Latest About Classification of Solids 2022

Crystal has been classified into four types on the basis of forces present between atoms, ions, or molecules in them. These forces are also called cohesive forces. These forces may be chemical bonds or some other interactions.

There are four types of crystalline solids on the basis of the nature of forces present in them.

A. ionic solids            B. Covalent solids            C. Molecular solids            D. Metallic solids


The crystalline solids, in which positively and negatively charged ions are held together by electrostatic forces of attraction, are called ionic solids. These forces are also called ionic bonds example NaCl, BaCl₂, K₂SO, etc.

Properties of Ionic Solids

1.  These are crystalline solids at room temperature, due to the definite arrangement of cations and anions

2.  Under normal temperature and pressure, these never exist as liquids or gases.

3.  These are stable compounds. In ionic crystals, ions are held together by strong electrostatic forces, which are difficult to break. Hence these are stable.

4.  These have high melting points and boiling points due to strong electrostatic forces.

5.  These have low volatility due to strong electrostatic forces.

6.  These are very hard. It is because, in these crystals, ionic forces are present which are non – directional. As a result, ions are attracted from all sides. Hence ions are closely packed and there is not much free space in these crystals.

7.  The structure of an ionic crystal depends upon the radius ratio. lonic crystals having the same radius ratio have the same geometry, e.g. both NaCl and CsF have the same radius ratio hence they have the same geometry i.e. cubic.

8. lonic crystals do not exist as individual, neutral, independent molecules. Therefore, the term formula mass is used to express their mass instead of Molecular mass.

9. lonic crystals do not conduct electricity in a solid state. Because in solid form, ions have a fixed position and therefore can not conduct electricity.

However, these conduct electricity in a molten state or in an aqueous solution. Because in these forms, crystal lattices are broken up and ions become free. These free ions conduct electricity. 10 ionic crystals are highly brittle.

10.  Ionic crystals are highly brittle.


It is because ionic crystal consists of parallel layers in which cations and anions are present in alternate positions. Thus, when a force is applied to the crystal, one layer of ions slides a little bit over the other layer. In this way like ions come in front of each other, which repel each other and thus a crystal is broken and shows brittleness.

11. Due to the close packing of ions in a crystal, these have high densities.

12. These are soluble in polar solvents like H₂O and produce ions

13. Since these crystals produce ions dissolving in polar solvents. Thus their reactions in polar solvents are very fast.

14. They show isomorphism and polymorphism.

Structure of NaCl Crystal

The structure of ionic crystals depends upon the structure and size of their ions.

NaCl crystal is face-centered cubic and consists of Na+ and Cl- ions.

Na+ has 10 electrons, while Cl- has 18 electrons. Thus CI is larger in size than Na+ ion.

Similarly each Cl- ion is also surrounded by six Na ions. Its coordination number is also six.

Calculation of the number of Na+ and Cl- ions per unit cell.

The number of Cr¹ ions.


The figure shows that in NaCl crystal two types of CI – ions are present.

(i)  8 Cl – ions are present at the corners. Each of these is shared by 8 unit cells.

      So share of 1 unit cell = 8/8=1

 (ii) 6 Ci – ions are present on the faces. Each of these is shared by 2 unit cells.

      So share of 1 unit cell =6/2=3

Hence one unit cell has 3+1=4 Cl- ions.

Number of Na+ ions

The figure shows that in NaCl crystal two types of Nations are present.

(i) 12 Na+ ions are present along the edges, each of which is shared by four unit cells.

      So share of 1 unit cell =12/4=3

(ii) 1 Na * ion is present at the center, which is shared only by one unit cell.

      Hence one unit cell has 3 + 1 = 4 Na+ ions .

Total number of Na ” and CI – ions

One unit of NaCl cell has 4 Na+ ions and Cl- ions. Thus four NaCl formula units are present in one unit cell. Hence, the ratio of Na+ and Cl- ions is 1:1.

Facts about NaCI  

In NaCl, the distance between two similar ions is 5.63 A˚.

Thus, the distance between two different ions is 5.63/2=2.185 A˚.

In NaCl crystal, no independent NaCl molecule is present. However, the NaCl molecules exist in the vapor phase.

The radius ratio of NaCl is 0.53.

Lattice Energy

The amount of energy released when one mole of the ionic crystal is formed from the gaseous ions is called lattice energy.

In this case, the energy is given a negative value.

OR The energy required to break one mole of solid into isolated ions in the gas phase is called lattice energy.

In this case, the energy is given a positive value.

Units: Lattice energy is measured in kJ mol-¹.


Na+        + Cl          →       NaCl             ∆H = -787 kJ mole – ¹

NaCl       →      Na+        + Cl                 ∆H = + 787 kJ mole-¹  

Lattice energy decreases with increase in size of cations or anions.

It is because due to the increase in size, the packing of ions becomes less tight. Thus, more empty spaces are present and the structure is loose. So, less amount of energy is required to break it. Hence, the lattice energy of the crystal decreases with an increase in the size of cations or anions.

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