Question

hydrogen+chlorine - hydrogen chloride

= H+Cl to form HCl.

then why you wrote in answer that ,

H2+Cl2 to form 2HCl

Answer 1

because hydrogen and chlorine exist in stable form that is diatomic form.

Answer 2

First,
There are 2 H and Cl 2
and we added 2H +2Cl = 2Hcl