Solid State

$ \begin{aligned} &\\ &\begin{aligned} & \text { Density }\left(\rho_\alpha\right) \text { for } \alpha \text {-form }=\frac{M Z_1}{N_0\left(a_1\right)^3} \\ & \text { and density }\left(\rho_\beta\right) \text { for } \beta \text {-from }=\frac{M Z_2}{N_0\left(a_2\right)^3} \end{aligned} \end{aligned} $

Given, side length $\left(a_1\right)$ for $(\alpha)=2 \mathrm{pm}$ side length $\left(a_2\right)$ for $(\beta)=4 \mathrm{pm}$ where, $M=$ molar-mass

$N_0=$ Avagadro number

$Z_1$ and $Z_2=$ number of particles in $\alpha$ and $\beta$ form.

$a_1$ and $a_2$ are side lengths.

Here, $Z_1=4, Z_2=2, a_1=2, a_2=4$

$ \begin{aligned} \therefore \quad \frac{\rho_\alpha}{\rho_\beta} & =\frac{M Z_1 / N_0\left(a_1\right)^3}{M Z_2 / N_0\left(a_2\right)^3}=\frac{Z_1 \times\left(a_2\right)^3}{Z_2 \times\left(a_1\right)^3} \\ \frac{\rho_\alpha}{\rho_\beta} & =\frac{4 \times(4)^3}{2 \times(2)^3}=\frac{256}{16}=16 \end{aligned} $

Hence, option (b) is the correct answer.

At atmospheric pressure ice crystallises in the hexagonal form, but at very low temperature water condense to cubic form.

Miller indices (hkl) of crystal are the reciprocal of intercepts along crystallographic axes ( $a, b, c$ ). Here, $h=\frac{1}{1}, k=\frac{1}{\frac{1}{2}}=2, l=\frac{1}{3}$

$ \text { } \quad \begin{aligned} So,\,\,\,\,\,\,\,\,\, h k l & =1: 2: \frac{1}{3} \\ & =3: 6: 1 \text { or }(361) \\ \Rightarrow \quad(h k l) & =(361) \end{aligned} $

∵ For bcc lattice, radius ratio for $r^{+} / r^{-}$is 0.7 , i.e. radius ratio of cation by anion ranges from 0.732 to 1.0 . Also, for bcc $r^{-}$ions are at corners,

$ \text { Thus, } \quad 4 \times r^{-}=\sqrt{3} a \quad(\because \sqrt{3} a=1.7 \times 200) $

$ \begin{aligned}and\,\,\, 4 \times r^{-} & =1.7 \times 200 \\ r^{-} & =\frac{1.7 \times 200}{4}=85 \mathrm{pm} \end{aligned} $

$\therefore \quad \frac{r^{+}}{r^{-}}=\frac{70}{85}=0.82$, which is close to 0.7. Hence, option (a) is the correct answer.

Given, copper crystallises in ccp manner

$ \text { radius }=1.41 \mathop {\rm{A}}\limits^{\rm{o}}=141 \mathrm{pm} $

Atomic weight of copper $=64$

Avogadro's number $=6 \times 10^{23}$, Density $=$ ?

$ \rho=\frac{Z \times M}{a^3 \times N_a} $

Here, edge length of unit cell can be calculate as

$ \begin{aligned} & r=a / 2 \sqrt{2} \Rightarrow a=r \times 2 \sqrt{2} \\ & a=141 \times 2 \times 1.414 \\ & a=398.74 \mathrm{pm} \text { or } 398.74 \times 10^{-10} \mathrm{~cm} \end{aligned} $

Volume of unit cell

$ =\left(398.74 \times 10^{-10} \mathrm{~cm}\right)^3 $

For $\mathrm{ccp}=2=4, N_a=6.0 \times 10^{23}$

$ \begin{aligned} & f=\frac{4 \times 64}{\left(398.74 \times 10^{-10} \mathrm{~cm}\right)^3 \times 6.0 \times 10^{23}} \\ & f=6.73 \approx 6.67 \end{aligned} $

Hence, option (a) is correct.

Z_eff for fcc = 4

Z_eff for bcc = 2

Atomic volume of $\alpha$-form = ${{{{(3.68 \times {{10}^{ - 8}})}^3}} \over 4}$ $\times$ N_A

Atomic volume of $\beta$-form = ${{{{(2.92 \times {{10}^{ - 8}})}^3}} \over 2}$ $\times$ N_A

As atomic weight is same, element is same, so the density ratio is

${\rho _\alpha }:{\rho _\beta } = {V_\beta }:{V_\alpha } = {{{{(2.92)}^3}} \over 2}:{{{{(3.68)}^2}} \over 4}$

$ = 12.448:12.459 = 1:1$

Coordination number cannot be 12, for any atom in the topmost layer, as there is no layer above it. Thus, each atom is in contact with six atoms in the same layer and three atoms from the layer below it.

For cubic close packing, we have

Packing fraction = ${{Volume\,of\,four\,spheres\,in\,the\,unit\,cell} \over {Total\,volume\,of\,the\,unit\,cell}}$

$ = 4 \times {{(4/3)\pi {r^3}} \over {16\sqrt 2 {r^3}}} = {\pi \over {3\sqrt 2 }} = 0.74 = 74\% $

In fcc unit cell, the effective number of atoms is

One atom at each corner = 8 corner atoms $ \times {1 \over 8} = 1$

Atoms at each of the six face centres :

6 face centered atoms $ \times {1 \over 2} = 3$.

Number of octahedral voids = 4.

Number of tetrahedral voids = 8.

Therefore, per atom, there is one octahedral void and two tetrahedral voids.

In fcc (or ccp), the unit edge length is given by

$\sqrt 2 a = 4r \Rightarrow a = {{4r} \over {\sqrt 2 }} = 2\sqrt 2 r$

For ccp, Z = 4 = no. of O-atoms

No. of octahedral voids = 4

No. of tetrahedral voids = 2 $\times$ 4 = 8

No. of Al³⁺ ions = m $\times$ 4

No. of Mg²⁺ ions = n $\times$ 8

Thus, the formula of the mineral is Al_4m Mg_8nO₄

4m(+3) + 8n(+2) + 4($-$2) = 0

12m + 16n $-$ 8 = 0

4(3m + 4n $-$ 2) = 0

3m + 4n = 2

Possible values of m and n are ${1 \over 2}$ and ${1 \over 8}$ respectively.

From the figure, it can be seen that the cation A⁺ occupies octahedral void formed by the anion X^$-$. The radius ratio for an octahedral void is r_A⁺ / r_X^$-$ = 0.414. Now, given that the radius of anion X^$-$ is 250 pm, so the radius of A⁺ is

r_A⁺ = 0.414 $\times$ 250 = 103.50 $ \simeq $ 104 pm

8 X atoms present at the corners.

Atoms contribute to 1 unit cell $ = {1 \over 8} \times 8 = 1$

6 X atoms present at the face centres.

Atoms contribute to 1 unit cell $ = 6 \times {1 \over 2} = 3$

Total X atoms $ = 3 + 1 = 4$

4 M atoms present at edge centres.

Atoms present in 1 unit cell $ = 4 \times {1 \over 4} = 1$

1 M atom present at body centre and it contribute completely to 1 unit cell.

Thus, total M atoms in one unit cell $ = 1 + 1 = 2$

Ratio is $M:X::2:4::1:2$

Thus, empirical formula is $M{X_2}$.

The diagonal is given as 4r, where r is the radius of atom or sphere forming close packed structure.

$ \begin{aligned} \text { Diagonal } & =\sqrt{\mathrm{L}^2+\mathrm{L}^2}=\sqrt{2 \mathrm{~L}^2} \\\\ 4 r & =\sqrt{2 \mathrm{~L}^2} \text { or } \mathrm{L}=\frac{4 r}{\sqrt{2}}=2 \sqrt{2} r \\\\ \text { or } \text { Total area } & =\mathrm{L}^2 \\\\ (2 \sqrt{2} r)^2 & =8 r^2 \end{aligned} $

Number of spheres inside the square is

$ 1+4\left(\frac{1}{4}\right)=2 $

Area of each sphere $=\pi r^2$

Total area of spheres $=2 \times \pi r^2$

$ \begin{aligned} \text { Packing fraction } & =\frac{\text { Total area of spheres }}{\text { Total area }} \\\\ & =\frac{2 \times \pi r^2}{8 r^2}=\frac{\pi}{4}=0.785 \end{aligned} $

So, the percentage fraction is $78.5 \%$.

Frenkel defect occurs in compounds in which the anions are much larger in size than cations. It is a dislocation effect. The presence of Schottky defects lowers the density of the crystal, it hence affects the physical properties of the crystal. In the metal excess defects, the electrons trapped in anion vacancies are known as F-centres.

The number of atoms in a HCP unit cells is 6.

3 atoms are at the middle of the unit cell and each atom contributes 1 atom to the unit cell.

2 atoms are at the two face centres of the unit cell and each atom contributes 1/2 atom to the unit cell.

12 atoms are at the 12 corners of the unit cell and each atom contributes 1/6 atom to the unit cell.

The number of atoms in one unit cell

(3 $\times$ 1) + (2 $\times$ 1/2) + (12 $\times$ 1/6) = 3 + 1 + 2 = 6

Volume of unit cell = Area of base $\times$ Height of the unit cell

The (HCP) is a hexagonal prism structure. The base of (HCP) is a hexagon. We know that the hexagon is made of the 6 equilateral triangles.

Area of equilateral triangle $ = {{\sqrt 3 } \over 4}\,{a^2}$

Therefore, the area of the base is given as,

Area of base $ = 6\times{{\sqrt 3 } \over 4}\,{a^2}$

Let's the 'h' be the height of the hexagonal unit

Therefore, we have

${h \over a} = \sqrt {{8 \over 3}} $

Now, substitute these values in the volumes of (HCP) we have,

Volume of HCP = Area of base $\times$ height of the hexagon

$\therefore$ Volume of HCP $ = 6 \times {{\sqrt 3 } \over 4}\,{a^2} \times \sqrt {{8 \over 3}} a$

Since, spheres touch each other, the edge length 'a' is equal to twice the radius of the atom. That is a = 2r

Substitute these values in the volume we get,

Volume of HCP $ = 6 \times {{\sqrt 3 } \over 4}\,{a^2} \times \sqrt {{8 \over 3}} a$

$ = 6 \times {{\sqrt 3 } \over 4}\,{(2r)^2} \times {{\sqrt 8 } \over {\sqrt 2 }}2r = 24\sqrt 2 {r^3}$

Packing fraction = $\frac{\mathrm{Total~volume~of~unit~cell}}{\mathrm{Volume~occupied}}$

Substituting the values of occupied sphere and volume of the unit cell we have:

$ \Rightarrow {{6 \times {4 \over 3} \times \pi {r^3}} \over {24\sqrt 2 {r^3}}} = 0.74$

Rearranging and solving % occupied space = 74%

The space in the HCP unit cell = 100 $-$ 74 = 26%

(A) Simple cubic and face centered cubic unit cells have cell parameters as $a = b = c$ and $\alpha = \beta = \gamma= 90$

They belong to same crystal system.

(B) Cubic and rhombohedral unit cells have cell parameters $a = b = c$ and $\alpha = \beta = \gamma$. They are two crystal systems.

(C) Cell parameters of cubic unit cell: $a = b = c$ and $\alpha = \beta = \gamma= 90$

b. Rhombohedral unit cell has cell parameters:

$a = b = c$ and $\alpha = \beta = \gamma= 90$ not equal to 90

(D) Hexagonal and monoclinic unit cells are two crystal system:

The have only two crystallography angles of 90$^\circ$.