Here below some basic MCQ’s about “Solids, Unit cells and Packing Efficiency” with Answer explanation which is explained in details. Let’s check one by one which is given below.
1. Which of the following is a property of ionic solids?
a) High conductivity in solid state
b) High melting point
c) Malleability
d) High electrical conductivity in molten state
Answer:
d) High electrical conductivity in molten state
Explanation:
Ionic solids are composed of positive and negative ions held together by strong electrostatic forces. In their solid state, ions are fixed in place, and ionic solids do not conduct electricity. However, when molten or dissolved in water, the ions are free to move, allowing them to conduct electricity.
2. What is the coordination number of atoms in a body-centered cubic (BCC) unit cell?
a) 4
b) 6
c) 8
d) 12
Answer:
c) 8
Explanation:
In a body-centered cubic (BCC) structure, each atom at the corner of the unit cell is surrounded by 8 other atoms: one atom at the center of the unit cell and the rest are at the corners of adjacent unit cells. Hence, the coordination number is 8.
3. Which type of unit cell has atoms only at the corners?
a) Simple cubic
b) Face-centered cubic
c) Body-centered cubic
d) Hexagonal close-packed
Answer:
a) Simple cubic
Explanation:
In a simple cubic unit cell, atoms are located only at the corners of the cube. Each corner atom is shared among eight adjacent unit cells. This arrangement leads to a lower packing efficiency compared to other types of unit cells like face-centered cubic or body-centered cubic.
4. The packing efficiency of a face-centered cubic (FCC) structure is approximately:
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
c) 74%
Explanation:
In a face-centered cubic (FCC) structure, the atoms are packed more efficiently than in a simple cubic or body-centered cubic structure. The packing efficiency is about 74%, which means that 74% of the volume of the unit cell is occupied by atoms, and the remaining 26% is empty space.
5. What is the number of atoms per unit cell in a face-centered cubic (FCC) structure?
a) 1
b) 2
c) 4
d) 6
Answer:
c) 4
6. Which of the following unit cells has the highest packing efficiency?
a) Simple cubic
b) Body-centered cubic
c) Face-centered cubic
d) Hexagonal close-packed
Answer:
c) Face-centered cubic
Explanation:
The face-centered cubic (FCC) structure has a packing efficiency of 74%, which is the highest among the given options. Hexagonal close-packed (HCP) also has a packing efficiency of 74%, but since only cubic structures were compared, FCC is the answer.
7. In hexagonal close packing (hcp), what is the coordination number of each atom?
a) 4
b) 6
c) 8
d) 12
Answer:
d) 12
Explanation:
In hexagonal close-packed (hcp) structures, each atom is surrounded by 12 nearest neighbors: 6 atoms in the same plane, 3 atoms in the layer above, and 3 atoms in the layer below, making the coordination number 12.
8. Which of the following statements is true for molecular solids?
a) They conduct electricity in both solid and molten states
b) They have strong covalent bonds between molecules
c) They have relatively low melting points
d) They are always hard
Answer:
c) They have relatively low melting points
Explanation:
Molecular solids are held together by weak intermolecular forces such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds. As a result, they typically have low melting points and do not conduct electricity.
9. The packing efficiency in a simple cubic structure is:
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
a) 52%
Explanation:
In a simple cubic structure, atoms occupy 52% of the total volume of the unit cell, and the remaining 48% is empty space. This is because atoms only occupy the corners of the cube, leading to a relatively low packing efficiency.
10. In a body-centered cubic (BCC) unit cell, what fraction of the total volume is occupied by atoms?
a) 52%
b) 68%
c) 74%
d) 100%
Answer:
b) 68%
Explanation:
The packing efficiency of a body-centered cubic (BCC) structure is 68%, meaning 68% of the volume of the unit cell is occupied by atoms, and the rest is empty space.
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