TYPES OF CRYSTALS
Crystals are classified in the following four types:
(i) Atomic crystals
(ii) Ionic crystals
(iii) Covalent crystals
(iv) Molecular crystals
This classification is based on the nature of forces or bonding between the atoms, ions or molecules constituting the crystal.
(1) Atomic crystals:
Examples of such crystals are metals; hence they are also called metallic crystals. They consist of atoms packed and held together by metallic bond.
In metals, the valency electrons move freely in crystal lattice and are said to form an electron gas, in which positive ions are immersed. Such crystals are characterized by:
(a) Lustre (b) High melting point (c) Electrical and thermal conductivity (d) Malleability (drawing into sheets) (e) Ductility (drawn into wires).
(ii) Ionic crystals:
Such crystals consist of positively and negatively charged ions, held together by electrostatic forces of attraction. Individual ions have no identity. They are characterized by:
(a) High melting point
(b) Conduction of electricity in fused stale; as well as in solution form.
(c)Brittleness and hardness
(d) Indefinite growth of crystal, until it is broken or is stopped from growing.
(iii) Covalent crystals:
Such crystals consist of atoms held together by covalent bonds. Covalent bonds are very strong. They require large amount of energy to break them. Examples of covalent crystals are: graphite, diamond, silica and carborundum.
In diamond each carbon atom is joined to four other carbon atoms at an angle of 109°, called tetrahedral angle. Due to close packing of atoms and large number of covalent bonds, diamond is very hard and has high melting point.
In graphite each carbon atom is joined to three other carbon atoms at an angle of 120°, forming layers of hexagons. Adjacent layers are held together by weak Vander waal’s forces. Due to space between layers graphite is soft and as layers easily slide over one another so graphite has greasy texture. Graphite conducts electricity in the plane of layers, as each carbon has one free electron. Covalent crystals are characterized by:
(a) High melting point
(b) Low density
(c) High refractive index
(iv) Molecular crystals:
Such crystals are composed of molecules, such as ice, I2, CO2. The molecules are held together either by the following forces:
(a) Hydrogen bonding (which is an electrostatic force of attraction between partially positive hydrogen atom of one molecule and an electronegative atom of other molecules).
(b) Weak Vander waal’s forces (which arise due to attraction between atomic nuclei of one molecule and electrons of other molecule). Such crystals are characterized by:
(a) Low melting point
(b) Non-conduction of heat and electricity.