Flexi Says: Single-replacement reactions only occur when the element that is doing the replacing is more reactive than the element that is being replaced. Therefore, it is useful to have a list of elements in order of their relative reactivities. The activity series is a list of elements in decreasing order of their reactivity. For a single-replacement reaction, a given element is capable of replacing an element that is below it in the activity series. For example, any element listed above iron (Fe) can replace iron, including chromium (Cr), zinc (Zn), aluminum (Al), and magnesium (Mg).
