Online Assignment for Exam
1. Allotropes differ in which of the following properties:
a) Atomic Number
b) Atomic Mass
c) Crystal Structure
Answer: c [Reason:] Allotropes are different crystalline structures of the same element. Hence, they have same atomic number, atomic mass and electronegativity. For e.g., graphite and diamond are allotropes of carbon.
2. Co-ordination number of a crystalline solid is:
a) Number of particles in the unit cell
b) Number of nearest neighbours of a particle
c) Number of octahedral voids in a unit cell
d) Number of tetrahedral voids in a unit cell
Answer: b [Reason:] Coordination number of a crystal structure is the number of particles with which a given particle is in direct contact.
3. Packing efficiency of a crystal structure is the ratio of:
a) Volume occupied by particles to total volume of unit cell
b) Volume occupied by particles to that by voids
c) Total volume of unit cell to volume occupied by particles
d) Volume occupied by voids to that by particles
Answer: a [Reason:] Packing efficiency represents the fraction of the unit cell volume that is utilized to hold the particles.
4. HCP and BCC are called close-packed structures. Close packed structures have:
a) Highest packing efficiency
b) Highest void fraction
c) Highest density
d) All of the mentioned
Answer: a [Reason:] Due to largest number of particles in a unit cell, close-packed structures have highest packing efficiency and hence, lest void fraction. Density is NOT a property of the crystal structure but the substance and depends also on molar mass.
5. An octahedral void is surrounded by:
a) 8 atoms
b) 18 atoms
c) 6 atoms
d) 16 atoms
Answer: c [Reason:] An octahedral void resembles a regular octahedron with atoms situated at all the six vertices.
6. Which of the following is a property of amorphous solids?
a) Sharp melting point
c) Long range order
d) Definite heat of fusion
Answer: b [Reason:] Due to irregularity in structure, the average of physical properties like density, thermal and electrical conductivity etc. is same along any direction for a particular amorphous solid.
7. Which of the following is a crystalline solid?
a) Copper wire
b) Glass bottle
c) Polythene bag
d) Rubber ball
Answer: a [Reason:] Copper wires are made of crystalline copper. In fact, most metallic objects are crystalline. However, research for large-scale production of amorphous metals is in progress.
8. The smallest portion of a crystal which when repeated in different directions generates the entire crystal is called:
a) Lattice points
b) Crystal lattice
c) Unit cell
d) None of the mentioned
Answer: c [Reason:] Unit cell is the smallest unit of a crystal which repeats itself to generate the crystal. Lattice point represents the centres of atoms in a unit cell. Crystal lattice refers to the entire structure of a crystalline solid.
9. Which of the following is not a property of metal glass?
b) Poor thermal conductivity
c) High magnetic susceptibility
d) None of the mentioned
Answer: a [Reason:] Metal glass is a form of metal which like glass, is amorphous but not transparent. Most of the metal glasses are alloys, having high yield strength and high elastic strain limits. High magnetic susceptibility, low coercivity and comparatively higher electrical resistance than their crystalline counterparts make them ideal material for power transformers.
10. Grain boundaries are one of the causes of corrosion of metals?
Answer: a [Reason:] Grain boundaries are the regions that separate two grains in polycrystalline metallic solids. Absence of properly defined structure results in residual stresses. Hence, grain boundaries are the weak spots in the structure and are often the site of fracture.
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