Top 10 Properties OF Crystalline Solids 2022
1. Geometrical shape
Crystalline solids have a definite, regular three-dimensional arrangement of particles. Each crystal has definite faces (sides) and definite angles (interfacial angles), between the faces.
For a given crystal, the interfacial angles, at which the surfaces Intersect, are always the same no matter in which shape they are grown. The faces and angles of crystals are not changed even if it is ground to a fine powder.
2. Melting point
Crystalline solids have a definite melting point and can be identified by this. Thus crystals have definite heats of fusion.
3. Cleavage and Cleavage Plane.
CRYSTALLINE SOLIDS have properties that
The breaking of crystalline solids into smaller identical crystals due to external pressure is called cleavage.
The plane in which a crystalline solid can be broken into smaller identical crystals is called the cleavage plane.
The cleavage planes are inclined to one another at a particular angle for a crystalline solid. The angle is different for different solids.
Some of the crystals show variation in physical properties depending upon the direction Such properties are called anisotropic properties and the phenomenon is called anisotropy.
A substance is called anisotropy.
Crystals show the property of anisotropy. It is because crystals have different arrangements of particles in different directions.
In crystals, electrical conductivity, thermal conductivity, co-efficiency of thermal expansion and refractive index are anisotropic properties.
The electrical conductivity of graphite is greater in one direction than another. It is because in graphite mobile electrons can move easily parallel to its layers rather than perpendicular to their layer
Cleavage itself is an anisotropic behaviour.
The properties, which do not depend upon the direction, are called isotropic properties. e.g. melting point etc.
The substance which possesses isotropic property is called isotropy.
The repetition of faces, angles and edges when a crystal is rotated by 360 ° along its axis is called symmetry.
Crystals have various types of symmetry elements. e.g. centre of symmetry, the plane of symmetry, the axis of symmetry etc.
6. Habit of crystals
The shape in which a crystal usually grows is called the habit of a crystal.
e.g. when a saturated solution of NaCl is cooled, cubic crystals of NaCl are formed. Thus NaCl has a cubic habit.
Crystals are obtained either by cooling their saturated solution or by slow cooling of liquid substances. These are formed by growing in various directions. In particular conditions, the shape of the growing crystal is not changed. However, if conditions are changed then the shape of the crystal may also change. e.g. Cubic crystal of NaCl becomes needle-like when 10 % urea is present in the solution as an impurity.
The phenomenon in which two different substances exist in the same Isomorphism.
These different substances are called isomorphs of each other.
• Isomorphous substances have different physical and chemical properties.
• The formula of isomorphous substances shows that they generally have the same ratio of their atoms
• A crystal structure depends only on the number of atoms and their way of combination. It does not on the chemical nature of atoms.
• Isomorphous substances crystallize together in all proportions in a homogeneous mixture.
• The structure of negative ions like NO, and CO² are the same. Both are triangular planar.
• Similarly the Structure of SO, and CrO2 are also the same. Both are tetrahedral.
The phenomenon in which a compound exists in more than one crystalline form is called Polymorphism.
• The substance, which exists in more than one crystalline form, is called polymorphic.
• Different crystalline forms of a substance are called polymorphs.
• Polymorphs have different physical properties. It is due to the different structural arrangements of their particles.
• AgNO3 exists as Rhombohedral, Orthorhombic
• AgNO3 exists as Rhombohedral, Orthorhombic
The phenomenon in which an element exists in more than one crystalline form is called allotropy.
Different crystalline forms of an element are called allotropes or allotropic forms
• Carbon can exist in graphite (Hexagonal) and diamond (cubic) forms.
• Sulphur can exist in Rhombic and Monoclinic forms.
• Tin (Sn) can exist as grey tin (cubic) and white tin (tetragonal)
10. Transition temperature
The temperature at which two crystalline forms of the same substance can co-exist in equilibrium with each other is called transition temperature.
• At this temperature, one crystalline form of a substance changes into another.
• Above and below this temperature, only one form exists.
• The transition temperature of the allotropic form of an element is always less than its melting point.