Difference in metal nobility

A metal that is noble is good at resisting chemical reactions. Like copper. A copper object can stay bright and shiny for years. A metal that is less noble reacts more easily. Like magnesium, which reacts quickly with the oxygen in air, becoming dull and grey.

We can compare the nobility of metals with an experiment. We put a piece of magnesium into a solution containing copper ions. The copper ions will pull electrons from the magnesium atoms, since copper is more noble than magnesium. The copper ions become copper atoms ... ... and the magnesium atoms become magnesium ions, that dissolve into the solution.

The more noble the metal ions are, the stronger they pull on the electrons. And the less noble the metal atoms are, the weaker they hold back the electrons. When the difference in nobility is large, the electrons have a strong "urge" to move from one metal to the other. The electrons’ "urge" to move, is the voltage they are subjected to. This means we can not only compare, but also measure differences in nobility.

A large difference in nobility means a high voltage between the metals. When there is only a slight difference in nobility, like between tin and lead, these opposing forces almost cancel each other out. Lead is only slightly more noble than tin, so the electrons don’t have a strong "urge" to move. So, there is a low voltage between these metals. So, how can we measure the voltage?

Let’s look at the experiment with magnesium and copper. Hmm… there is a problem here. The magnesium and the copper ion solution are in contact with each other. The electrons go directly from the magnesium atoms to the copper ions. And we can’t get close enough in - to the atomic level - to measure the voltage.

We need to modify the experiment a bit. The magnesium and the copper ions need to be separated from each other. We place the magnesium in one beaker of water ... ... and keep the copper ion solution in a separate beaker. And then we connect the magnesium and the copper ions with a wire - an electrical conductor.

The electrons can move from the magnesium, through the conductor, to the copper ions. The chemical reaction is exactly the same as before: Magnesium atoms release electrons, and become magnesium ions which move into the water. The copper ions pick up the electrons, and become copper atoms. But now that the electrons are traveling through the conductor, the voltage can be measured. Connect the wire to a device that measures volts – a voltmeter – and now we can read the voltage between the metals.

The more noble the metal ions are, the stronger they pull the electrons through the wire. And the less noble the metal atoms are, the more weakly they hold back the electrons. The voltmeter measures the difference in nobility between the two metals. The greater the difference in nobility, the higher the voltage.