Question

Hgf2 ,alf3, and snf4 are ionic but their chloride are covalent

Answer 1

The ionic or covalent nature of a compound depends on the electronegativity difference of the two compounds . if the electronegativity difference is greater than 1.8 then the compound is ionic otherwise if it is smaller than 1.8 then compound is covalent. Now
Electronegativity of fluorine is 4.0 while that of chlorine is 3.0 . So obviously the electronegativity difference in HgF2 or SnF4 or AlF3 is greater than that in HgCl2 or AlCl3 or SnCl4 . Hence surely flourides are ionic but chlorides are covalent . For further details refer NCERT chemistry class 11 chapter 3.