BONDING

The study of the solid structure shows that there are three strong principle types of primary bonding

1. Ionic bond.
2. Covalent bond.
3. Metalic bond.

These bonds are distinguished on the basis of the positions assumed by bond electrons during the formation of bond. There are typical example of secondary bond are –

1. Van-der Waals bond
2. Hydrogen bond

• Ionic bond

In ionic crystals one or more electrons of one type of atoms are transferred from one atom to another and the result being a crystal that is composed of positive and negative ions.

Example : NaCl

Characteristics of ionic bond

1. The ionic bond is fairly strong.
2. Due to high binding energies, the ionic crystal have high melting and boiling point.
3. The electrical conductivity of ionic crystals is much lower than that of a metal and the conductivity increases with increasing temperature.
4. The ionic crystals are often quite soluble in ionising solvents such as water, the solutions being highly dissociated into free ions.
5. Ionic crystals are easily soluble in polar solvent,  exm – water
6. The ionic crystals are usually crystallize in relatively closed packed stucture.
7. Due to the spherical charge distribution this bond is non-directional.

• Covalent bond

In This bonding, the valence electrons are not transferred from one atom to the other as in ionic crystal but the neighbouring atoms share their valence electrons under the formation of strong homopolar or covalent bonds. Thus covalent bonds are formed by the shared electron pairs between the valence electrons in the incomplete outer shell of atoms. .

Example: CH4, Cl2, HCl.

Characteristics of covalent bond

1. Covalent crystals are usually hard, brittle materials with quite high binding energies and thus have high melting, and boiling points.
2. Covalent bonds are strongly directional in character.
3. Covalent bonds have saturation property.
4. Covalent subtances are insoluble in polar solvents like water.
5. Most of covalent subtances do not conduct electricity because of the non availability of free electrons on charged ions to carry the current.

• Metalic bond

In metalic bonding, the valence electrons are not bound to individual atoms or pair of atoms, but move freely throughout the whole metal.

Example: Na, K, Cu.

Characteristics of metalic bonding

1. The metalic crystal have high electrical and thermal conductivity.
2. Metalic bonds being weak, metals have a melting point moderate to high.bor
3. Metals are opaque to light since the light energy is absorbed by free electrons.
4. Due to the symmetrical arrangements of the positive ions in a space lattice, metals are crystalline.

• Van-der-waals’ bond

The Van-der-walls bonding is used which arises from dipolar forces between the atoms or molecules of the crystal. It occours in elements or compounds with electronic configuration such that little electrons transfer occours between atoms.

Characteristics of Vander-walls bond (or molecular bonding)

1. The molecular have small binding energy.
2. They have low melting and boiling points.
3. They are poor electrical conductors.
4. They are usually transparent to electromagnetic radiations.
5. Their bond structure is almost unknown but the energy gap is large.

Essential difference between ionic bonding and covalent bonding in crystal 

1. The ionic bond is fairly strong and hence the ionic crystals have very high melting and boiling points. But covalent binding energy is strong but not as so strong like ionic bond and hence the covalent crystals have high melting and boiling point.
2. The electrical conductivity of ionic crystals is much lower than that of a metal and the conductivity increases with increasing temperature. The electrical conductivity of covalent crystals veries over a wide range. Some are excellent insulators like diamond. Others are fair conductors like Germenium Whose conductivity increases with increase in temperature.
3. Due to spherical charge distribution, ionic bond is non – directional. But covalent bonds are strongly directional in character.
4. The ionic crystals are often quite soluble in ionizing solvents such as water. But covalent crystals are hard, brittle material and insoluble.

 

• Similarity – Both the ionic and covalent crystals are transparent for all frequencies upto the point called the fundamental absorption frequency. At freequencies higher than fundamental absorption frequency, they are opaque

 

Read more –

CRYSTAL STRUCTURE | Classification of solids| Lattice and lattice points, Crystal structure| Bravais space lattice| Unit cell, Primitive cell| Miller indices.

CRYSTAL STRUCTURE | Packing fraction| sc , bcc, fcc| CRYSTAL DIFFRACTION| Bragg’s law |Von Laue equation of scattering vector

Crystal Structure | Reciprocal Lattice

Crystal Structure | Geometrical structure factor