The reactivity series.

Step 1 of 6

Down the group

The Scene shows two troughs of water with indicator. Into the left one drops a metal from group 1 of the table, an alkali metal, and into the right its neighbour from group 2, an alkaline earth metal. Each reacts with the water, giving off hydrogen, and the hydroxide that forms turns the indicator pink:

$2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{NaOH} + \mathrm{H_2}$⁠

In this reaction the sodium is oxidised: it loses its outer electron. Row of the table is now fixed at Na · Mg.

Step 2 of 6

Why

Zoom to the atoms is on Atoms: above each trough is an atom of the metal, with its shells and its outer electrons lit. This is the sixth row: caesium and barium. Compare their size with lithium and beryllium in the second row.

Step 3 of 6

Group 2

Group 2 metals have two outer electrons, and they must lose both. So they are less reactive than their group 1 neighbours. Magnesium hardly reacts with cold water. Water temperature is now fixed at 293 K.

Step 4 of 6

Displacement

All the metals can be ranked by how easily they lose electrons. That ranking is the reactivity series:

$\mathrm{K} > \mathrm{Na} > \mathrm{Ca} > \mathrm{Mg} > \mathrm{Al} > \mathrm{Zn} > \mathrm{Fe} > \mathrm{Pb} > \mathrm{(H)} > \mathrm{Cu} > \mathrm{Ag} > \mathrm{Au}$$\mathrm{K} >$$\mathrm{Na} >$$\mathrm{Ca} >$$\mathrm{Mg} >$$\mathrm{Al} >$$\mathrm{Zn} >$$\mathrm{Fe} >$$\mathrm{Pb} >$$\mathrm{(H)} >$$\mathrm{Cu} >$$\mathrm{Ag} >$$\mathrm{Au}$⁠

A more reactive metal gives electrons to the ions of a less reactive one, and displaces it from the solution.

Step 5 of 6

Not the other way

Displacement works only one way: from higher in the reactivity series to lower. A metal low in the series cannot displace one above it.

Step 6 of 6

Metals and acids

Hydrogen has a place in the reactivity series too, though it is not a metal. An acid holds $\mathrm{H^+}$⁠ ions, and a metal above hydrogen gives them electrons, releasing hydrogen gas. A metal below hydrogen does not react with dilute acid.