Copper is a metallic solid having metal - metal bonds white diamond is a covalent solid having covalent bonds. Metallic bonds are weaker than covalent bonds and thus less amount of energy is required to break metallic bonds than covalent bonds.

No. of atoms of X at the corners = 8 - 1 = 7
(since 1 is missing)
Contribution of X towards unit cell
\(=7\times \frac { 1 }{ 8 } =\frac { 7 }{ 8 } \)
No. of atoms of Y at the face centres = 6
Contribution of Y towards unit cell
= 6 x \(\frac{1}{2}\) = 3
Ratio of X : Y = \(\frac{7}{8}\) : 3 = 7:24
Formula of the compound is X₇ Y₂₄.

The defects which are caused by missing or misplaced atoms or ions in the crystal.

(i) In ionic compounds, electricity is conducted by ions.
(ii) In solid state, ions are held together by A strong electrostatic forces and are not free to move about within the solid.
(iii) Hence, ionic solids do not conduct electricity in solid state
(iv) However, in molten state or in solution form, the ions are free to move and can conduct electricity.

The imperfection occurs due to missing atoms, displaced atoms or extra atoms, is named as a point defect. Such defects arise due to imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures.

a. P₄ -  Covalent solid
b. Brass - Metallic solid
c. Diamond - Covalent solid
d. NaCl - Ionic solid
e. Iodine - Covalent solid

(i) Similarities between ionic and metallic crystals:
(a) Ionic and metallic crystals have electrostatic forces of attraction.
(b) Both the crystals exhibit high melting point.
(c) The bonds in metallic and ionic crystals are non-directional.
(ii) Differences between ionic and metallic crystals:

AAAA type of three dimensional packing:

1. This is simple cubic arrangement.
2. Three dimensional packing arrangement can be obtained by repeating the AAAA type two dimensional arrangements in three dimensions.
3. Spheres in one layer sitting directly on the top of in the previous layer so that all layers are identical.
4. All spheres of different layers of crystal are perfectly aligned horizontally and also vertically.
5. In simple cubic packing, each sphere is in contact with 6 neighbouring spheres
6. Four in its own layer, one above and one below and hence the coordination number of the sphere in simple cubic arrangement is 6.
ABABA type of three dimensional packing:

(i) This is body centered cubic arrangement.
(ii) The spheres in the first layer are slightly separated and the second layer is formed by arranging the spheres in the depressions between the spheres in layer A.
(iii) The third layer is a repeat of the first.
(iv) This pattern ABABAB is repeated throughout the crystal.
(v) Each sphere has a coordination number of 8, four neighbors in the layer above and four in the layer below.
ABCABC type of three dimensional packing:

(i) This is face centered cubic arrangement.
(ii) In this arrangement (FCC) second layer spheres are arranged at the dips of first layer. Third layer spheres are arranged in a manner such that it cover the octahedral void.
(iii) Then no longer third layer is similar to first or second layer.
(iv) Third layer gives different arrangement. Fourth layer spheres are similar to first layer.
(v) If the first, second and third layer are represented as A, B, C then this type of packing gives the arrangement of layers as ABCABC.. and the sequence is repeated.

Volume of unit cell = (2.88\(\mathring { A } \))³ = 23.9 x 10^-24cm³
Volume of 100 g of the metal \(=\frac { m }{ A } =\frac { 100g }{ 7.2{ gcm }^{ -3 } } \) = 13.9 cm³
Number of unit cells in this volume \(=\frac { 13.9{ cm }^{ 3 } }{ 23.9\times { 10 }^{ -24 }{ cm }^{ 3 } } \) = 5.82 x 10²³

(i) Compressibility is the ability of the substance to change its shape by applying external pressure.
(ii) Solids form closed packed structure with negligible intermolecular space and possesses very strong intermolecular forces of attraction.
(iii) Thus, they do not change their shape in the presence of external pressure.

\(\operatorname{Density}(\rho)=\frac{\mathrm{nM}}{\mathrm{a}^{3} \mathrm{~N}_{\mathrm{A}}} \)
\(\mathrm{n}=2, \mathrm{~N}_{\mathrm{A}}=6.023 \times 10^{23} ; \mathrm{a}=288 \mathrm{pm}=2.88 \times 10^{-8} \mathrm{~cm}, \rho=7.2 \mathrm{~g} \mathrm{~cm}^{-3} \)
\(\therefore M=\frac{\rho \times a^{3} \times N_{A}}{n} \)
\(=\frac{7.2 \times\left(2.88 \times 10^{-8}\right)^{3} \times 6.023 \times 10^{23}}{2} \)
\(=517.95 \times 10^{-1} \)
\(=51.795 \mathrm{~g} \mathrm{~mol}^{-1} \)
Number of moles (n) \(=\frac{\text { Mass }}{\text { Molar mass }}=\frac{208}{51.795}\)=4.02 moles
No. of atoms = No. of moles \(\times\) Avogadro number
=n \(\times\) N_A
\(=4.01 \times 6.023 \times 10^{23} \)
\(=24.15 \times 10^{23} \text { atoms }\)

sc ⇒ 2r = a
⇒ r = a/2
bcc ⇒ 4r = \( \sqrt{3a} \) ⇒ r = \(\frac { \sqrt{3a}} {4}\)
fcc ⇒ 4r ⇒  \( \sqrt{2a} \) ⇒ r = \(\frac { \sqrt{2a}} {4} = \frac { a} {2\sqrt{2a}}\)
\(\left( \frac { a }{ 2 } :\frac { \sqrt { 3 } }{ 4 } a:\frac { a }{ 2\sqrt { 2 } } \right) \)

For a fcc structure = r_x+ / r_y- = 0.414
Given that r_x⁺ = 100 pm
r_y = 100pm/0.414

In bcc unit cell,
2 atoms = 1 unit cell
Number of atoms in 8 g of element is, Number of moles = 8g / 40 g mol^-1 = 0.2 mol
1 mole contains 6.023 x10²³ atoms 
0.2 mole contains 0.2 x 6.023 x 10²³ atoms
[1 unit cell / 2 atoms] x 0.2 x 6.023 x 10²³
= 6.023 x 10²² unit cells.

b) Both the assertion and reason are true but the reason is not the correct explanation of the assertion.

a) Both assertion and reason are true and the reason is the correct explanation of the assertion.

c) Assertion is true but reason is false.

d) Non-stoichiometric compound is formed

b) Co-ordination number of each sphere is 8 in ccp arrangements