![]() 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. 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.įrom 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.Įxample: The melting point of KCl is higher than that of AgCl though the crystal radii of Ag + and K + ions are almost the same. 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. 8 electrons in outermost shell) will be more polarising. Thus covalency increases in the order : Na + Cl -, Mg 2+ (Cl 2) 2-, Al 3+ (Cl 3) 3 -Įlectronic 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. 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. ![]() This explains why iodides, among halides, are most covalent in nature. 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. Large Negative Ion (Anion): The larger the anion, the greater is its polarisability, i.e. This explains why LiCl is more covalent than KCl. Small Positive Ion (Cation): Due to greater concentration of positive charge on a small area, the smaller cation has high polarising power. The polarising power, or polarisability and hence formation of covalent bond is favoured by the following factors: Greater the polarization power or polarisability of an ion, greater will be its tendency to form a covalent bond. 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. If the polarization is quite small, an ionic bond is formed, while if the degree of polarization is large, a covalent bond results. This results in the distortion, deformation or polarization of the anion. 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. 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. An explanation for the partial covalent character of an ionic bond has been given by Fajan.
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