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Chemistry: Chemical Bonding: Covalent Character of Ionic Bond

Covalent Character of Ionic Bond

  • Most of the covalent bonds have some ionic character. Similarly, ionic bonds also have some covalent character.
  • The origin of covalent character of an ionic bond lies in the distortion of electron cloud under the influence of an electric charge, this is called polarizability and this process is called as polarization.
  • The power of an electric charge to distort an electron cloud is called as polarizing power.
  • In an ionic compound, the cations and anions are formed as a result of loss and gain of electrons, respectively.
  • The cations are smaller in size, so they have more polarizing power and high charge density.
  • The anions which are larger in size are easily polarized.
  • The electrons in anions are more loosely bound to the nucleus than in cations.
  • The ionic bond sometimes has covalent character when the cation attracts the electron cloud of the anion that result in polarization which causes the electron clouds to spread out towards the cation that are present in between the two ions.
  • Thus, the electron lost by cation does not fully belong to the anion but partially comes back towards cation and is shared by it.
  • Fajan proposed some empirical rules that deal with the polarization process. These rules are known as Fajan՚s rules which are given below:
    • The cations should be small.
    • Large size anion
    • The presence of high positive charge on cation
    • The cations with electronic configuration (n – 1) as compared to cations with same size and charge but having ns2np6 (noble gas) configuration.
  • The above factors increase the covalent character of an ionic bond.

Covalent Bond Parameters

Each covalent bond is characterized by the following parameters which are called covalent bond parameters they are:

Bond Order

  • The bond order is the number of bonds present between two atoms in a molecule or ion.
  • The bond order has an integral value.
  • For examples:

Bond order = 1

  • Bond order is 1 (one) when there is only one covalent bond or sigma bond between the two bonded atoms. Eg: H2, Cl2, NH3 .
  • These are also called as single bonds.
  • In NH3 molecule, there are three single N – H bonds are present.
Illustration: Bond Order

Bond order = 2

  • Bond order is 2 (two) when there are two covalent bonds between the two bonded atoms.
  • The bonded atoms have one sigma bond and one pi bond. These are called as double bond. Eg:
Illustration: Bond Order

Bond Order = 3

  • Bond order is 3 (three) when there are three bonds between the two bonded atoms.
  • Out of these, one is a sigma bond and two are pi bonds. These are called triple bonds. Eg:
Illustration: Bond Order

Bond Length

  • Bond length is the distance between the nuclei of the two bonded atoms.
  • It is expressed in picometer (pm) .
  • The bond length depends upon the sizes of the bonded atoms and the bond order of the bond which are linking them.
  • The greater the bond order the smaller is the bond length.
  • The bond length for some bonds are shown below:
Illustration: Bond Length

Bond Angle

  • It is the angle between the two bonds in a molecule.
  • The covalent bonds are formed by overlapping the orbitals.
  • The angle between two orbitals that contain bonding electrons and overlap with atomic orbitals of other atoms.
  • The bond angle between two bonds or bonding orbitals is shown below
  • The bond angle between two O – H bonds in H2O molecule is , between N – H bonds in NH3 molecule is and between C – H bonds in CH4 is .
Illustration: Bond Angle