Chemical bonding and their types

Chemical bonding and their types-

Chemical bond -
A bond is any force which holds two atoms together. The formation of bond between two atoms is due to some redistribution or regrouping of electrons to form a more stable configuration. Several atoms are assembled and held together to form thousands of molecules which participate in the building and function of physical and biological systems.

Types of chemical bond-
The regrouping of electrons in the combining atoms may take place in either of three ways-

(1) Electrovalent or polar or ionic bond======> by a transfer of one or more electrons from one atom to another.

(2) Covalent bond =======> by a sharing of one or more pairs of electrons between the combining atoms.

(3) Co-ordinate Bond =======> by a combination of the two processes of transfer and sharing of electrons.



(1) Electrovalent or polar or ionic bond-

Ionic bond formation takes place between atoms of strongly electropositive and strongly electronegative elements. An element preceding and inert gas in the periodic table is strongly electronegative and the element immediately following the inert gas is strongly electropositive. For example- chlorine is electronegative while sodium is electropositive. According to W. kossel (1916),a transfer of electrons takes place from the outermost shell of the electropositive atom to the outermost shell of the electronegative atom, resulting in the formation of stable positive and negative ions respectively which are held together by electrostatic forces of attraction to form a molecule or more precisely and ion pair .


A   +   B ------> A+  +  B-

The atoms involved are electrically neutral before combining. The element A which has lost its electrons is known as electropositive where as element B which has gained the electrons is termed electronegative element. The compound formed by electron transfer is termed as electrovalent by Langmuir (1919) because the resulting compound is electrolyte. It is also called Polar since the molecules develops a positive and a negative pole.
               The electrovalent compounds always exist in ionic form, are hard and nonvolatile, have high melting and boiling points because of stronger nature of the bond and are soluble in polar solvents and because of the presence of ions conduct electricity in solution or in the fused state. The electrovalent compounds having identical electronic configuration exhibit the phenomenon of isomorphism.

 Na   +   Cl  ---->   Na+    +     Cl-

 Ca   +   O  -----> Ca2+   +   O2-

(2) Covalent bond (Nonpolar Bond)-

Covalent bond formation first suggested by G.N. Lewis (1916) consists in sharing or holding a pair of electrons in partnership between two combining atoms, so that the pair counts towards the electronic grouping of both atoms. By this mechanism also, the stability akin to the inert gas is attained by each atom. For each pair of electrons to be shared between two atoms in each of the constituent atom contributes one electron.

   A•  +   B• ----->  A : B

This type of linkage which is the result of equal contribution and equal sharing of electrons is known as covalent bond. The compound formed by electron sharing is termed as covalent or or nonpolar by Langmuir (1919).
              The covalent compounds always exist in molecular form, are non electrolytes or non ionizable, soluble in organic solvents and have low melting and boiling points because of weaker nature of the bond. They are usually liquids or gases and are generally soft, easily fusible and volatile. They are non conducting in the fused state or in solution. The covalent bond is rigid and directional and as such there is a  possibility of position isomerism and stereoisomerism amongst these compound.
              covalent linkage is common in organic compounds, although inorganic compounds also have it. In covalent compounds one pair of shared electrons corresponds to a single bond, two pairs to double bond, three pairs of electrons or 6 shared electrons to triple Bond.
        Some common examples from three categories are-

 H •  +  H•  ------> H-H

   :            :
 :O:   +   :O:   --------> O=O

                                    _
 N:.  +   .:N   ------>  N=N
                                 

(3) Co-ordinate Bond (Semipolar or Dative Bond)-

Co-ordinate bond is also formed by mutual sharing of electrons but in this case the two electrons that are shared come from the same atom. The shared pair of electrons is called lone pair. The atom which provides the pair of electrons is called the donor and the atom accepting this pair is called the acceptor. After the formation of the bond, the lone pair of electrons is held in common. This sort of bonding is called co-ordinate (Sidgwick) or dative (Menzies). In this mechanism although the sharing is equitable the contribution is one sided and therefore a slight polarity develops in the molecule. For this reason this bond is also called semipolar (Sugden). This type of linkage is represented by an arrow pointing away from the donor atom (or pointing towards the acceptor atom). Usually the donor is an atom which has already acquired stable electronic configuration and the acceptor is generally two short of the stable configuration.

    :         :                  :   :
  :A:  +  B:  ------>   :A:B:
    :         :                  :   :
In this case atom A is dinor and atom B is acceptor because atom A is given a lone pair of electrons to atom B.

For example- In O3(Ozone) form a co- ordinate bond.

   :         :        :              :    :      :
 :O:  + :O: + :O: -----> :O::O:->O:
                                                  :