Intermolecular forces and thermal energy are the two factors on which physical states of matter depend. While the intermolecular forces of attraction tend to keep the particles closer; the thermal energy tends to keep the particles apart from each other by making them move faster.
When the net resultant of these two opposing forces, i.e. intermolecular forces and thermal energy, makes the particles cling together and forces them to occupy fixed positions, matters exist in solid state.
Characteristic properties of solid state:
- Solids have definite mass, volume and shape
- Solids are incompressible and rigid
- In solids, intermolecular distances are very short
- In solids, intermolecular forces are very strong
- The constituent particles of solids have fixed positions.
- The constituent particles of solids can only oscillate about their mean positions.
Classification of Solids
Solids can be classified into two types on the basis of the arrangements of their constituent particles (atoms, molecules or ions). These two types are Crystalline Solid and Amorphous Solid.
Solids having large number of crystals; each with definite characteristic geometrical shape; are called crystalline solids.
The constituent particles of crystalline solid are arranged in regular pattern which is repeated periodically over the entire crystal. Such type of arrangement is called long range order. Crystalline solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive index, etc. are different along different axes. Crystal of NaCl, Quartz, Ice, HCl, Iron, etc. are some examples of crystalline solid.
Characteristics of crystalline solid
- Crystalline solids have definite characteristic geometrical shape.
- Crystalline solids have sharp characteristic melting point.
- Crystalline solids have definite and characteristic heat of fusion.
- Crystalline solids produce pieces with plain and smooth surface when cut with a tool of sharp edge.
- Crystalline solids are anisotropic in nature.
- Crystalline solids are true solid.
- Constituent particles of crystalline solids are arranged in long range order.
Solids having irregular shapes of particles are known as Amorphous Solids. The word ‘Amorphous’ came from Greek ‘Amorphos’ which means no shape.
The constituent particles of amorphous solids have only short range order of arrangement, i.e. regular and periodical arrangement of particles is seen to a short distance only. The structures of amorphous solids are similar to that of liquids. Glass, rubber, plastics, etc. are some of the examples of amorphous solids. Amorphous solids are isotropic in nature, i.e. physical properties of amorphous solids are same in all directions.
In old buildings, it is often seen that glasses of windows get slightly thickened at bottom, this happens because glass which is an amorphous solid; flows down very slowly. Some very old glasses get milky appearance because of some crystallization. This happens because on heating, glasses get crystallized at some temperature. This is the cause; amorphous solids are also known as Pseudo Solids or Super Cooled Liquids.
Characteristic of amorphous solid
- Particles of amorphous solids are irregular in shape.
- Amorphous solids soften gradually over a range of temperature.
- Amorphous solids produce pieces of irregular shapes when they are cut into two pieces.
- Amorphous solids do not have definite heat of fusion.
- Amorphous solids are isotropic in nature, i.e. they have same physical properties in all directions.
- Amorphous solids are not true solids and hence these are also known as Pseudo Solid or Super Cooled Liquid.
- The arrangement of constituent particles is in short range order.
Amorphous solids are very useful but most of the solids are crystalline in nature. Crystalline solids are classified into four types; based on the intermolecular forces operating in them.
- Molecular Solids
- Ionic Solids
- Metallic Solids
- Covalent solids
Molecular Solids Solids having molecules as their constituent particles are called Molecular solids. For, example, Hydrogen, Chlorine, Water, HCl, solid carbon dioxide, sucrose, etc.
Molecular solids are classified into three types on the basis of their bond:
- Non-Polar Molecular solids
- Polar Molecular Solids
- Hydrogen Bonded Molecular Solids
(a) Non Polar Molecular Solids Solids which are comprised of atoms only, such as helium and argon or molecules; formed because of the non polar covalent bonds are known as Non-Polar Molecular Solids. For example – H2, Cl2, I2, etc.
Characteristic of Non-Polar Molecular Solids
- The molecules of non-polar molecular solids are held together by weak dispersion forces or London forces.
- Non-Polar Molecular Solids are soft.
- Non-polar molecular solids are non-conductor of electricity.
- Non-polar molecular solids have low melting points.
- Non-polar molecular solids are usually in liquid or gaseous state at the room temperature and pressure.
(b) Polar Molecular Solids The solids which are formed by polar covalent bonds are known as Polar Molecular solids. For example – HCl, SO2, NH3, etc
Characteristic of Polar Molecular Solids
- The molecules in polar molecular solids are held together with dipole-dipole interactions.
- Polar molecular solids are generally soft in nature.
- Polar molecular solids are non-conductor of electricity.
- Polar molecular solids have higher melting points in comparison to non-polar molecular solids.
- Most of the polar molecular solids are gases or liquids at room temperature and pressure.
- Solid SO2 and solid NH3 are some examples of polar molecular solids.
(c) Hydrogen bonded Molecular Solids The molecules of hydrogen bonded molecular solids contain polar covalent bond between H and O, F or N. In solids such as H2O (ice) molecules are bound together strongly with hydrogen bond. HF, H2O (ice), etc are the examples of hydrogen bound molecular solids.
- Hydrogen bound molecular solids are generally volatile liquid or soft solids at room temperature and pressure.
- Hydrogen bound molecular solids are non-conductor of electricity.
Ionic Solids Solids, in which ions are the constituent particles, are called ionic solids. These solids are formed because of three dimensional arrangements of cations and anions bound together with strong electrostatic forces (coulombic forces). For example NaCl.
Characteristics of Ionic Solids
- High melting and boiling points.
- Non-conductor of electricity in solid state.
- Conductor of electricity in molten state.
- Conducted electricity when dissolved in water.
Metallic Solids All metals are referred as Metallic solids. Their constituent particles are positive ions. These positive ions are surrounded by free moving electrons. For example – iron, aluminium, etc.
- High melting points.
- Good conductors of electricity and heat.
- Lustrous, and are of specific colors.
- Hard but malleable and ductile in nature
Covalent Solids Crystalline solids are formed by non metals because of formation of covalent bonds between the adjacent molecules throughout the crystal. These are also known as Network Solids. These are also called giant molecules. For example – diamond, graphite, silicon carbide, etc.
Characteristic of Covalent Solids
- They are very hard and brittle except graphite which is soft.
- Very high melting points.
- Do not conduct electricity except graphite.
- Also called giant molecules.