Material Science & Engineering : Chemical Bond

Introduction

Types of Bonds

Seitz in 1940 classified solids into five types according to the bonding of atoms, which has becomes a generally adapted classified 

Table below Classification of solids according to the bonding of atoms

The atoms of different types come closer and join together during chemical reaction and usually these bonds are referred to as chemical bonds.

FORCES BETWEEN ATOMS: MECHANISM OF BOND FORMATION AND BOND ENERGY

We have already stated that the interatomic forces exist amongst the atoms of a crystal and are responsible for holding atoms together to form solid structure. We have also said that the, forces between atoms can be of two kinds: (i) attractive forces which keep the atoms together, and (ii) repulsive forces which come into play when the solid is compressed. The force of attraction between atoms brines them closer together until the individual election clouds begin to overlap and a strong repulsive force arises to comply with Pauli’s exclusion principle. The attractive forces decrease the potential energy of the system and repulsive forces increase it Obviously, the net energy of the system is equal to the algebraic mm of these two energies. When the attractive force and the repulsive force between any two atoms are equal, the two atoms should be in a stable situation with a minimum potential energy. The forces between two atoms or ions in a solid as a function of their distance of separation r are shown in Figure below.

Bond Length

The separation distance (r) between the centre to centre of two bonding atoms which gives the stable bond is called bond length. Obviously, greater the force of attraction between the two bonding atoms, smaller will be the bond length (r). We must note that the primary bonds are stronger than the secondary bonds. One can use the bond length for finding the diameter of ions or atoms as follows:

r = r_c + r_a, where r_c → radii of a cation and r_a → radii of anion

IONIC BONDING

Perhaps ionic or hetropolar bonding which is formed by the actual transfer of electrons from one atom to the other so that each atom acquires a stable configuration similar to the nearest inert gas atoms, is the simplest type of chemical bonding to visualize since it is almost totally electrostatic in nature. Ionic bonding occurs between electropositive elements (metals, i.e., those elements on the left side of the periodic table) and the electronegative elements (non-metals, i.e. on the right hand of the periodic table). These bonds are formed mainly in inorganic compounds, e.g., sodium chloride (common salt NaCl), MgO, CuO, CrO₂, and MOF₂. In MgO the ions are doubly ionized leading to a stronger interatomic bond and hence a higher melting point (), compared to 800°C for NaCl.

An ionic bond is truly speaking the attractive force existing between a positive and negative ion when they are brought into close proximity. These ions, of course, are formed when the atoms involved lose or gain electrons in order to stabilise their outer shell electron configuration. The cohesive energy of ionic crystal is quite large, it is of the order of 5 to 10 eV.

A good example of ionic crystals is the crystal of sodium chloride (NaCl). The electronic configurations of white soft metal Na and Cl atoms are as follows :

Na : 1s² 2s² 2p⁶ 3 s¹ [K (2) L (8) M(1)]

Cl : 1s² 2s² 2p⁶ 3s² [K (2) L (8) M(7)]

Na atom has a low ionisation energy and hence easily loses an electron; where as C₁ atom has a high electron affinity and strongly tends to acquire an electron. The reaction can be represented by

Na + Cl → Na⁺ + Cl^- → NaCl

Since chlorine exists as molecules, we can write the chemical reaction as

2Na + Cl₂ → 2Na⁺ + 2Cl^- → 2NaCl

After the transfer of an electron from 3s orbital of Na to 3p orbital of C1, the electronic configurations become Na⁺ : 1s² 2s² 2p⁶ (same as Ne)

Cl^- : 1s² 2s² 2p⁶ 3s² 3p⁶ (same as Ar)

Covalent Bond

This type of bonding is formed by an equal sharing of electrons between two neighbouring atoms each having incomplete outermost shells. The atoms do so in order to acquire a stable electronic configuration in accordance with the octet rule. Unlike ionic bonding the atoms participating in the covalent bond have such electronic configurations that they cannot complete their octets transfer of electrons from one atom to the other. Obviously, there is no charge associated with any atom of the crystal, The majority of solids incorporating covalent bonds are also bound by either ionic or Vander Waals bonds.

Characteristics of Covalent Compounds

(i) Covalent compounds are mostly gases and liquids

(ii) They are usually electric insulators.

(iii) They are directional in nature.

(iv) They are insoluble in polar solvents like water but are soluble in non-polar solvents, e.g., benzene, chloroform, alcohol, paraffin sets,

(v) Covalent compounds are homopolar, i.e. the valence electrons are bound to individual or pairs of atoms and electrons cannot move freely through the material as in the case metallic bonds.

(vi) Covalent compounds are soft, rubbery elastomers, and form a variety of structural materials usually termed as plastics. The melting and boiling points of these compounds are low.

(viii) In the exceptional case of diamond covalent bonding is very strong due to the very large cohesive energy. Usually the binding energy ranges from 1 to 5 eV. The strong covalent bonding makes diamond very hard and with a high melting point.

Metallic Bond

Metallic bond is formed by the partial sharing of valence electrons by the neighbouring atoms. They are formed by the elements of all subgroups A and I-II, subgroups B. Unlike the case of covalent bond, the sharing in metallic bond is not localized. Hence metallic bond nay considered as delocalized or unsaturated covalent bond. Metallic bonds are electropositive. When interacting with elements of other groups, atoms in a metallic crystal can easily give off their valence electrons and change into positive ions.

Comparison between ionic, covalent and metallic bonds

Hydrogen Bonds

This is a special type dipole bond, but it is considerably stronger. It is a special case of inter-molecular attraction produced between certain covalently bonded hydrogen atoms and one pairs of electrons of another atom. When a hydrogen atom is covalently bonded to a relatively large atom such as nitrogen, oxygen or fluorine, a powerful permanent dipole is set up. There is a positive field adjacent to the hydrogen and a negative field around the electron pair. One finds that this force is very important in plastics and in biological molecules such as DNA. A good example of hydrogen bond is water molecule.

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