When one element replaces another element in a compound, a single replacement (also known as displacement) reaction has taken place. When this happens a new compound is formed and an element is released. The general formula for this reaction is A + BX --> AX + B.
A single replacement reaction will only take place if a more reactive element is replacing a less reactive one.
In order to know if one element is more reactive than another, we must look at an activity series chart like the one pictured at the right. (*Activity Series taken from Chemistry: The Molecular Science). A metal activity series is a ranking of relative reactivity of metals in displacement and other kinds of reactions that have been developed through studies. The most reactive elements are at the top of the activity series, and as you go down the chart the elements become less reactive.
Usually a single replacement reaction consists of a solid being added to a solution.
Just because a formula is written doesn't necessarily mean the reaction can take place. Single Replacement reactions can not be reversed. Meaning, the reaction AX + B --> A + BX will not take place. For example:
This equation works: Fe(s) + 2HCl(Aq) --> FeCl2(Aq) + H2(g) Whereas the reverse of this equation will not take place: H2(g) + FeCl2(Aq) --> Fe(s) + 2HCl(Aq)