Answer: Depending upon the nature of intermolecular forces present in the constituent particles, crystalline solids are classified into the following four classes:
i. Molecular solids
ii. Ionic solids
iii. Metallic solids
iv. Covalent solids
a. Molecular solids are those solids in which the constituent particles are molecules of the same compound.
b. These are further subdivided into the following three categories:
1. Polar molecular solids
2. Non-polar molecular solids
3. Hydrogen bonded molecular solids.
Polar molecular solids:
a. In these crystalline solids, the constituent particles are covalently bonded polar molecules like HCl, SO2, etc.
b. Polar molecules possess permanent dipole moment and in solid state they are held together by strong dipole-dipole interaction.
c. There is a separation of positive and negative charges because of the polar nature of molecule.
Hence, the polar molecules arrange themselves in such a way that opposite charges of the neighbouring molecules are brought closer.
Polar molecular solids : Characteristics:
a. They are soft.
b. They do not conduct electricity.
c. The melting points of the solids are relatively low, as the bonding is relatively weak. eg. Solid SO2,, solid NH3 and solid HCl.
Non-polar molecular solids:
a. They comprise of either atoms or molecules formed by non-polar covalent bonds.
b. In these solids, the atoms or molecules are held by weak dispersion forces or London forces.
Non-polar molecular solids : Characteristics:
a. They are soft.
b. They do not conduct electricity.
c. They have very low melting points (lower than polar molecular solids) and are usually in liquid or gaseous state at room temperature and pressure. Iodine (I2) exists in solid state even at room temperature.
eg. Non-polar molecules like CO2, H2, Cl2, CH4 H2O, and weakly polar molecules like CO and other hydrocarbons form non-polar molecular solids at relatively lower temperatures.
Hydrogen bonded molecular solids:
a. These crystalline solids consist of hydrogen atom covalently bonded to strongly electronegative atom like oxygen, nitrogen or fluorine.
b. In these molecules, the hydrogen atoms acquire additional positive charge and form additional bond with strongly electronegative atoms in the vicinity. This additional bond is called hydrogen bond.
Hydrogen bonded molecular solids: Characteristics:
a. They do not conduct electricity.
b. These solids exist as liquids or gases at room temperature and pressure. c. On cooling, the liquids solidify.
eg. H2O, NH3
a. Ionic solids are crystalline salts formed by the three dimensional arrangement of cations and anions bound by electrostatic forces.
b. Two types of electrostatic forces are present. One is the force of attraction between the ions carrying opposite charges. The other is the force of repulsion between the ions carrying same charges.
c. The arrangement of ions depends on following factors:
1. Sizes of the cation and anion
2. The charges on the ions
3. The ease of polarisability of the anion
Ionic Solids: Characteristics:
a. These solids are hard and brittle in nature. They have high melting and boiling points.
c. However, in the molten or fused state or when dissolved in water, the ions become free to move about and thereby conduct electricity.
d. When shearing force is applied, ionic crystals distort and the crystal structure is fractured.
eg. NaCl, ZnS, CuCl, etc.
a. Metallic solids are the crystalline solids formed by atoms of the same metallic element.
c. These electrons are mobile and are evenly spread throughout the crystal. Each metal atom contributes one or more electrons towards the sea of mobile electrons.
d. The force of attraction between positively charged metallic ion and negatively charged sea of delocalised electrons is called metallic bond. Metallic bonds are stronger than ionic and covalent bonds.
Metallic solids: Characteristics:
a. Metallic solids are good conductors of heat and electricity due to the presence of mobile electrons.
b. Metallic solids are tougher than other solids due to presence of strong metallic bonds. The nature of metallic bonds is such that any shape can be given to the metallic article.
c. The metallic solid contains several layers of the metallic ions arranged over one another. These layers can slide on other layers. Hence, the metallic solids are malleable and ductile. They can be hammered or rolled into thin sheets of desired thickness and can be drawn into thin wires of required size.
d. Alloys can be formed by fusing the mixture of metals. Alloys show all properties of metals.
e. Metallic solids posses metallic lustre due to which the surface appears grey or silvery.
eg. The surface of gold appears yellow whereas the surface of copper appears reddish.
Covalent or network solids:
a. Covalent solids are those in which the constituent particles are non-metal atoms linked to the adjacent atoms by covalent bonds throughout the crystal.
b. A network of covalent bonds is formed and the covalent solids are also called giant solids.
Covalent or network solids:Characteristics:
a. Depending on the type of covalent bonding present between the atoms, covalent solids are either hard or brittle.
b. They have extremely high melting points. Covalent solids are good conductors of electricity if mobile electrons are available, otherwise they are insulators.
eg. Three allotropic forms of carbon i.e. diamond, graphite and fullerene.