A covalent bond in which electrons are shared unequally and the bonded atoms acquire a partial positive and negative charge, is called a polar covalent bond. Bond polarity is described in terms of ionic character.
Similarly in ionic bond, some covalent character is introduced because of the tendency of the cation to polarise the anion. The magnitude of covalent character in the ionic bond depends upon the extent of polarization caused by cations.
In general :
(i) Smaller the size of cation, larger is its polarizing power.
(ii) Larger the anion, more will be its polarisability.
(iii) Among two cations of similar size, the polarizing power of cations with pseudo - inert gas configuration (ns 2 np 6 nd 10 ) is larger than cation with noble gas configuration (ns 2 np 6 ) e.g. polarizing power of Ag + is more than K + .
(i) Which of the following will be most covalent ?
Text Solution
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(i) For more covalent character, small cation and large anion are favourable factors. In MgS, Mg 2+ will have higher polarising power and S 2– will have higher polarisability. Hence there will be higher polarisation of anion resulting in higher covalent character.
(ii) Due to smaller size of Be 2+ and largest size of Ι – amongst all anions (i.e. F – , Cl – , Br – and Ι – ) there will be greater polarisation of anion. Thus Be Ι 2 will be most covalent i.e. least ionic.
(iii) As the size of the cations increases in the order : Si 4+ < Sn 4+ < Sn 2+
and for size of anions
F – < Cl –
so the order of increasing ionic character is : SiCl 4 < SnCl 4 < SnF 4 < SnCl 2 < SnF 2
(iv) As polarizability of anion increases covalent character increases.
(v)
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