Lesson Notes By Weeks and Term - Senior Secondary School 1

SUBJECT: PHYSICS

CLASS:  SS 1

DATE:

TERM: 3rd TERM

 
WEEK 6

TOPIC: CRYSTAL STRUCTURE OF MATTER- AMORPHOUS AND CRYSTALLINE SUBSTANCES

CONTENT :  Crystal Structure

 Crystalline and Amorphous Substance

The term crystal structure is generally used to describe the internal structure of solids. The particles of a solid are orderly arranged in parallel planes. In other words, solid crystals consist of particles arranged in parallel planes. Crystals are built up from simple structural units called unit cells. A unit cell (also called a space lattice) is made up of few atoms, ions or molecules.

There are seven types of unit cells. Each type gives rise to a crystal system. They include cubic, rhombic, monoclinic, triclinic, tetragonal, rhombohedra and hexagonal crystal system. X- ray analysis of crystal shows that three pairs of opposite parallel faces bund the unit cells in all crystal systems, except in hexagonal system. We can draw three imaginary lines between the centres of the opposite parallel faces.

These imaginary lines represent three axes of symmetry. The length of these axes of symmetry and the angles between them determines the system to which a crystal belongs. This is demonstrated using cubic and monoclinic crystal systems.

In a cubic crystal system, the axes of symmetry have equal length and at right angles to each other. This type of crystal system is found in copper, sodium chloride, silver, gold, iron, sodium, potassium etc. however, the type of cubic crystal system found in copper, silver, gold and sodium chloride is different from the type found in sodium, iron and potassium. The former is called face centred cubic, while the later is called body centred cubic. In a monoclinic system, the axes are not equal in length and only two of them are at right angles to each other. These crystal systems are in sugar, washing soda, ferrous sulphate etc.

EVALUATION

1. What is a unit cell?
2. With the aid of diagram, distinguish between cubic crystal system and monoclinic crystal system.

DIFFERENCES BETWEEN CRYSTALLINE AND AMORPHOUS SUBSTANCES

On the basis of internal structure of solid, we can classify a solid as either a crystal substance (e.g. common salt) or an amorphous substance (e.g. glass). The differences between the two substances are:

  Crystalline substance          Amorphous substances                   1. Have a definite internal arrangement      Have a haphazard distribution of particles

of particles. 

2. Have a cleavage planes along which       Do not have cleavage planes 

 they fracture when struck

3. Meets sharply at a definite temperature  Do not meet sharply at a definite 

 when heated.                                            temperature

4. Regarded as true solid                             Regarded as super cooled liquids 
5. Have a long range order of arrange-        Have short range order of arrangement

ment of particles                                       of particles 

6. Are efflorescence substance                     Are not efflorescence substance 

EVALUATION  

1. Differentiate between crystalline and amorphous substances
2. Define crystal lattice 

Reading Assignment : Read more on crystal structure of matter – New school physics (pg99- 101)

  WEEKEND ASSIGNMENT

1.   crystal structure is generally used to describe (a) the external structure of solids (b) the internal structure of solids (c) the internal structure of liquids   (d) the external structure of liquids
2. Particles of a solid are orderly arranged in .,……….. planes (a) parallel (b) horizontal (c) vertical (d) none of the above
3. Unit cell is also known as (a) monoclinic system (b) polyclinic system (c)space lattice (d) none of the above  
4. ………… substance has a definite internal arrangement of particles (a) Amorphous (b) Crystalline (c) Elastic (d) Atomic
5. ………….. are not efflorescence substance  (a) Amorphous (b) Crystalline (c) Elastic (d) Atom

THEORY

1. Differentiate between crystalline and amorphous substances
2. Define crystal lattice