Bravais Laces. The following are the characteriscs of a crystal lace: (a) Each point point in a lace is called lace point or lace site. (b) Each point in a crystal lace represents represents one constuent constuent parcle which which may be an atom, a molecule (group of atoms) or an ion. i on. (c) Lace points are joined by straight lines to bring out the the geometry of the lace. lace. Unit cell is the smallest poron of a crystal lace which, when repeated in dierent direcons gives crystal lace. A unit cell is characterised by: (i) Its dimensions along the three edges, a, b and c. These edges may or may not be mutually perpendicular. (ii) Angles between the edges, α (between b and c), β (between a and c) and γ (between a and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ. These parameters of a typical unit cell are shown in Fig. 1.6. Fig 1.6: Illustraon of parameters o a unit unit cell cell

Primitive and Centred Unit Cells Unit cells are divided into two categories, primive and centred unit cells. (a) Primive Unit Cells: When constuent parcles are present only on the corner posions of a unit cell, it is called as primive unit cell. (b) Centred Unit Cells: When a unit cell contains one or more constuent parcles present at posions other than corners in addion to those at corners, it is called a centred unit cell. Centred unit cells are of three types: (i) BCC: Such a unit cell contains one constuent parcle (atom, molecule or ion) at its bodybody-centre besides the ones that are at its corners. (ii) FCC: Such a unit cell contains one constuent parcle present at the centre of each face, besides the ones that are at its corners. (iii) EndEnd-Centred Unit Cells: In such a unit cell, one constuent parcle is present at the centre of any two opposite faces besides the ones present at its corners.










Number of Atoms in a Unit Cell: 1. Primitive Cubic Unit Cell A simple cube has 8 lace points. A parcle present at each corner of the cube is shared by eight other unit cells. Hence each unit cell gets a share of

∴ Total number of atoms per unit cell =  x 8 = 1 atom.

 th  of that parcle.

2. Body- Centred Cubic Unit Cell

A body centred cube has 9 lattice points. A particle present at each corner of the cube is 1 th shared by eight other unit cells. Hence each unit cell gets a share of /8 of that particle. A particle present at the centre of the cube is not shared by any other unit cell. Hence, its contribution for the unit cell is 1. Thus in a bodybody-centered cubic (bcc) unit cell:

 

(i) 8 corners x per corner atom =

 x 8 

(ii) 1 body centre atom = 1 × 1 Total number of atoms per unit cell

∴

= 1 atatom = 1 atom = 2 atoms

3. Face-Centred Cubic Unit Cell

A face centred cube has 14 lattice points. A particle present at each corner of the cube is th shared by eight other unit cells. Hence each unit cell gets a share of 1/8 of that particle. A particle present at the centre of each face of the cube is shared by two unit cells. Hence, each unit cell gets a share of ½ of that particle. Thus, in a faceface-centred cubic (fcc) unit cell: (i) 8 corners x






















UNIT 1: THE SOLID STATE Close Packed Structures: In solids, the constuent parcles are closeclose -packed, leaving the minimum vacant space. Consider the constuent parcles as idencal hard spheres and build up the three dimensional structure in three steps. (a) Close Packing in One Dimension: In one dimension close packing arrangement, the spheres are arranged in a row touching with each other. In this arrangement, each sphere is in contact with two of its neighbours. The number of nearest neighbours of a parcle is called its coordinaon number. Thus, in one dimensional close packing arrangement, the coordinaon number is 2. (b) Close Packing in Two Dimensions: There are two dierent ways. (i) The second row may be placed in contact with the rst one such that the spheres of the two rows are aligned horizontally as well as vercally. Say, rst row as ‘A’ type row, the second row being exactly the same as the rst one, is also of ‘A’ type. This is called AA type arrangement. In this arrangement, C.No. of each sphere is 4. The centres of these 4 immediate neighbouring spheres when joined, a square is formed. Hence this packing is called square close packing in two dimensions.

(ii) The spheres of the second row are placed in the depressions of the rst row. If the arrangement of spheres in the rst row is called ‘A’ type and the second row is dierent and may be called ‘B’ type. When the third row of spheres is placed in the depressions of second row, its spheres are aligned with those of the rst layer. Hence this layer is also of ‘A’ type. The spheres of fourth row will be aligned with those of the second row (‘B’ type). Hence this arrangement is of ABAB type. In this arrangement, free space is less and packing eciency is more than the square close packing. Each sphere has a C.No. of 6. The centres of these six spheres are at the corners of a regular hexagon (Fig. 1.14b), hence this packing is called two dimensional hexagonal closeclose-packing. In this arrangement there are some triangular voids (empty spaces). The triangular voids are of two dierent types. In one row, the apex of the triangles is poinng upwards and in the next layer downwards. (c) Close Packing in Three Dimensions: All real structures are three dimensional structures. As far as three dimensional close packing is concerned, they can be obtained by stacking two dimensional layers one above the other. (i) Three dimensional close packing from two dimensional square closeclose-packed layers: The second layer is placed over the rst layer such that the spheres of the upper layer are exactly above those of the rst layer. In this arrangement spheres of both the layers are perfectly aligned horizontally horizontally as well as vercally as shown in Fig. 1.15. Similarly, we may place more layers one above the other. If the arrangement of spheres in the rst layer is called ‘A’ type, all the layers have the same arrangement. Thus this lace has AAA.... type paern. The lace thus generated is the simple cubic lace, and its unit cell is the primive cubic unit cell (See Fig. 1.9). (ii) Three dimensional close packing from two dimensional hexagonal close packed layers: Three











UNIT 1: THE SOLID STATE (a) Placing second layer over the rst layer Consider a two dimensional hexagonal close packed layer ‘A’ and place a similar layer above it such that the spheres of the second layer are placed in the depressions of the rst layer. Since the spheres of the two layers are aligned dierently, let us call the second layer as B. It can be observed from Fig. 1.16 that not all the triangular voids of the rst layer are covered by the spheres of the second layer. This gives rise to dierent arrangements. Wherever a sphere of the second layer is above the void of the rst layer (or vice versa) a tetrahedral void is formed. These voids are called tetrahedral voids because a tetrahedron is formed when the centres of these four spheres are joined. They have been marked as ‘T’ in Fig. 1.16. One such void has been shown separately in Fig. 1.17.

At other places, the triangular voids in the second layer are above the triangular voids in the rst layer, and the triangular shapes of these do not overlap. One of them has the apex of the triangle poinng upwards and the other downwards. These voids have been marked as ‘O’ in Fig. 1.16. Such voids are surrounded by six spheres and are called octahedral voids. One such void has been shown separately in Fig. 1.17. The numbers of these two types of voids depend upon the number of close packed spheres. Let the number of close packed spheres be N, then: The number of octahedral voids generated = N The number of tetrahedral voids generated = 2N (b) Placing third layer over the second layer When third layer is placed over the second, there are two possibilies. (i) Covering Tetrahedral Voids: Tetrahedral voids of the second layer may be covered by the spheres of the third layer. In this case, the spheres of the third layer are exactly aligned with those of the rst layer. Thus, the paern of spheres is repeated in alternate layers. This paern is oen wrien as ABAB ....... paern. This structure is called hexagonal close packed (hcp) structure (Fig. 1.18). This sort of arrangement of atoms is found in many metals like magnesium and zinc.









UNIT 1: THE SOLID STATE (ii) Covering Octahedral Voids: The third layer may be placed above the second layer in such a way that its spheres cover the octahedral voids. In this, the spheres of the third layer are not aligned with those of either the rst or the second layer. This arrangement is called “C’ type. Only when fourth layer is placed, its spheres are aligned with those of the rst layer as shown in Figs. 1.18 and 1.19. This paern of layers is oen wrien as ABCABC ........... This structure is called cubic close packed (ccp) or faceface -centred cubic (fcc) structure. Metals such as copper and silver crystallise in this structure. Both these types of close packing are highly ecient and 74% space in the crystal is lled. In either of them, each sphere is in contact contact with twelve spheres. Thus, the C.No is 12.

Formula of a Compound and Number of Voids Filled: In ccp or hcp structure, two types of voids are generated. The number of octahedral voids present in a lace is equal to the number of close packed parcles; the number of tetrahedral voids is twice this number. In ionic solids, the bigger ions (usually anions) form the close packed structure and the smaller ions (usually caons) occupy the voids. If the anion is small enough then tetrahedral voids are occupied, if bigger, then octahedral voids. Not all octahedral or tetrahedral voids are occupied. In a given compound, the fracon of octahedral octahedral or tetrahedral tetrahedral voids that are occupied depends upon the chemical formula of the compound, as can be seen from the following examples. Example 1.1: A compound is formed by two elements X and Y. Atoms of the element Y (as anions) make ccp and those of the element X (as caons) occupy all the octahedral voids. What is the formula of the compound? Soluon: The ccp lace is formed by the element Y. The number of octahedral voids generated would be equal to the number of atoms of Y present in it. Since all the octahedral voids are occupied by the atoms of X, their number would also be equal to that of the element Y. Thus, the atoms of elements X and Y are present in equal numbers or 1:1 rao. Therefore, the formula of the compound is XY. 2 rd Example 1.2: Atoms of element B form hcp lace and those of the element A occupy /3 of tetrahedral voids. What is the formula of the compound formed by the elements A and B? Soluon: The number of tetrahedral voids formed is equal to twice the number of atoms of 2 rd element B and only /3 of these are occupied by the atoms of element A. Hence the rao of the number of atoms of A and B is 2 × (2/3):1 or 4:3 and the formula of the compound is A4B3.

Locating Tetrahedral and Octahedral Voids: We know that close packed structures have both tetrahedral









UNIT 1: THE SOLID STATE Thus, there is one tetrahedral void in each small cube and eight tetrahedral voids in total. Each of the eight small cubes has one void in one unit cell of ccp structure. The ccp structure has 4 atoms per unit cell. Thus, the number of tetrahedral voids is twice the number of atoms. (b) Locang Octahedral Voids: Consider a unit cell of ccp or fcc lace *Fig. 2(a)+. The body centre of the cube, C is not occupied but it is surrounded by six atoms on face centres. If these face centres are joined, an octahedron is generated. Thus, this unit cell has one octahedral void at the body centre of the cube. Besides the body centre, there is one octahedral void at the centre of each of the 12 edges. *Fig. 2(b)+. It is surrounded by six atoms, three belonging to the same unit cell (2 on the corners and 1 on face centre) and three belonging to two adjacent unit cells. Since each edge of the cube is shared between four adjacent unit cells, so is the octahedral void located on it. 1 th Only /4 of each void belongs to a parcular unit cell.

Thus in cubic close packed structure: Octahedral void at the bodybody-centre of the cube = 1 12 octahedral voids located at each edge and shared between four unit cells

∴ Total number of octahedral voids = 4

= 12 x  = 3

We know that in ccp structure, each unit cell has 4 atoms. Thus, the number of octahedral voids is equal to this number.

Packing Efficiency: Packing eciency is the percentage of total space lled by the parcles.

Packing Efficiency in hcp and ccp Structures Both types of close packing (hcp and ccp) are equally ecient. Let us calculate the eciency of packing in ccp structure. In Fig. 1.20 let the unit cell edge length be ‘a’ and face diagonal AC = b. In Δ ABC 2

2

2

2

AC = b = BC  AB 2 2 2 = a a = 2a or  b=

√ 2

If r is the radius of the sphere, we nd









UNIT 1: THE SOLID STATE Efficiency of Packing in Body-Centre Body-Centred d Cubic Structures: From Fig. 1.21, it is clear that the atom at the centre will be in touch with the other two atoms diagonally arranged. arranged. In Δ EFD, 2 2 2 2 b = a  a = 2a b= a Now in Δ AFD, 2 2 2 2 2 2 c = a  b = a  2a = 3a c = a The length of the body diagonal c is equal to 4r, where r is the radius of the sphere (atom), as all the three spheres along the diagonal touch each other.  Therefore, a = 4r OR

√ 2

√ 3

√ 3

Also we can write,

r=

a=

√   

√ 

In this type of structure, total number of atoms is 2 and their volume is Volume of the cube, Therefore,



a = √  r

2 x  πr

by two spheres in the unit cell x 100 % Packing Packing eficiency eficiency = Volume occupied Total volumeof the unit cell            =   = 68 % =    

√  √  Packing Efficiency in Simple Cubic Lattice:

In a simple cubic lace the atoms are located only on the corners of the cube. The parcles touch each other along the edge (Fig. 1.22). Thus, the edge length or side of the cube ‘a’, and the radius of each parcle, r is related as a = 2r. 3 The volume of the the cubic unit unit cell = a 3 3 = (2r) = 8r Since a simple cubic unit cell contains only 1 atom.

   πr      Pack Packin ingg efic eficieienc ncyy =  %      4 r3 100 = 6  100 = 52.4 % = 38 r3 x 10

The volume of the occupied space = Fig 1.22: Simple cubic unit cell. The spheres are in contact with each other along the edge of the cube

∴

π

Thus, we may conclude that ccp and hcp structures have maximum packing eciency.

Calculations Involving Unit Cell Dimensions: From the unit cell dimensions, we can calculate the volume of the unit cell. Knowing the density of the metal, we can calculate the mass of the atoms in the unit cell. The determinaon of the mass of a single atom gives an accurate method of determinaon of









UNIT 1: THE SOLID STATE Example 1.3: An element has a bodybody -centred cubic (bcc) structure with a cell edge of 288 3 pm. The density of the element is 7.2 g/cm . How many atoms are present in 208 g of the element? 3 Soluon: Volume Volume of the unit cell = (288 pm) -12 -10 3 -23 3 = (288×10 m) = (288×10 cm) = 2.39×10 cm

   = 28.888    7.   = 28.8 .  12.08 x 10 10 unit cells Number of unit cells in this volume = .  −  /   = 12.08

Volume Volume of 208 g of the element

=

Since each bcc cubic unit cell contains 2 atoms, therefore, the total number of atoms in 208 g 23 23 = 2 (atoms/unit cell) × 12.08 × 10 unit cells = 24.16×10 atoms. Example 1.4: X-ray diracon studies show that copper crystallises in fcc unit cell with cell -8 edge of 3.608 × 10 cm. In a separate experiment, experiment, copper is determined to have a density of 3 8.92 g/cm , calculate the atomic mass of copper. Soluon: In case of fcc lace, number of atoms per unit cell, z = 4 atoms. Therefore,



M = ..   − (.  − ) = .    .   = 63.1 g/mol  

Atomic mass of copper = 63.1u Example 1.5: Silver forms ccp lace and XX-ray studies of its crystals show that the edge length of its unit cell is 408.6 pm. Calculate the density of silver (Atomic mass = 107.9 u). Soluon: Since the lace is ccp, the number of silver atoms per unit cell = z = 4 –1 -3 –1 Molar mass of silver = 107.9 g mol = 107.9×10 kg mol –12 Edge length of unit cell = a = 408.6 pm = 408.6×10 m Density,

=

.   (7.   −) = . (.  − ) ) (.   −) = 10.5 10.5  10  ; = 10.5 g cm –3

Imperfections in Solids:

The defects are basically irregularies in the arrangement of constuent parcles. The defects are of two types, namely, point defects and line defects. Point defects are the irregularies or deviaons from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularies or deviaons from ideal arrangement in enre rows of lace points. These irregularies are called crystal defects.

Types of Point Defects: Point defects can be classied into three types: (a) Stoichiometric defects (b) Impurity defects and (c) NonNon -stoichiometric defects. (a) Stoichiometric Defects: These are the point defects that do not disturb the stoichiometry of the solid. They are also called intrinsic or thermodynamic defects. These are of two types, vacancy defects and intersal defects. (i) Vacancy Defect: When some of the lace sites are vacant, the crystal is said to have vacancy defect (Fig. 1.23). This results in decrease in density of the substance. This defect can also develop when a substance is heated.











UNIT 1: THE SOLID STATE (iii) Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually caon) is dislocated from its normal site to an intersal site (Fig. 1.25). It creates a vacancy defect at its original site and an intersal defect at its new locaon. Frenkel defect is also called dislocaon defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large dierence in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to small size 2  of Zn and Ag ions. (iv) Schoky Defect: It is a vacancy defect in ionic solids where caons and anions are of almost similar sizes. The number of missing caons and anions are equal (Fig. 1.26) and hence solid is electrical neutral. In this defect the density of the substance decreases. Number of such defects in ionic solids is quite signicant. For example, 6 3 in NaCl there are approximately 10 Schoky pairs per cm at 3 22 room temperature. In 1 cm there are about 10 ions. Thus, 16 there is one Schoky defect per 10 ions. Eg: NaCl, KCl, CsCl and AgBr. However AgBr shows both, Frenkel as well as Schoky defects. (b) Impurity Defects: If molten NaCl containing a lile amount of SrCl 2  is crystallised, some of the sites of Na ions are 2 2 occupied by Sr (Fig.1.27). Each Sr replaces two  Na ions. It occupies the site of one ion and the other site remains vacant. The caonic vacancies 2 thus produced are equal in number to that of Sr ions. Another similar example is the solid soluon of CdCl 2 and AgCl. (c) NonNon-Stoichiometric Defects: A large number of nonnon-stoichiometric inorganic solids contain the constuent elements in nonnon-stoichiometric rao due to defects in their crystal structures. These defects are of two types: (i) metal excess defect and (ii) metal deciency defect. (i) Metal Excess Defect: Metal excess defect due to anionic vacancies: NaCl and KCl show this type of defect.









UNIT 1: THE SOLID STATE (ii) Metal Deciency Defect There are many solids which are dicult to prepare in the stoichiometric composion and contain less amount of the metal as compared to the stoichiometric proporon. A typical example of this type is FeO which is mostly found with a composion of Fe 0.95O. It may 2 actually range from Fe0.93O to Fe0.96O. In crystals of FeO some Fe caons are missing and 3 the loss of posive charge is made up by the presence of required number of Fe ions.

Electrical Properties: Solids can be classied into three types on the basis of their conducvies. 4 7 –1 –1 (i) Conductors: The solids with conducvies conducvies ranging between 10 to 10 ohm m are called 7 –1 –1 conductors. Metals having conducvies in the order of 10 ohm m are good conductors. –20 –10 –1 –1 (ii) Insulators: The solids with very low conducvies ranging between 10 to 10 Ω m . (iii) Semiconductors: These are the solids with conducvies in the intermediate range from –6 4 –1 –1 10 to 10 ohm m .

Conduction of Electricity in Metals A conductor may conduct electricity by movement of electrons or ions. Metals conduct electricity by movement of electrons and electrolytes by the movement of ions. Metals conduct electricity in solid as well as molten state. The conducvity 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 parally lled or it overlaps with a higher energy unoccupied conducon band, then electrons can ow easily under an applied electric eld and the metal shows conducvity (Fig. 1.29 a). If the gap between lled valence band and the next higher unoccupied band (conducon band) is large, electrons cannot jump to it and such a substance has very small conducvity and it behaves as an insulator (Fig. 1.29 b).










(b) Electron – decit impuries OR pp-type semiconductors: semiconductors: Silicon or germanium when doped with a group 13 element like B, Al or Ga pp -type semiconductor semiconductor is obtained. Boron has only 3 valence electrons. Hence boron can form bonds with three silicon atoms. The fourth electron in silicon is le unbounded and hence each boron atom produces a posive hole and the valency remains unsased. Electrons from the adjacent bonds jump to ll the posive hole. As a result the posive hole shis to the adjacent site from where the electron has jumped. The process connues. Hence, pp-type semiconductor conducts electricity by the movement of posive holes. Applicaons of n-type and p-type semiconductors: Various combinaons of nn -type and pp-type semiconductors are used for making electronic components. Diode is a combinaon of nn -type and pp-type semiconductors and is used as a recer. Transistors are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semiconductor. npn and pnp type of transistors are used to detect or amplify radio or audio signals. The solar cell is an ecient photo -diode used for conversion of light energy into electrical energy.

Magnetic Properties: Every substance has some magnec properes due to electrons. Each electron in an atom behaves like a ny magnet. Its magnec moment originates from two types of moons (i) its orbital moon around the nucleus and (ii) i ts spin around its own axis (Fig. 1.31).









UNIT 1: THE SOLID STATE (iii) Ferromagnesm: Substances like iron, cobalt, nickel, gadolinium and CrO2 are aracted very strongly by a magnec eld are called ferromagnec substances. These substances can be permanently magnesed. In solid state, the metal ions of ferromagnec substances are grouped together into small regions called domains and each domain acts as a ny magnet. In unun-magnesed piece of a ferromagnec substance the domains are randomly oriented and their magnec moments get cancelled. In a magnec eld all the domains get oriented in the direcon of the magnec eld (Fig. 1.32 a) and a strong magnec eect is produced. This ordering of domains persists even when the magnec eld is removed and the ferromagnec substance becomes a permanent magnet. (iv) Anferromagnesm: Substances like MnO showing anferromagnesm have domain structure similar to ferromagnec substance, but their domains are oppositely oriented and cancel out each other's magnec moment (Fig. 1.32 b). (v) Ferrimagnesm: Ferrimagnesm is observed when the magnec moments of the domains in the substance are aligned in parallel and anan -parallel direcons in unequal numbers (Fig. 1.32 c). They are weakly aracted by magnec eld as compared to ferromagnec substances. Fe3O4 (magnete) and ferrites like MgFe2O4 and ZnFe2O4 are examples of such substances. These substances also lose ferrimagnesm on heang and become paramagnec.










1.1 Dene the term 'amorphous'. 'amorphous'. Give a few examples examples of amorphous solids. 1.2 What makes a glass dierent from a solid such as quartz? Under what condions quartz could be converted into glass? 1.3 Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous. (i) Tetra phosphorus decoxide (P 4O10) (ii) Ammonium phosphate (NH 4)3PO4 (iii) SiC (iv) I2 (v) P4 (vi) Plasc (vii) Graphite (viii) Brass (ix) Rb (x) LiBr (xi) Si 1.4 (i) What is meant by the term 'coordinaon 'coordinaon number'? (ii) What is the coordinaon number of atoms? (a) In a cubic closeclose -packed structure? (b) In a bodybody-centred cubic structure? 1.5 How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain. 1.6 'Stability of a crystal is reected reected in the magnitude of its melng points'. Comment. Comment. Collect melng 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? 1.7 How will you disnguish disnguish between between the following pairs pairs of terms: (i) Hexagonal closeclose-packing and cubic closeclose -packing? (ii) Crystal lace and unit cell? (iii) Tetrahedral void and octahedral void? 1.8 How many lace points points are there in one unit cell of each of the following following lace? (i) FaceFace-centred cubic (ii) FaceFace-centred tetragonal (iii) BodyBody-centred 1.9 Explain (i) the basis of similaries and dierences between metallic and ionic crystals. (ii) Ionic solids are hard and brile. 1.10 Calculate the eciency of packing in case of a metal crystal for (i) Simple cubic (ii) bodybody-centred cubic (iii) faceface-centred cubic (with the assumpons that atoms are touching each other). –8 1.11 Silver crystallises in fcc lace. If edge length of the cell is 4.07 × 10 cm and density is 10.5 –3 g cm , calculate the atomic mass of silver. 1.12 A cubic solid is made of two elements P and Q. Atoms of Q are at the corners of the cube and P is at the bodybody-centre. What is the formula of the compound? What are the coordinaon numbers of P and Q? –3 1.13 Niobium crystallises in bodybody-centred cubic structure. If density is 8.55 g cm , calculate atomic radius of niobium using its atomic mass 93 u. 1.14 If the radius of the octahedral void is r and radius of the atoms in close packing is R, derive relaon between r and R. –8 1.15 Copper crystallises into a fcc lace with edge length 3.61 × 10 cm. Show that the –3 calculated density is in agreement with its measured value of 8.92 g cm . 1.16 Analysis shows that nickel oxide has the formula Ni 0.98O1.00. What fracons of nickel exist as 2 3 Ni and Ni ions?












1.

Sodium crystallises in a bcc unit cell. Calculate the approximate approximate number of unit cells in 9.2 g of sodium. (Atomic mass of sodium = 23) Soluon: In a bcc unit cell there are eight atoms at corners of the cube and one atom at the body centre.

∴, Number of atoms per unit cell = 8 x   1 x 1 = 2.

.  .   = 2.41 2.41  10.   ∴, Number of unit cells in 9.2 g of sodium = .   = 1.205 205  10. Number of atoms in 9.2 g of sodium =

2.

Relaon between the nearest neighbour distance (d), the edge length of unit cell (a) and the radius of atom (r) for pure elements. Soluon: Relaonship between nearest neighbours distance (d) and the edge length (a) of the unit cell. Lattice Type

1. 2.

Simple FCC

3.

BCC

=  = √   =  = √  

Relationship

 √ 

Relaonship between nearest neighbours distance (d) and the edge length (a) of the unit cell. Lattice Type

1.

Simple

2.

FCC

3.

BCC

Relationship

 =  =   =  = √    =  = √  

Unit 1 the Solid State

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