I want to know how to arrange the covalent character of ZnCl2, CdCl2, and HgCl2. By Fajans' rules, We know that a smaller cation equals higher covalent character. We also know that atomic size increases as we go down a group. So, covalent character should decrease as we go down the group. Meaning covalent character should be in the order ZnCl2 > CdCl2 > HgCl2. However, ZnCl2 and CdCl2 are ionic whereas HgCl2 is covalent. What is the explanation behind this?


1 Answer 1


I think you misinterpreted that rule. When atoms have a similar size (when big) in a compound they will behave as covalent bond character. Also, is good to remember that they usually have similar electronegativity values. So for example check the diatomic molecules as Cl2 or O2. They obviously are not compounds like you mentioned but you can take them as an example.

When you have smaller atoms in a compound such as NaF, you sure have similar size but their outermost shell are different from HgCl2. Sodium has low electronegativity and fluorine has a large energy gain when receive an electron. Also is good to remember that in NaF you have ions, differently from compounds with covalent character.

Electronegativity for the cited atoms:

  • chlorine: 3,16
  • mercury: 2
  • fluorine: 4
  • zinc: 1,65
  • sodium: 0,93
  • oxygen: 3,44
  • cadmium: 1,68

EDIT: also you have to check the polarizability of certain atoms. The bigger the atoms the higher polarizability. So in compounds with different in size between the atoms you have to consider the polarizability and when you have compounds such as NaF and HgCl2 you have to check the electronegativity.


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