Distinguish between the items in the following pairs. a . crystalline solid; amorphous solid b. ionic solid; molecular solid c. molecular solid; network solid d . metallic solid; network solid

Interpretation:

                 Differences between crystalline solid and amorphous solid, ionic solid and molecular solid, molecular solid and network solid, metallic solid and network solid have to be explained.

Concept introduction:

• Solids constitute the major part of the matter in the universe. Beneath the earth and above the sky in our Universe, as well as in our everyday life, solids can be found. Solids do have such a profound significance since the beginning of the Universe and human evolution.

• A solid is anything that is firm and stable in shape. Physics and Chemistry do provide clear cut explanation for the structure of solids. The firm and dense nature of solids is due to the strong intermolecular forces between their components which are nothing but molecules or ions.

• On the basis of the arrangement of the components of a solid, there are two distinct types of solids viz., crystalline solids and amorphous solids. These two types differ in the arrangement of their respective components and so in their properties.

• Crystalline solids have their own sub-classification. The types of solids can be summarized as follows –

• Crystalline solids have well-defined regular, compact, orderly arrangements of their components in a very long range order. They are termed as true solids. Amorphous solids lack such well defined arrangement of its components thus disordered or random arrangement does exist in them. They are termed as pseudo solids or super cooled liquids.

• Ionic solids, molecular solids, covalent solids and metallic solids are the types of crystalline solids.  The components that are found to be arranged in regular, compact, three dimensional patterns are ions, then it is an ionic solid.

• If molecules are arranged in such a fashion, then it is molecular solid whereas metal atoms are arranged in such a manner, it is metallic solid. In Covalent solids the components are atoms bonded by covalent bond repetitively and thus forms huge network form of solid.

Analyze and  contrast the properties of crystalline and amorphous solids.

• Arrangement of components:

• Crystalline solids have a regular, compact and well-defined orderly arrangement of their components in a long range order whereas amorphous solids do not have such an orderly arrangement instead a random arrangement of components is observed.

• Intermolecular forces:

             Crystalline solids have relatively strong intermolecular forces than amorphous solids.

• Melting point:

            Crystalline solids melt at definite temperature whereas amorphous solids do not.

• Cleavage:

• Crystalline solids give clear cleavage when cut with a sharp object whereas amorphous solids give irregular cut.

• Rigidity and deformation:

Crystalline solids are very rigid and not compressible and high force is needed to deform them. Amorphous solids are non-rigid and can be deformed by applying moderate force.

• Isotropy and anisotropy:

            Crystalline solids exhibit anisotropy whereas amorphous solids exhibit isotropy.

              Crystalline and amorphous solids do differ widely in the arrangement of their respective components. The structural variation results in the difference between their respective properties. Since the components are regularly and orderly stacked in a crystalline solid, the intermolecular forces are stronger and sustained. In amorphous solid the intermolecular forces are weaker due to disorderly arrangement.

             Each component in a crystalline solid is considered as a “unit cell” which is the simplest and basic unit of a crystal. The unit cell repeats itself orderly in a regular fashion and builds a plane known as lattice plane. A crystal has several lattice planes likes this arranged orderly.

            The strong intermolecular forces exist in the crystalline solids provides resistance to deform to a certain extent by mild forces. In case of amorphous solids, deformation occurs easily than in the crystalline solids.

             The evenly distributed intermolecular forces make crystalline solids to melt at definite temperature whereas in amorphous solids there is no such existence and hence melt over a varying range of temperature. For example, glass, being an amorphous solid melts gradually over a certain range of temperatures around 1400o to 1500o C but NaCl a crystalline solid has definite melting point of 801oC.

            Crystalline solids when cut with a sharp object, the neatly stacked unit cells in the lattice plane gives clean cleavage to the crystal. Amorphous solids do lack such feature and gives irregular cleavage.

            Crystalline solids are generally anisotropic that mean their properties differ with respect to directions but amorphous solids are isotropic and exhibit same properties in all directions.

Analyze and contrast the properties of ionic solids and molecular solids.

• Appearance:

            Ionic solids are hard and brittle. Molecular solids are soft to some extent.

• Intermolecular forces:

The intermolecular force exist in ionic solids is known as “electrostatic force of attraction” whereas in molecular solids it is known as “Van der Waals forces”.

• Density:

Ionic solids are denser than the molecular solids.

• Melting point:

Melting point of ionic solids is high whereas that of molecular solids is low to moderately high.

• Mechanical strength:

• Ionic solids are rigid, brittle and cannot be deformed easily whereas molecular solids are not very rigid and can be deformed.

• Solubility:

Ionic solids are soluble in polar solvents and insoluble in non-polar solvents.                   Molecular solids are soluble either in polar or non-polar solvents.

• Conductivity:

Ionic solids have good conductivity but molecular solids have poor conductivity.

          Both ionic and molecular solids belong to the category of crystalline solids. Ionic solids have ions neatly stacked in an orderly fashion of a long range order. In molecular solids, molecules are regularly and orderly arranged in a long range order.

         Ionic solid consists of ions of opposite charges that are cation and anion. These two ions are held together as a compound by strong intermolecular force called electrostatic force of attraction. Molecular solids consists molecules as their components. The like molecules are held together by weak Van der Waals forces such as London dispersion forces, dipole-dipole forces and hydrogen bonding. Among these three, “hydrogen bonding” is the strongest.

         The strong intermolecular force is responsible for the rigidity, brittleness and high density of the ionic solids and such instance is not found in molecular solids so they are not hard, brittle and dense as ionic solids and deformed quite easily.

         Electrostatic attraction is the strongest known force. The ions in the unit cells must be held together so tightly and strongly that high energy is required to break the lattice points in the crystal of ionic solids. Hence, they melt under high temperature. But in case of molecular solids the weak Van der waals force does not make their lattice sites so strong that, low to moderate temperature is enough to break the lattice sites. Hence, they melt under low to moderate temperature itself.

       Ionic solids shows good conductivity because, when in dissolved and molten state they exist as individual ions of positive and negative charges which are highly mobile enough to exhibit conductivity. Many ionic solids in aqueous state are good electrolytes. But molecular solids are made of molecules which are almost neutral and unable to exhibit conductivity