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.

There are seven primitive crystal systems; cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic and rhombohedral.

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.

The empty spaces present between the metal atom or the ions when they are packed within the crystal are called voids.

(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.

ZnS exhibits Frenkel defect.

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) A basic repeating structural unit of a crystalline solid is called a unit cell.
(ii) A crystal is consisted of large number of unit cells.

  Tetrahedral voids Octahedral voids. 1. A void surrounded by 4 spheres occupying the corners of a tetrahedron. A void surrounded by 6 spheres along the corners of an Octahedron 2. It is formed when a sphere of second layer placed above the void of first layer. All the voids of first row which remain unoccupied from octahedral. 3. Co-ordination number is 4. Co-ordination number is 6. 4. The number of tetrahedral voids are equal to 2n. The number of Octahedral voids are equal to n.

	hcp structure	ccp structure
1.	This is 'aba' pattern of arrangement.	This is 'abc' pattern of arrangement.
2.	The spheres can be arranged so as to fit into the depression in such a way that the third layer is directly over a first layer.	The third layer may be placed over the second layer in such a way that all the spheres of the third layer fit in octahedral voids.
3.	The tetrahedral voids of the second layer are covered by the spheres of the third layer.	This arrangement of the third layer is different from other two layers and the stacking of layers continued.
4.	6 spheres are present	4 spheres are present

There are seven types of unit cell, Cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic and rhombohedral. They differ in the arrangement of their crystallographic axes and angles.
i) Cubic: a = b = c; α = β = ૪ = 90^o.

ii) Tetragonal: a = b ≠ c; α = β = ૪ = 90°.

iii) Orthorhombic:  a  ≠ b  ≠ c; α = β = ૪ = 90°.

iv) Hexagonal: a = b ≠ c; α = β = 90^o, ૪ = 120^o. 

v) Monoclinic: a ≠ b ≠ c; α = ૪ = 90^o, β ≠ 90^o, 

vi) Triclinic: a ≠ b ≠ c; α  ≠ β ≠ ૪ ≠ 90^o.

vii) Rhombohedral:  a = b = c; α = β = ૪ ≠ 90^o. 

(i) Ionic solids have high melting points.
(ii) These solids do not conduct electricity, because the ions are fixed in their lattice positions.
(iii) They are hard so strong external force can change the relative positions of ions.