|Unit – II B|
Classification of Crystalline Solids
B) Ionic Solids:
- Ions are the constituent particles of ionic solids.
- The ionic salts are formed by molecules containing positively charged smaller in size cations and negatively charged relatively bigger anions
- The charges on cations and anions balance each other hence the solid is electrically neutral.
- Such solids are formed by the three dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces.
- These solids are hard and brittle in nature.
- They have high melting and boiling points.
- Since the ions are not free to move about, (due to strong electrostatic force) they are electrical insulators in the solid state. However, in the molten state or when dissolved in water, the ions become free to move about and they conduct electricity.
- Examples: Sodium chloride, potassium bromide, copper nitrate, copper sulphate
C) Metallic Solids:
- Metals are orderly collection of positive ions (called kernels) surrounded by and held together by a sea of free electrons(delocalised). These electrons are mobile and are evenly spread out throughout the crystal.
- Each metal atom contributes one or more electrons towards this sea of mobile electrons. These free and mobile electrons are responsible for high electrical and thermal conductivity of metals. When an electric field is applied, these electrons flow through the network of positive ions. Similarly, when heat is supplied to one portion of a metal, the thermal energy is uniformly spread throughout by free electrons.
- Another important characteristic of metals is their lustre and colour in certain cases. This is also due to the presence of free electrons in them.
- Metallic bonds are stronger than covalent and ionic bonds. It is due to the adjustment of sea of electrons to the new arrangement of kernels.
- Metals are highly malleable and ductile.
- The malleability and ductility of metals is due to the adjustment of the sea of electrons to the new arrangement of kernels in the solid.
- Examples: All metals