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 particle

    of particles.

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

fracture when struck

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

when heated.

4. Regarded as true solid                                Regarded as super cooled liquids

5. Have a long range order of arrangement    Have short range order of particles of arrangement

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)

GENERAL EVALUATION

1. A body of mass 25Kg moves on a horizontal straight road with a velocity of 15m/s. calculates the height above the road at which it possesses a potential energy equal to its kinetic energy. [g= 10m/s²]
2. An engine of a car of power 80KW moves on rough road with velocity 32m/s the force required to bring it to rest is?

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

• Unit cell is also known as (a) monoclinic system (b) polyclinic system (c)space lattice (d) none of the above
• ____ substance has a definite internal arrangement of particles (a) Amorphous

(b) Crystalline (c) Elastic (d) Atomic

• ____ are not efflorescence substance  (a) Amorphous (b) Crystalline (c) Elastic

(d) Atom

THEORY

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