Classification of crystalline solids and General Characteristics of ionic solids

   General Characteristics of Ionic solids       and Classification of crystalline solids 

Hello students  , 

     Today is chemistry Class .I hope this           note is helpful you .

        * Classification of crystalline solid :-

Most of the  chemical compounds that we some across in our daily life are solids at room temperature  . The particles constituting the solids may be atoms , ion or molecules . One of the most important feature of solids is that the constituent particles are most closely packed . As a result , they posses strong intermolecular forces of attraction . These strong molecular forces of attraction accounts for the rigidity , definite shape and definite volume in case of solids .

                    Solids can be broadly divided into two types - the crystalline solids and the amorphous solids . A crystalline solid consists of regular arrangement of atoms , ions or molecules throughout the Crystal , Whereas in case of an amorphous solid , the regular arrangement of particles occurs over a small regions . In other words , a crystalline solid has the regular arrangement over a long range whereas an amorphous solid has the regular arrangement over a short range.

                        Crystalline solids can be classified into four major types depending upon :- 

            Bonding                    Constituent

1 -  Intermolecular.               Molecules

2 - Covalent Network.           Atoms

3-  Metallic                           Positive ions 

                                              and electrons

4- Ionic                                 Positive ions                                                   and negative                                                     ions

 

Table :- 1 . Classification of crystalline                             solids by type of bonding 

 *General Characteristics of Ionic Solids :-

            

  The properties of ionic solids are different from those of covalent solids . Some of the Characteristics properties of ionic. Solids are described below :-

1- Crystalline nature , Ionic solids are               crystalline in nature :-

                                              X - ray studies reveal that ionic solids do not exist as molecules , but consist of ions with opposite charges .The ions are arranged in a regular pattern . 

        Each ion in the Crystal is surrounded by a fixed number of oppositely charged ions . This fixed number is called the

Coordination number . The arrangement of ions is different ions solids is different . For example, in sodium chloride , each Na+ ion is surrounded by six Cl- ion and each chloride ion is surrounded by six Na+ ions.

 2- Hardness :-

                            Ionic solids are hard . The hardness of the ionic solids is due to the strong electrostatic forces of attraction between the oppositely charged ions . Hardness in ionic solids increases with decrease in the interionic distances and increase in ionic charges .

3 High melting and boiling points :- 

                                                              Ionic solids have relatively high melting and boiling points . Ionic solids have high melting and boiling point because of the strong electrostatic interactions between the cation and the anion. As a result , a large amount of energy is needed to overcome the inter - ionic attraction to break the Crystal lattice . This accounts for their higher melting and boiling points .

                The melting and boiling points of ionic solids depend upon the electrostatic forces of attraction between the oppositively charged ions. The larger the electrostatic forces of attraction , higher is the melting points . The magnitude of the force of attraction depends upon the charges on the ions as well as ionic radii of the ions . In other words , the closer the ions in the ionic crystal , the larger will be the attractive forces and higher will be the melting point . 

 Table :-2. Melting point of alkali metal                           halides

An inspection of the table reveals that the melting points decrease from NaF to NaI . This is because as the size of the anion increases , the corresponding force of attraction between the Na+ ion and the corresponding anion decreases . This results in the decrease of melting point . Thus , NaI has the least melting point in the halides . 

 4 - Brittleness :- 

                             Ionic solids are brittle and are easily broken when a stress is applied on them.

An ionic crystal is built up of closely packed ions of opposite charged . When a stress is applied to the ionic solid there is a movement of layers of ions across the next layer . During this movement , the ions with similar charges come together . This case strong repulsions among the similar ions and results in the breakdown of the Crystal .

 Ions of like charges come near to each       other on the application of stress :-

5- Conductance :- 

                               Ionic solids do not conduct electric current at room temperature .

                        In an ionic solid , the cation and the anion occupy the fixed positions in the lattice and are held together by strong electrostatic forces . It means that the ions are not free to move . As a result , ionic solids do not practically conduct electricity in solid state .

              However , ionic solids in solution or molten state do not have a crystal lattice and the ions are not held together by strong electrostatic forces at all. Thus , the ions are set free to move in an electric field . As a result , the ions conduct electricity due to their movement in the solution or in the molten state .

6- Ionic reactions :- 

                                   ionic solids when dissolve in water , break down into oppositively charged ions . When aqueous solutions of two ionic solids are mixed together , the oppositively charged ions combine instantaneously to give the product . Becouse of their immediate reaction , these reactions are quite fast .

    Dissolution of ionic solid in water :-

7-  Solubility of ionic solids :-

                                               Ionic solids are soluble in water and in polar solvents with high dielectric constant . Ionic solids are generally soluble in water. The dielectric constant of water is high ( 80 ). The role of dielectric constant is to decrease the electrostatic forces of attraction between the ions in an ionic solid . When an ionic compound is dissolved in water , the electrostatic forces of attraction between the ions reduces to 1/80 the force of attraction between the ions in the solid state. As a result , the ion get separated from one another and from a solution . The ions are now free to move . Another important feature of water is that it acts as a dipole . with a positive and  a negative end . When an ionic solid is dissolved in water , the positive end of the water dipole interacts with the negative ion of the ionic solid while the negative end of the water dipole interacts with the positive and a negative end . When an ionic solid is dissolved in water , the positive end of the water dipole interacts with the negative ion of the ionic solid while the negative end of the water dipole interacts with the positive end of the ionic solid .  As a result , the orderly arrangement of the ions in the Crystal breaks and the ionic solid goes into  solution . It is important to mention here that the ionic solids do not dissolve in organic solvents organic solvents are either non - polar and have low dielectric constant .

     Dissolution of ionic solid in water :-

8- Non-directional nature :-

                                   In an ionic solid , the ions are arranged in a perfectly regular way , repeating over and over again in all the directions . As a result , the electrostatic forces holding the ions together are uniformly distributed around the ions . Thus , the bonding in ionic solids is non - directional .

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