Question

what elment is substance X?

Answer 1

any halogen , it's what i study any subs for exmaple, HX , X represents a halogen group 7 element and H is hydrogen

Answer 2

Yes. It is often used in organic chemistry and sometimes inorganic. In lower level courses, it is usually written as HA.
But for instance, you have H\(_3\)C-X
where like usman stated above, can be any halogen, whether it be Fluorine, Iodine, Chlorine, or Bromine.