Question

why halogens represented by x?

Answer 1

because it's an element having only one less than inert gas...

Answer 2

Halogens are represented by the symbol ―X
Because they are placed in the 17th group of the modern periodic table and all have a valency of -1 .
Halogens are - F , Cl ,Br , I ,At
They are given the symbol ―X by the IUPAC