Out of fluorine displacing chlorine from sodium chloride, and chlorine displacing fluorine from sodium fluoride, only the first occurs. Which factor decides this feasibility?

1. In displacement reactions, a more active element can displace a less active one from its compound. 2. Fluorine sits above chlorine in the periodic table and is more active, so fluorine can displace chlorine. 3. Chlorine, being less active than fluorine, cannot displace fluorine, so the reverse reaction does not occur; this is decided by activity, not colour or mass. 4. Therefore the relative activity and periodic position of the elements decide feasibility. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 10 Types of Chemical Reactions "Single Displacement Reactions", p.148_