1. Define the term “amorphous”. Give few examples of amorphous solids.
Soild in which constituent particles (ions, atoms, molecules) have no regular arrangement means they have disordered atomic structure is called amorphous soild (sometimes referred as pseudo soilds or supercooled liquid). Amorphous solid includes – Glass, Rubber, plastic etc.
• They have short range order.
• Isotropic in nature.
• No definite heat of fusion.
• They melt over a range of temperature.
2. What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?
A glass is different from solid such as quartz in the arrangement of constituents particles. Glass is a amorphous soild in which constituents particles have short range order whereas in quartz, the constituent particles have short range order and long order order.
The conditions in which quartz converted into glass –
• By rapidally cooling
• By heating
3. Categorize the given soilds as metallic, molecular, amorphous or network (covalent) –
a) Tetra phosphorus decoxide (P4O10)
b) Ammonium Phosphate
There are some points which is used to differentiate the covalent, metallic, ionic solids –
• Covalent solids – Covalent bond takes place between non metal and non-metal.
• Ionic solids – Ionic bond takes place between metal and non-metal.
• Metallic solids – Metallic bond takes between takes place metal and metal.
Amorphous – Plastic
Covalent – SiC, Graphite, Si
Molecular – P4O10, I2, P4
Metallic – Brass, Rb
Ionic – Ammonium phosphate, LiBr
4. a) Define coordination number.
b) What is the coordination number of the atoms –
I. In cubic-closed packed structure.
II. In body centered cubic structure.
a) Coordination Number –
Coordination number means the number of nearest neighbours of any constituent particle in crystal lattice.
b) Coordination number of cubic-closed packed structure is 12. Coordination number of body centered cubic structure is 8.
5. How can you determine the atomic mass of an unknown metal if you its density and dimensions of unit cell? Explain.
The atomic mass of an unknown metal can be determined if density or dimensions of the unit cell is known –
6. Stability of crystal is reflected in the magnitude of its melting point.Comment. Collect melting points of the solid water, ethyl alcohol, diethyl ether and methane from data book. What can you say about the intermolecular forces between these molecules.
Stability of crystal is reflected in the magnitude of its melting point because –
✓ Melting point increases
✓ Intermolecular forces of attraction increases
✓ Stability increases
Hence, a substance having higher melting point is more stable . The melting point of given substances are –
1. Solid water – 273K
2. Ethyl alcohol – 158.8K
3. Diethyl ether – 156.8K
4. Methane – 89.34K
By observing melting points of these substances, it can be seen that the solid water has strong intermolecular force of attraction (hydrogen bonding) and methane has weak intermolecular forces of attraction (vanderwaal forces or London dispersion forces).
7. How will you distinguish between following pairs of the terms –
a) Cubic closed packing or hexagonal closed packing
b) Crystal lattice and unit cell
c) Tetrahedral or octahedral void
a) Cubic closed packing and hexagonal closed packing –
Cubic closed packing –
i. When the second layer of octahedral void is covered by sphere of third layer but the sphere of third layer is not as first layer. This arrangement is called ABCABC….. and structure is called CCP (Cubic closed packing).
ii. The coordination number of CCP is 12.
iii. It contains 4 atoms per unit cell.
iv. It has both tetrahedral or octahedral holes.
Hexagonal Closed packing —
i. When the second layer of tetrahedral void covered by spheres of third layer but the sphere of third layer is same as first layer. This is repeated in alternate layers and form ABAB…… rearrangement and structure is called HCP (Hexagonal closed packing).
ii. The coordination number of HCP is 12.
iii. It contains 6 atoms per unit cell.
iv. It has tetrahedral void but octahedral holes are covered by layers of spheres.
Crystal lattice and unit cell –
Crystal lattice –
i. Crystal lattice is the three dimensional representation of constituent particles (atoms, ions, molecules) in space. It is represented by points.
ii. Each point represents a lattice site or lattice point. By joining these points, we will get a geometry of lattice.
Unit cell –
i. The smallest and the repeating unit of the crystal lattice is called unit cell. Many unit cell combine and form crystal lattice.
ii. The unit cell has three edges a, b and c and has three angles α, β and γ which may or may not be mutually perpendicular.
Tetrahedral and octahedral Void –
Tetrahedral Void –
i. The space is surrounded by four atomic spheres is called tetrahedral void.
ii. The coordination number of the tetrahedral void is four.
Octahedral Void –
i. The space is surrounded by six atomic spheres is called tetrahedral void. ii. The coordination number of the tetrahedral void is Six.
8. 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
a) There are 14 lattice points in the face – centred cubic. 14 lattice points – 8 at the corners + 6 at the faces
b) There are 14 lattice points in the face – centred tetragonal. 14 lattice points – 8 at the corners + 6 at the faces
c) There are 9 lattice points in the body centred cubic. 9 lattice points – 8 at the corners + 1 at the centre
9. Explain –
a) The basis of similarities and differences between metallic and ionic crystals.
b) Ionic solids are hard and brittle.
a) Similarities between metallic and ionic crystal –
I. Both metallic and ionic crystals are held by electrostatic force of attraction.
II. Both having high melting points.
III. Non- directional
Differences between metallic and ionic crystal –
I. In metallic crystal, ions are free to move and so, conduct electricity. Whereas in ionic crystal, ions are not free to move, they cannot conduct electricity. In molten or aqueous solution, they do conduct electricity.
II. In metallic crystal, metallic bond is present. It may be strong or weak depending upon the number of valence electrons and size of kernals. In ionic crystal, ionic bond is present. It is strong electrostatic force of attraction.
b) Ionic solids are hard because of presence of electrostatic force of attraction. They are brittle because due to presence of non-directional bonds in them.