Chlorine (Cl) becomes diatomic (Cl2) because two chlorine atoms bond together to achieve a full octet by sharing electrons. This bonding occurs as each Cl atom is short of one electron, allowing them to stabilize by forming a pair. The conservation of mass is maintained as the total number of atoms before and after the reaction remains the same. The reaction demonstrates how atoms can combine to fulfill their electron requirements while adhering to the principles of conservation. Understanding these concepts is crucial for grasping chemical bonding and molecular formation.