Reactivity Series
The reactivity series shows the list of metals starting with the most reactive at the top and the least reactive at the bottom.
A simple form of it is shown here:
sodium
magnesium
iron
copper
gold
We can add in other metals but this is good enough for now. We can also add in some non-metals (hydrogen goes between iron and copper; carbon goes between magnesium and iron).
The Reactivity Series helps us to predict the outcome of some reactions.
Think of the imaginary element Blobbium.
Does this reaction proceed? blobbium + iron oxide gives blobbium oxide + iron (A)
It looks OK but what about this one? blobbium oxide + iron gives blobbium + iron oxide (B)
Both COULD be OK but which one really happens? You cannot tell until you know the position of blobbium in the reactivity series.
If you are told that Blobbium is above iron in the reactivity series, then A is right. If you are told that Blobbium is below iron in the reactivity series, then B is right.
Think of it like the football league tables. The top team always beats the lower teams (there are no shock results!). Winning is represented by having the oxide.
This video looks at the Reactivity Series