Oxygen reactivity series

Did you know that the air contains a gas that is very reactive? It's a gas that can cause some metal objects to dissolve... and disintegrate, given enough time. This gas can also react violently with many substances if they're heated, and destroy them completely. And it's not just present in tiny amounts - it makes up more than 20 percent of the atmosphere.

This reactive gas - is oxygen. Here's a metal called magnesium. If you scrub it with some sandpaper, it gets a metallic shine. But it's enough to leave it in air for a few days for it to lose its shine. An oxide layer has formed on the surface of the metal.

If you want to speed up the reaction, set the magnesium on fire. Now the whole magnesium strip quickly turns into a white powder: magnesium oxide. Some metals are so reactive, that they need to be protected from the oxygen in the air. One example is this metal, Sodium. It's usually kept in jars filled with kerosene.

A few minutes in the air is enough for sodium to get an oxide layer. Other metals react much more slowly with oxygen. One such example is copper. At room temperature, a copper surface stays bright-red and shiny for years... ... but eventually, it will get a layer of dull brown copper oxide.

And if you want to burn copper... It won't catch fire in air, with only 20 per cent oxygen. You need to put the copper in pure oxygen gas. For some metals, an oxide layer makes it stronger and non-reactive. Aluminium for instance.

We use aluminium in many things, from soda cans to aeroplanes, and as a building material. Pure aluminium is almost as reactive with oxygen as magnesium is, but as soon as an oxide layer is formed... ... the reaction stops. This thin layer of aluminium oxide, only a few atoms deep... ... is enough to protect the rest of the aluminium underneath.

The oxygen can't get past the outer layer. Other metals - like iron - are weakened by reacting with oxygen. Iron in contact with oxygen forms rust, and unlike with aluminium, which was protected by its oxide layer... ... the rust that forms on iron is porous. Oxygen can penetrate deeper and deeper into iron objects.

Oxygen is a very reactive gas. Almost all elements, and many compounds, react with it, to form oxides. Non-metals and non-metal compounds usually form oxides by burning. Some metals can also burn, or they can form oxides just by being exposed to air at room temperature. The reactivity of metals can differ a lot: Sodium reacts very fast with oxygen.

Magnesium a bit more slowly. Aluminium does react with oxygen, but the reaction only affects the surface. The oxide layer protects the rest of the metal. Iron reacts more slowly than aluminium, but the reaction keeps going deeper until the entire object is destroyed. Copper and silver take months or even years to get a visible reaction.

Gold is one of the few metals that never forms an oxide.