Metal nobility

Wow, an old treasure chest! ...With coins of copper, silver and gold! When this treasure chest was brand-new, it had an iron padlock. But after several centuries under water, the lock is just a pile of rust. The iron has reacted with the water. How did it affect the other metals?

The copper coins have also reacted. When they were new they were bright and shiny, but now they are covered by a thick coating. They don’t even look like metal anymore. The coins made of silver have become a bit grey and dull, but we can still see that they are silver. However, the gold coins gleam and look almost like new.

They haven’t reacted at all. Because gold is very good at resisting chemical reactions with other substances: It is a very noble metal. Metals that are less noble react more easily to form chemical compounds. Like the iron, that forms rust. When a metal reacts with another substance, something happens to the atoms of the metal.

They lose some of their electrons, and become metal ions. The more noble a metal is, the better its atoms are at retaining their electrons. Of all atoms, gold atoms are the ones that hold on most tightly to their electrons. That’s why gold reacts with almost no other substance. The atoms in a less noble metal react more readily, since they aren’t as good at holding on to their electrons.

This gives us a way to compare metals: testing which is more noble. Let’s start with this piece of tin. And a solution containing copper ions. It’s the copper ions that make the liquid blue. Metallic tin consists of tin atoms, that are still holding on to all their electrons.

The copper ions lack two electrons and therefore have a positive charge. Let’s put the piece of tin in the copper ion solution. And now – there will be a contest. The copper ions want to take electrons from the tin. But the tin atoms want to keep them.

They are contesting for the electrons. The copper ions are stronger – copper is more noble than tin. As the copper ions pick up electrons, they become copper atoms. A dark layer of metallic copper starts forming on the surface of the tin. At the same time, the blue colour of the solution fades.

The tin atoms that lose electrons become tin ions that move into the solution. If we wait long enough, the metallic tin will disappear completely. Instead, we’ll have a colourless solution containing tin ions, and brown particles at the bottom of the beaker – metallic copper. We started out with metallic tin and a solution containing copper ions. After the reaction, we have metallic copper and a solution containing tin ions.

The more noble metal – copper – has won the contest over the electrons, and gets to be in atomic form. If we try the same thing with silver in a copper ion solution… ...nothing happens. The copper ions aren’t strong enough to pull electrons away from the silver atoms. When the more noble metal – silver – starts out in atomic form, the electrons stay put. The more noble metal remains in atomic form.

So copper is less noble compared to silver, but more noble compared to tin. Metal nobility is a measure of their ability to withstand chemical reactions. One way of comparing the nobility of metals, is by letting atoms of one metal meet ions from another metal. Then there’ll be a contest over the electrons, to decide which metal gets to be in atomic form. And it’s the more noble metal that wins.