Classification Of Crystalline Solid online notes for 12th class chemistry

Classification Of Crystalline Solid online notes for 12th class chemistry

A crystal is defined as a homogenous, anisotropic substance, having a definite geometrical shape with surfaces that are usually planar and have sharp edges. Several types of binding forces are involved in holding crystals together; hence the physical properties of a crystal depend on the type of bonding.

Depending upon the type of constituent particles and the nature of intermolecular forces between them, the crystalline solids are classified into following categories:

Molecular Solids: In these solids the particles that are involved are molecules. Depending upon the nature of the molecules they are further divided into three types.

1. Non-Polar Molecular Solids -They possess the particles either atoms like those of noble gases or non-polar molecules like H2, Cl2, I2 etc. The attractive force between them is weak van der Waal force or dispersion force. Due to this weak force, they have low melting and boiling points, are soft in nature and non-conductors of electricity (no ions are present).
2. 2. Polar Molecular Solids – These are those solids in which constituent particles are polar molecules like HCl, SO2, etc., the force which held them together is dipole-dipole force of attraction. This force is stronger than the dispersion force of non-polar molecular solids. They are generally gaseous or liquid in nature at room temperature and pressure. These solids are soft, non-conductors of electricity and exist in gaseous or liquid form at room temperature and pressure. Their melting and boiling point is higher than non-polar molecular solids but not so much.
   Hydrogen-bonded molecular solids– In these types of solids, the constituent particles are bonded with a hydrogen bond. For example H2O, NH3, HF, etc. So they possess the strong hydrogen bond as an inter-molecular force. Due to this, they have a high melting and boiling point which are higher than the other two types of molecular solids. They exist as a volatile liquid or soft solids at room temperature and pressure.

Ionic Solids: In the ionic solids, the constituent particles are anions and cations. Each participating ion is surrounded by a typical number of opposite charges. The number of ions that surround a particular ion of the opposite charge is called the coordination number of the ion.

For example, in NaCl crystal, each Na+ ion is surrounded by 6 Cl- ions and each Cl- ion is surrounded by 6 Na+ ions. So, the coordination number of Na+ and Cl- is 6. Similar in CaF2 crystal, the coordination number of Ca+ and F- ions are 8 and 4 respectively. Some other examples are CaCl, ZnS, etc. These ions are held together with the strong electrostatic force of attraction.

Some common characteristics of ionic solids are given below:

1. They are hard and brittle.
2. The heat of evaporation is very high.
3. The melting points and boiling points are very high.
4. They are insulators in the solid state as the ions are fixed at one position, but in molten state or solution form, they are good conductors of electricity due to the mobility of ions.
5. The vapor pressures of such crystals at ordinary temperatures are very low

They are soluble in polar solvents as well as water, but insoluble in non-polar solvents.

Continue.... Classification Of Crystalline Solid online notes

For more information, videos lectures, study material, CBSE sample papers, Chemistry notes for Class 12, register with Takshila Learning.

Call us : 8800999280/8800999284