COVALENT CHARACTER IN IONIC BOND
Just as covalent bond develops ionic character due to the difference of electronegativities of bonded atoms, the ionic bond also develops covalent character as described below:
When two oppositely charged ions come close, positive ion tends to attract the electron cloud of negative ion towards itself because the electron cloud in the anion is loosely held by its nucleus. The anion no longer remains spherical but undergoes some distortion. The electron cloud of negative ion gets polarised and electron density is pulled in between the nuclei of the two ions The build up of electron density in between the two nuclei is shown in Fig. 7.15. The ionic bond does not remain 100% ionic but develops some covalent character.
• The power of cation to cause distortion in the electron cloud of anion is referred to as its polarising power.
• The ability of anion to undergo distortion is called polarisability .
• The extent of covalent character in ionic bond depents on the polarising power of cation and polarisability of anion which are decided on the basis of set of rules called Fajan rules. These are described as follows:
(i) Smaller the size of cation, larger will be its polarizing power. The polarising power of cation depends on the electrical field used to pull electrons away from the anion. The electrical field strength depends on the charge density (q/r) i.e., charge (q) to size (r) raido
of the cation larger the charge density of cation, higher is its polarising power. For example, let us compare the polarising power of Na+, Ma2:+” and A13+ ions.
Na+ Mg2+ AI3+
Charge ( q): + 1 + 2 + 3
Size (r) (nm): 0.102 0.072 0.053
q/r: 9.8 27.8 -56.6
Thus, polarising power increases from
Na+ à Mg2+ à AI3+.
Hence covalent character increases among their respective chlorides as NaCl < MgC12 < AIC13.
(ii) For two cations of similar size, the one with pseudo noble gas configuration ns2np6nd10 has large polarising power than the one with noble gas configuration n2p6. For example, CuCI·is more covalent than NaCI because polarising power of Cu+ ion which has pseudo noble gas configuration, is more than Na+ ion.
(iii) Larger the size of anion, higher will be its polaris ability.
The polarisability of anion, in fact depends on the product qr. Larger the value of qr product of the anion, higher will be its polarisability. For example, the comparison ofpolarisability of F-, Cl- and I- ions are given as follows:
-F Cl- I-
Charge (q): -1 -1 -1
Size (r) (nm): 0.136 0.181 0.216
(qr) product: 0.136 0.181 0.216
As evident qr product increases from p- to I- ion (Here, 15 taken in magnitude only). Thus, polarisability increases m p=- tO Cl- to I-.
Hence covalent character increases from MgF2 to MgC12 –MI2.
Why NazC03 is thermally more stable than CaC03? The stability of metal carbonate towards heat depends upon the relative stability of the resulting metal oxide. More the stability of the resulting metal oxide lesser is the stability f metal carbonate.
Now large anion is stabilised by large cation whereas mall anion is stabilised by small cation. Na+ ion being larger in size stabilises co3 2- (a larger anion) more than Q2- (a smaller anion) ion. As a result Na2O is less stable than Na2 CO3. Therefore, Na2CO3 is stable towards heat and Doesnot decompose. On the other hand, Ca2+ ion being smaller in size stbilises 02- ion more than CO3 2- ion. Therefore, CaO more stable than CaCO3. Hence, CaCO3 easily decomposes into CaO on heating.