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Class-12th Unit-01 Solid State Chemist Past.l Vishwa Vivek Sharma Chemistry faculty
Classification of compounds on the basis of binding force The classification of solid substances on the basis of intermolecular attraction forces involved in them, can be made into four classes (1) Molecular solids (2) lonic solids (3) Metallic solids and (4) Covalent network solids. (1) Molecular solids : The molecules present in the molecular solid are its constituent particles. They can be classified into three parts as below: Non -Polar molecular solids Polar molecular solids Molecular solids containing hydrogen bond
(a) Nonpolar molecular solids: Elements like argon, helium or molecules formed by nonpolar covalent bonds. Elements like dihydrogen, dichlorine, diiodine etc. can be included in this type of solids The atoms or molecules in this type of solid possess the weak dispersion forces or London forces. You have studied about these types of forces in standard-11. Solids of this type possess low melting points and they are soft and non conductor of electricity. At normal temperature and pressure, they are in liquid or gaseous state. (b) Polar molecular solids : The solids like SO2 etc. generally possess polar covalent bond The molecules of such solids are bound by relatively strong polar-polar interactions. They are also soft and nonconductor of electricity. Their melting points are higher than those of nonpolar molecular solids Even then, they are in gaseous or liquid state at normal temperature. Soild SO, and solid NHz are solids of this type. (c) Molecular solids containing hydrogen bond In this type of solids, atoms like H form polar covalent bond by combining with electronegative atoms like F, O or N. The strong hydrogen bonding in them, keep them in bound state. Substance like ice is the example of this. They are also non conductors of electricity. At room temperature and pressure they are volatile liquids or soft solids. In the molecule of ice, four molecules of water are attracted by hydrogen bond. It gets separated from the hydrides of other elements of the same group because of hydrogen bond
(2) Ionic solids: The constituent particles of ionic solids are ions. The cations and anions of such substances are attracted by strong Coulombic forces and so they possess forced molecules in three dimensional directions. They are hard and brittle. Their melting points and boiling points are higher because of strong atraction between positive and negative ions in the solid. Though ions are present in such substances, even then, they do not conduct heat or electricity because they are bound by strong attraction forces between positive and negative ions. But if they are dissolved in water or are melted in solid state, their ions get separated and so they conduct electricity
(3) Metallic solids : Most of the metals are in solid state and so metal possesses positively charged nucleus around which free electrons are arranged. They are arranged in a systematic way. Electrons are spread in the form electron sea around the nucleus of atom of the metal. These electrons do not remain confined to any nucleus but are moving around the nuclei like the flow of water in the sea and these free electrons allow electricity and heat to conduct. When electric field is applied, they move, but in the new situation electron makes its arrangement around the nuclei. When heat is supplied to metal, the thermal energy is spread everywhere by the free electrons. In addition, metals have lustre and in some cases have colours. This is due to the presence of free electrons. They are ductile and malleable.
(4) Covalent solids: In the whole crystal, the adjoining atoms form covalent bond in larger proportion and so it results into different crystalline solids. They are called Giant molecules". Many diversities are found in these compounds because they possess covalent bond-directional properties. Their molecules are bound tightly and hence they possess very high boiling points. Some decompose before being melted. They are not conductors of electricity. Diamond and graphite are the examples of this type. Graphite is soft and good conductor of electricity because its structure is specific. Three atoms of carborn in graphite form three covalent bonds by sp2 hybridisation and the fourth electron remains free. It keeps one layer of graphite combined with other layer. Thus layers consisting of hexagonal structure having sp hybridisation are formed in graphite and the fourth electron keeps distance of 340 pm between the layers and it conducts electricity because it is free. The different layers of graphite can move creating friction with each other and so graphite is a good solid lubricant. In contrast to this, tetrahedral structure is formed by sp' hybridisation in diamond and all its four corners are combined with other carbon atoms of diamond and spread in three dimension. Hence, it is very hard. It cannot conduct electricity because it has no free electrons. Thus, it can be seen that in the two allotropes of the same element, if intermolecular forces and hybridisations formed are different, there is a great change in their properties.
Different types and properties of solid Electrical conductivity Type of Constituent Attraction Example Physical Melting solid particles forces nature point (1) Molecular (i) Non polar Molecules Dispersion or (ii) Polar iii) Hydrogen Molccules Hydrogen solid Ar, CC14 Soft Very low Non conductor London forces H. I2. CO Molecules Dipole-Dipole- HCI, sO2 Soft Low Non conductor inter attraction NH H2O (ice) Hard LoNon-conductor bond bond possessing (2) Ionic solid Ions Coulombic or NaCl, MgO,| Hard but High ElectrostaticZns, CaF2 brittle Solid state, non- conducting but molten or aqucous solution conductor Positive ion Metallic bondFe, Ca, in sea of delocalised electrons Atoms (3) Metallic solid Hard but ComparaConductors in tively very solid and molten high Mg. Ag and malleable Hard states Covalent bond Sio2 (Quartz) SiC (Carbo- randum C (Dimond) C (Graphite) (4) Covalent Very high Non conductor or Network solids Conductor (Exception)
Crystalline and amorphous solids Property Crystalline solid Amorphous solid Irregular shape (1) Shape (2) Melting pointDefinite and sharp melting point whichBecomes soft grandually during the (3) Fusion enthalpy Definite and characteristic fusion (4) Cleavage Definite shape and possessing characteristic gcometry is the characteristic of the crystal of small temperature range i.e. have no solid. definite and sharp melting point Not definite and characteristic fusion enthalpy Divided into two parts cutting the enthalpy Divided into two parts by cutting the crystal with sharp tool like knife. The crystal with sharp tool like knife but surface of the new part obtained is plain and soft i.e. it is as original one True solid The order is maintained till a long range property the surface of new part obtained is not as the original one. It is irregular. Pseudo solid or supercooled liquids The order is maintained in a short range (5) Nature () Order of Arrangement of constituent particles The graph (temperatureime) obtained on cooling after heating is a curvature. Temperature range is The graph (temperaturetime) obtained on cooling after heating is not a curvature. The temperature remains definite during crystallisation obtained during crystallisation Their properties like electrical conductivity, thermal conductivity, mechanical strength and refractive index strength and refractive index are are different in different directions (7 Effect of temperature (8) Properties Their properties like electrical condu- ctivity, thermal conductivity, mechanical same in all directions
Vishwa Vivek Sharma
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