Question

fluorine can not form FCl3 but chlorine can form clF3 why?

Answer 1

Fluorine cannot form FCl3 there are many reasons for that due high electronegativity difference as when it is written as FCl3 it refers that F is +ve charged and it donates the electron to the chlorine atom.
The other reason is due to the small size of fluorine it cannot hold the large size of chlorine.
One another reason is fluorine does not have d-orbital because of which its valency remains 1 while other have d-orbital due to which they can excite the electrons and make their valency greater than one.

Answer 2

Answer:

$FCl_3$cannot be manufactured because of the large electronegativity mismatch.

Explanation:

$FCl_3$  cannot be formed by fluorine. There are a variety of reasons for this, including the significant electronegativity difference; when written as $FCl_3$, F is positively charged and gives an electron to the chlorine atom. The other reason is that fluorine's tiny size prevents it from holding the huge size of chlorine.

An electron can be promoted from a$3p$ full orbital to a$3d$ orbital in chlorine because it possesses unoccupied$3d$ orbitals. As a result, chlorine can have a 3 covalency in its compounds and a $ClF_3$  molecule can exist. Fluorine, on the other hand, has no unoccupied $2d$  orbitals and can only have a covalency of one in its compounds. As a result, a molecule of $FCl_3$cant exist