Answer;
-Two chlorine atoms
Explanation;
A barium atom attains a stable electron configuration when it bonds with two chlorine atoms.
-Barium is an alkaline earth metal, in group two of the periodic table. Like other alkaline earth metal it has a valency of two which means it reacts by loosing two electrons.
-Chlorine on the other hand is a halogen (group seven element) it reacts by gaining an electron, thus two chlorine atoms will require two electrons. Therefore, Barium would attain a stable configuration by loosing two electrons to two chlorine atoms.