Covalent Character in Ionic Compounds

Fajan's Rule

Although atomic bond in a compound like M⁺X^- is considered to be 100% ionic, actually it also has some covalent character. An explanation for the partial covalent character of an ionic bond has been given by Fajan. According to Fajan, if two oppositely charged ions are brought together, the nature of the bond between them depends upon the effect of one ion on the other.

When two oppositely charged ions (say A⁺ and B^- ) approach each other the positive ion attracts electrons on the outermost shell of the anion and repels its positively charged nucleus. This results in the distortion,deformation or polarization of the anion. If the polarization is quite small, an ionic bond is formed, while if the degree of polarization is large, a covalent bond results.

Thus the power of an ion (cation) to distort the other ion is known as its polarization power and the tendency of the ion(anion) to get polarized by the other ion is known as its polarisability. Greater the polarization power or polarisability of an ion, greater will be its tendency to form a covalent bond.

The polarising power, or polarisability and hence formation of covalent bond is favoured by the following factors:

Small Positive Ion (Cation): Due to greater concentration of positive charge on a small area, the smaller cation has high polarising power. This explains why LiCl is more covalent than KCl.
Large Negative Ion (Anion): The larger the anion, the greater is its polarisability, i.e. susceptibility to get polarised. It is due to the fact that the outer electrons of a large anion are loosely held and hence can be more easily pulled out by the cation. This explains why iodides, among halides, are most covalent in nature.
Large Charge on Either of the Two Ions: As the charge on the ion increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases, with the result its ability for forming the covalent bond increases. Thus covalency increases in the order : Na⁺ Cl^-, Mg²⁺ (Cl₂)^2-, Al³⁺(Cl₃)^{3 -}
Electronic Configuration of the Cation : For the two ions of the same size and charge, one with a pseudo noble gas configuration (i.e., 18 electrons in outer-most shell) than a cation with noble gas configuration (i.e. 8 electrons in outermost shell) will be more polarising. Thus copper (I) chloride is more covalent than sodium chloride although Cu⁺ ion (0.96A°) and Na+ ion (0.95A°) have same size and charge.

The orbital overlapping involved in covalency reduces, the charge on each ion and so weakens the electrovalent forces throughout the solid, as is evident from the melting point of lithium halides.

LiF = 870°C LiCl = 613°C

LiBr = 547°C LiI = 446°C

From the above discussion, we find that greater the possibility of polarisation, lower is the melting point and heat of sublimation and greater is the solubility in non-polar solvents.

Example: The melting point of KCl is higher than that of AgCl though the crystal radii of Ag⁺ and K⁺ ions are almost the same.

Solution : Now whenever any comparison is asked about the melting point of the compounds which are fully ionic from the electron transfer concept it means that the compound having lower melting point has got lesser amount of ionic character than the other one. To analyse such a question first find out the difference between the 2 given compounds. Here in both the compounds the anion is the same. So the deciding factor would be the cation. Now if the anion is different, then the answer should be from the variation of the anion. Now in the above example, the difference of the cation is their electronic configuration. K⁺ = [Ar]; Ag⁺ = [Kr] 4d¹⁰. This is now a comparison between a noble gas core and pseudo noble gas core, the analysis of which we have already done. So try to finish off this answer.

Percentage of Ionic Character

Every ionic compound having some percentage of covalent character according to Fajan’s rule. The percentage of ionic character in a compound having some covalent character can be calculated by the following equation.

The percent ionic character = Observed dipole moment/Calculated dipole moment assuming 100% ionic bond × 100

Example: Dipole moment of KCl is 3.336 × 10^–29 coulomb metre which indicates that it is highly polar molecule. The interatomic distance between k⁺ and Cl^– is 2.6 ×10^–10m. Calculate the dipole moment of KCl molecule if there were opposite charges of one fundamental unit located at each nucleus. Calculate the percentage ionic character of KCl.

Solution: Dipole moment μ = e × d coulomb metre

For KCl d = 2.6 × 10^–10 m

For complete separation of unit charge

e = 1.602 × 10^–19 C

Hence μ = 1.602 × 10^–19 × 2.6 × 10^–10= 4.1652 × 10^–29 Cm

μ_KCl = 3.336 × 10^–29 Cm

∴ % ionic character of KCl = 3.336×10^–29/4.165×10^–29 = 80.09%

Example. Calculate the % of ionic character of a bond having length = 0.83 Å and 1.82 D as it’s observed dipole moment.

Solution: Tocalculate μ considering 100% ionic bond

= 4.8 × 10^–10 × 0.83 × 10^–8esu cm

= 4.8 × 0.83 × 10^–18 esu cm = 3.984 D

∴ % ionic character = 1.82/3.984 × 100 = 45.68

The example given above is of a very familiar compound called HF. The % ionic character is nearly 43.25%, so the % covalent character is (100 – 43.25) = 56.75%. But from the octet rule HF should have been a purely covalent compound but actually it has some amount of ionic character in it, which is due to the electronegativity difference of H and F. Similarly knowing the bond length and observed dipole moment of HCl, the % ionic character can be known. It was found that HCl has 17% ionic character. Thus it can be clearly seen that although we call HCl and HF as covalent compounds but it has got appreciable amount of ionic character. So from now onwards we should call a compound having more of ionic less of covalent and vice versa rather than fully ionic or covalent.

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Reference books of Physical Chemistry

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