Chemical Bonding and Molecular Structure

Fajan's rule and Dipole moment

Fajan's Rule : (For covalent bond formation)

  • smaller is the cation and larger is the anion covalent character ↑
  • Larger cation, smaller anion ionic character ↑
  • Charge on both cation and anion ↑ covalent character ↑
  • Cations with pseudo inert gas configuration covalent character ↑

Dipole moment :
used to express the polarity of molecule
\mu = \delta \times l , H^{\delta+} \xrightarrow[]{l} Cl^{\delta-}
δ = magnitude of charge on poles
\mu_{R} = \sqrt{\mu_1^2 + \mu_2^2 + 2\mu_{1}\mu_{2} \ cos\theta}

Part1: View the Topic in this Video from 0:07 to 4:25

Part2: View the Topic in this Video from 0:12 to 10:50

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1.Formal charge = \tt V-\begin{bmatrix}L+\frac{1}{2}S \end{bmatrix}
V= Number of valence electrons in free state
L= Number of lone pair of electrons that is total no. of e of lone pair
S=No. of shared electrons

2. Pauling Equation:%Ionic character = 18 (XA - XB)1.4 E of A > B in A-B bond

3. Hannay and Smith Equation : % Ionic character = 16 (XA - XB) + 3.5 (XA - XB)2 in A-B bond


μ=δ × d  CGS units  SI units 
δ  10-10 esu  10-19 Coulomb 
10-8 cm  10-10
μ  1D= 10-18 esu cm  1D = 3.33 × 10-30 Cm 


5. Percentage ionic character = \tt \frac{observed \ dipole \ moment}{dipole \ moment \ for \ 100\% \ ionic \ bond}\times 100 \ = \frac{\mu_{obs}}{\mu_{ionic}} \times 100