The element that does not show a disproportionation tendency is/are:
1.
Cl
2.
Br
3.
F
4.
I
Hint: For disproportionation tendency element must show both positive and negative oxidation number.
Being the most electronegative element, F can only be reduced and hence it always shows an oxidation number of -1. Further, due to the absence of d-orbitals it cannot be oxidised and hence it does not show positive oxidation numbers.
In other words, F cannot be oxidised as well as reduced simultaneously and hence does not show disproportionation reactions.
