An element with atomic number 12 and forms XCI, type of
compound.

Dear student, Your question appears to be incomplete.the element with atomic number 12 is Mg but it does not form XCl type of chloride. Your question is not clear and appears to be incomplete. Please recheck your question, specially XCl to which you are referring here so that we can provide you with some meaningful help.   Look forward to hearing from you again!   Regards ,