Bond Parameters II


  • All homonuclear diatomic molecules will have zero polarity. Example: O2, N2, F2, H2.
  • It mainly arises due to differences in electronegativity of bonded atoms.
  • A covalent bond, in which electrons are shared unequally and the bonded atoms acquire partial positive and negative charge, is called a polar covalent bond (or) a covalent bond \(^{\delta +}H-{{\,}^{\delta -}}F\) (or) \(H\,\underline{\to }\,F\)
  • Higher the electronegativity difference higher will be the polarity of the Bond.

Dipole moment:

  • The polarity of a molecule is indicated in terms of dipole moment.
    µ = electric charge x bond length
  • Dipole moment is a vector quantity.
  • A Symmetrical molecule is non – polar.
    Ex: CO2, BF2, CH4, CCl4
  • Unsymmetrical non – linear polyatomic molecules will have dipole moment.
    Ex: H2O, CH3Cl, NH3
  • Dipole moment is usually expressed in Debye units (D). 1 D = 3.33564 × 10 –30 C m.
  • In case of polyatomic molecules the dipole moment not only depends upon the individual dipole moments of bonds known as bond dipoles but also on the spatial arrangement of various bonds in the molecule.
  • In such case, the dipole moment of a molecule is the vector sum of the dipole moments of various bonds. % ionic character = μobserved/ μcalculated × 100 μcalculated = electric charge x bond length.

Fajan’s rule: The partial covalent character of ionic bonds was discussed by Fajans in terms of the following rules:

  • The smaller the size of the cation and the larger the size of the anion, the greater the covalent character of an ionic bond.
  • The greater the charge on the cation, the greater the covalent character of the ionic bond.
  • For cations of the same size and charge, the one, with electronic configuration (n-1)dnns0, typical of transition metals, is more polarising than the one with a noble gas configuration, ns2 np6, typical of alkali and alkaline earth metal cations.
  • The polarising power of the cation, the polarisability of the anion and the extent of distortion (polarisation) of anion are the factors, which determine the % covalent character of the ionic bond.

Polarization: It is a change of ionic character to covalent character.

Polarising Power: The ability of a cation to polarize the nearby anion is called polarizing power.

Polarisability: The tendency of an anion to get distorted (or) deformed (or) polarized by the caution is called its Polarisability.

Factors effecting polarization:

  • Charge on cation increases, polarizing power of cation also increases.
  • Polarizing power: Na+ < Mg2+ < Al+3
  • Higher the charge on cation higher will be the covalent characters.
  • Polarizing of cation increases as the size of cation decreases.
  • Larger the size of anion more easily it will be polarized by the cation.

Points to remember:

  • On moving to a group polarizing power of the cations goes on decreasing.
  • Polarizing power of the cations increases in moving left to right in a period.
  • Polarisability of the anions by a given cation decreases in moving left to right in a period.
  • The polarizability of the anions by a given cation increases moving down the group.