Bonding in Solids Viva Questions

Bonding in Solids Viva Questions

Bonding in Solids Viva Questions, Short Questions on Bonding in Solids, Viva Questions on Bonding in Solids, Engineering Viva Questions, Material Science Engineering Viva Questions

Material Science Engineering Viva Questions

Crystal Structure of Materials Viva Questions

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Q.1. Define the primary and secondary bonds.

Ans. A primary bond is formed only if there is a pronounced lowering of the energies of the electrons involved during the formation of the bond. Primary bond is stronger and more stable than secondary bond. There are three principal types of primary bonds:

1. Metallic bond
2. Ionic bond
3. Covalent bond

Typical examples of secondary bonds are:

1. van der Waals bond
2. Hydrogen bond
3. Dispersion bond
4. Dipole bond

Few materials have pure bonds of one type or the other. Mixed bonds are mostly found in common materials.

Q.2. Describe the ionic bond and also describe the characteristics of bonds.

Ans. Ionic bonding is formed between two oppositely charged ions, which are produced by the transfer of electrons from one atom to another. An ionic bond is really the attractive force existing between a positive and a negative ion when they are brought into close proximity. These ions are formed when the atoms involved lose or gain electrons in order to stabilize their outer shell electron configuration. Elements are classified as either electropositive or electronegative, depending upon whether they tend to lose or gain electrons in order to achieve this stable outer-shell electron configuration. Thus, metals are termed Electropositive while non-metals are Electronegative, these terms referring to the abilities of the elements to form positive and negative ions respectively under favorable conditions during chemical reaction.

An ionic bond should possess the following characteristics:

1. There is no fixed directed force of attraction in ionic crystals.
2. The crystal planes strongly resist any tendency to slip past each other.
3. Although the ionic crystals are strong, they are likely to be brittle.
4. They have very little elasticity and cannot easily be bent or worked.
5. Melting points of these crystals are generally high.

Q.3. What do you understand by the term covalent bond?

Ans. The covalent bond is found in a wide variety of materials since it can be formed between atoms of the same or different elements. Stable covalent bonds are formed in the solid state primarily between non-metallic atoms like nitrogen, oxygen, carbon, and chlorine families. Other elements like silicon, germanium, arsenic, and selenium form bond that are partly covalent, partly metallic. It is also found in many of the organic compounds such as benzene, alcohol, turpentine, chloroform and members of the paraffin series which contain mainly the elements carbon, hydrogen and oxygen. An important example of covalent bonding is found in polymeric materials, which include rubbery elastomers, high strength fibres and a variety of structural materials denoted by the term plastic.

The number of covalent bonds that can be formed by an element is determined by the number of electrons that can be added to the valence shell without exceeding δ. According to this rule the maximum number of covalent bonds is (8 – N), where N is the number of valence electrons.

In covalent bonding, the crystals are strong, hard and have high melting points and low electrical conductivity.

In comparison to ionic bonds, covalent bonds have fixed direction. However, covalent crystals are similar to the ionic crystals in that there are no larger groups of atoms that differ from the fundamental unit of which the lattice is made, so that the whole crystal may be considered as a molecule.

Q.4. Describe the difference between ionic bond and covalent bond.

Ans. An ionic bond is formed between two types of atoms:

1. Atoms which have low ionization energies. Such atoms lose their valence electrons easily and form positive ion.
2. Atoms which have high electron affinities. Such atoms readily accept an extra electron or two to form stable negative ions. An ionic bond is the result of attraction between these charged ions.

Covalent bonds are formed due to presence of electrons between the adjacent atoms.

1. Ionic solids are crystalline in structure. Covalent solids are also crystalline.
2. Ionic solids have high melting and boiling points as compared to the covalent solids.
3. When solid, ionic compounds arc good insulators of electricity. Same is the case with covalent solids. However, when certain impurities are added to covalent solids they become reasonably good conductors and are termed as semi-conductors.
4. Ionic solids are soluble in water, liquid ammonia, etc Covalent solids are not soluble in water; however, they are easily soluble in organic solvents like benzene etc.

Q.5. Define the mixed bonds.

Ans. Bonding between atoms in many materials cannot be classified as one of the four ideal types but rather a mixture of those types. The metals gradually change from pure metallic bonding as in sodium to the less perfect metals such as tellurium and arsenic finally reaching the pure covalent bonding of carbon in the form of diamond. Starting at diamond, we find graphite, benzene rings and high polymers. Ultimately, we reach the rare gases and pure Van der Waals bonding. Layer lattices develop between ionic and metallic bonding with various detective structures formed between ionic and metallic bonding. In general, there are no mixtures between metallic and van der Waals bonding.

Q.6. Define bond length. Explain the factor affecting bond length.

Ans. When the two atoms of a material approach each other under the influence of externally applied force, they are drawn together until equilibrium spacing is reached. This spacing between the two atoms is called bond length is denoted by r₀. The force of attraction just balances the force of repulsion. hence (r) = 0. The bond length is smaller in primary bond than those in secondary bonds. Range of bond lengths are of the order of

r₀ = 1 to 2 Ȧ for primary bonded materials,

r₀ = 2 to 5 Ȧ for secondary bonded materials.

If the atoms are pulled apart from their equilibrium spacing (r > r_d), an attractive force is built-up. Contrary to this, repulsive force dominates when atoms are pushed closer (r < r₀). This repulsive force pushes the atoms back to their equilibrium spacing (r = r₀). Thus, atoms in a material cannot remain in the condition of r < r₀.

Factors affecting the bond length: Bond length is influenced by temperature. The bond length r₀ at 0 K has no thermal energy. At higher temperatures, the bond length increases. This is due to thermal expansion of the material.

The bond length is helpful in determining diameters of atoms and ions. Bond formation may be between the atoms or the ions. Hence, bonding may also be referred as

1. Atomic bonding
2. Ionic bonding

When two adjoining atoms of the same type form a bond. it is termed as atomic bonding. In case of ionic bonding, the two adjoining atoms are of different types.