For a metal to be more reactive means that the metal will form its stable ion very easily - by contributing 1 or more electrons to anything other non-metallic element or ion it comes into contact with. Hence potassium atoms form potassium ions easier than sodium atoms do. Potassium ion also is more stable than sodium ion, which in turn is more stable than calcium ion and so on.

To be more reactive also means that once the metal ion is formed, it is difficult to put electrons back into the stable ion to form back the elemental metal again. Hence, it is therefore, very difficult to make sodium metal out of its salt, sodium chloride. It requires an electrical method (by the process of Down's Cell) to obtain sodium metal. However it is much easier to obtain metallic iron, reducing iron ion back into its metallic form. This is done by a chemical method, as in the Blast Furnace Process - using only a chemical reducing agent (carbon monoxide).

If a more reactive metal like iron is put into contact with copper ions present in copper sulphate solution, the former will donate 2 electrons per atom to copper ion, displacing the latter out as metallic copper. Hence, we say that iron is more reactive than copper.

In the series of photographs shown below, metallic grey iron filings are able to displace copper out of blue copper sulphate solution and visible changes are very noticeable.

The following changes are observed:

1. Blue coloration of copper ions (Cu²⁺) disappears
2. Red brown copper metal appears at the top of the grey iron filings
3. Light green coloration of iron (II) ions (Fe²⁺) is seen

The net ionic equation is:

Cu2+(aq)	+	Fe0(s)	→	Cu0(s)	+	Fe2+(aq)
Blue solution		Grey powder		Reddish brown metal		Light green solution

Basically what the iron atom did was to contribute 2 electrons (xx)per atom to the copper ion and let the latter ion come out as red brown copper metal.