Vidaara.orgClass 12 · Chemistry
CodeVID-C12-01-T1-01
Assignment — Crystalline Solids & Unit Cells
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Which property is shown only by crystalline solids?
- A.Isotropy
- B.Anisotropy
- C.Short-range order
- D.No sharp melting point
2.
The total number of Bravais lattices is:
- A.7
- B.14
- C.4
- D.8
3.
Atoms per body-centred cubic unit cell:
- A.1
- B.2
- C.4
- D.6
4.
Dry ice is an example of a:
- A.ionic solid
- B.metallic solid
- C.molecular solid
- D.covalent solid
5.
A corner atom contributes how much to a unit cell?
- A.$\tfrac{1}{2}$
- B.$\tfrac{1}{4}$
- C.$\tfrac{1}{8}$
- D.$1$
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Distinguish between crystalline and amorphous solids on the basis of melting point and order.
7.
Define unit cell and name the parameters used to describe it.
8.
Calculate the number of atoms in a simple cubic and an fcc unit cell.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Classify ionic, covalent, molecular and metallic solids by their constituent particles and binding force.
10.
In a compound, anions X form fcc and cations Y occupy all octahedral voids (body centre + 12 edge centres). Derive the formula.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Define crystal lattice and unit cell. Explain primitive and centred unit cells, and compute the number of atoms in simple cubic, bcc and fcc cells with full counting.
Answer Key
Section A — Multiple Choice Questions
- (B) Anisotropy
- (B) 14
- (B) 2
- (C) molecular solid
- (C) $\tfrac{1}{8}$
Section B — Short Answer (2 marks)
- Crystalline solids have long-range order and a sharp melting point; amorphous solids have short-range order and soften over a range of temperature.
- A unit cell is the smallest repeating portion of a crystal lattice; it is described by three edge lengths $a,b,c$ and three angles $\alpha,\beta,\gamma$.
- Simple cubic $z=8\times\tfrac{1}{8}=1$; fcc $z=8\times\tfrac{1}{8}+6\times\tfrac{1}{2}=4$.
Section C — Short Answer (3 marks)
- Ionic — ions, electrostatic force; covalent — atoms, covalent network; molecular — molecules, van der Waals/H-bonds; metallic — kernels in a sea of delocalised electrons.
- X $=4$; Y at body centre $=1$ plus edges $12\times\tfrac{1}{4}=3$, so Y$=4$. Ratio $4:4=1:1$, formula XY.
Section D — Long Answer (5 marks)
- A crystal lattice is a 3-D regular array of lattice points; the unit cell is its smallest repeating part. Primitive cells have particles only at corners; centred cells have extra particles (bcc — body centre; fcc — face centres; end-centred — two opposite faces). Counting: simple cubic $z=8\times\tfrac{1}{8}=1$; bcc $z=8\times\tfrac{1}{8}+1=2$; fcc $z=8\times\tfrac{1}{8}+6\times\tfrac{1}{2}=4$.
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