CLASSIFICATION OF SOLIDS SOLIDS ARE OF TWO TYPES: CRYSTALLINE SOLIDS AMORPHOUS SOLIDS

A true solid must have- A definite and sharp melting point A definite heat of fusion A regular arrangement of particles i.e. ions atoms or molecules. Only crystalline state satisfies these criteria.Substances which are rigid but do not satisfy these criteria are called amorphous solids

CLASSIFICATION OF CRYSTALLINE SOLIDS Crystalline solids can be classified into four categories based on the type of constituent particles and the nature of binding forces. These are IONIC SOLIDS METALLIC SOLIDS COVALENT SOLIDS MOLECULAR SOLIDS

IONIC SOLIDS Ionic solids are three dimensional arrangement of cations and anions held together by strong electrostatic force of attraction This type of crystal is that in which the units occupying the lattice points are positive and negative ions which are held together by coulombic forces of attraction between the oppositely charge ions. Examples: CsCl, ZnS, NaCl etc.

CRYSTAL STRUCTURE OF IONIC SOLIDS The ions are held together in a closely packed, perfect, three-dimensional pattern called the CRYSTAL LATTICE. Each ion is surrounded by a definite number of ions of opposite charge and this number is called CO-ORDINATION NUMBER. The smallest three dimensional recurring unit present in a crystal is called a UNIT CELL. Each solid has its own specific crystal lattice. The crystal as a whole is electrically NEUTRAL.

CRYSTAL LATTICE OF NaCl

CRYSTAL LATTICE OF ZnS

CHARACTERISTICS OF IONIC SOLIDS They are hard, strong and brittle in nature and have low volatility. They have high melting and boiling points. They are poor conductors of electricity in the solid state as ions present in them are not free to move from their fixed positions. They are generally soluble in polar solvents like water, ammonia etc.

METALLIC SOLIDS Metallic solids is orderly collection of positive ions of metals surrounded by free delocalised electrons and these ions are held together by free electrons with the help of a metallic bond. The electrostatic force of attraction between the positive ions and the delocalised electrons is called the metallic bond. Examples: All common metals like Na, Al , Mg etc.

CRYSTAL STRUTURE OF Na Sodium has body centred cubic structure.

CRYSTAL STRUCTURE OF Al Al has face centred cubic structure.

CHARACTERISTICS OF METALLIC SOLIDS The nature of these solids range from soft to very hard. They are malleable and ductile. They are good conductors of heat and electricity. They possess bright shining metallic lustre. They have high melting and boiling points. They have moderate heat of fusion.

COVALENT SOLIDS The crystalline solids of non-metals in which the constituent particles are atom of same or different elements held together by covalent bonds are covalent solids. EXAMPLES:Diamond, graphite, SiC , Silica, Si etc.

CRYSTAL STRUCTURE OF DIAMOND In diamond every carbon atom is surrounded by 4 carbon atoms tetrahedrally.

CHARACTERISTICS OF COVALENT SOLIDS They are very hard, having very high melting points and boiling points. They are poor conductors of heat and electricity in solid state except graphite. They have high heats of fusion.

MOLECULAR SOLIDS Molecular solids consist of molecules which are held together by weak Vander waals forces of attraction. The crystal in which the molecules with no charge are the lattice points which are held together by weak force of attraction. Examples: P4, S8, solid CO2 etc.

CHARACTERISTICS OF MOLECULAR SOLIDS These solids are generally soft in nature. They have low melting and boiling points due to weak Vander waal’s forces. These solids are generally bad conductor of heat and electricity. They have generally low density. These solids are generally bad conductors of heat and electricity.