Molecule of the Day

Corundum is a funny one. It is aluminum (III) oxide, or Al₂O₃. This means that it is composed of aluminum atoms, three electrons shy of the zero oxidation state (the shiny, silvery, regular metal). You could also call this an Al³⁺ compound, instead of an aluminum (III) compound. The charge is balanced by the oxygen (-2) atoms. In a crystal, it looks like this:

The funny thing about corundum is, when you have it in a clean single crystal, you get something much different. You know them as sapphires…

and rubies:

Yup, all the same stuff. Rubies are actually a type of sapphire. Any of this single-crystal aluminum oxide is called sapphire. A touch of iron, and you get a blue sapphire. A touch of chromium, and you get pink or red rubies. Other metal impurities can give all sorts of colors. Pure aluminum (III) oxide in a single crystal gives you clear sapphire. These are all nearly as strong as diamond and can be used as windows in high-pressure spectroscopy cells, or your car, if you are the pope, a CEO, or anyone else prone to encountering gunfire.

The very first laser was made from ruby. I have made little balls of ruby lase with a green laser in a lab. If you happen to have one of those green laser pointers (and a ruby) at home, you can try by just shining the light on the ruby. The ruby emission is red. A flashlamp will also work, this is how people got the first ruby laser to work (obviously they didn’t all have green lasers to turn into red lasers). It will be much harder to tell if you’re getting it to lase, though - when you pass a green laser through a regular red filter, you just get much dimmer green light. When you pass it through a ruby, you get red light. If you get a red emission from your ruby and a green laser, it is probably the ruby line.

We don’t really use ruby lasers today, but like I said, you can have just about any color sapphire, and another class of sapphire lasers - Ti:sapphire lasers - is still used today.
The same stuff, when it’s not crystalline, is an ugly old stone called bauxite. This is the source for essentially all the world’s aluminum. Making bauxite into aluminum is very, very hard. As the Wikipedia article on aluminum mentions, we have only had widely available aluminum stuff for 100 years! This is not true of many things, especially an element we knew about for so long. Aluminum very rarely occurs “natively” - that is, as the free metal. It typically exists as an ore. That means the metal is bound up with something else, and it’s usually missing some electrons. The “something else” here is oxygen, but sulfur and silicon are also common. Al was so rare for so long that it’s almost incomprehensible to us today. You know how a lot of capital domes are layered with gold? The Washington monument is layered with aluminum! Theodore Gray, author of the amazing wooden periodic table website, remarks on his aluminum page:

Aluminum is incredible stuff. If it wasn’t so common, its combination of desirable properties, great strength, lightness, corrosion resistance, and easy workability, would make it seem incredibly useful. People would say, man, if only we could get more aluminum, we could make all kinds of things out of it! In fact, that’s exactly what happened: When it was first isolated the only process for getting it was extremely difficult, and as a result it was rare and expensive. Napoleon had dinnerware made out the amazing new precious metal. But people knew it was common in many rocks and minerals, and before too long a quick, cheap process using electricity was discovered for refining large quantities inexpensively.

The process by which we make aluminum (III) into aluminum (0) he mentions is called electrolysis. This requires loads and loads of power; Aluminum loves to lose those three electrons and exists as that oxide you find in corundum and bauxite. He goes on to mention that refining aluminum is so expensive, we don’t do it where you mine the aluminum (as you would with, say, iron), we take it somewhere else where we can get loads of cheap electricity.

Finally, there’s a third place you find aluminum (III) oxide - on the surface of anything aluminum you own! I mentioned above that aluminum metal would much prefer to be in its 3+ oxidation state - that is, it corrodes easily. Any aluminum left out in air does, in fact, corrode. It turns out that the aluminum oxide that forms, though, is a clear hard stuff - sapphire, as opposed to the red flaky iron oxide - rust. Anything aluminum left in air will develop a thin (millionths of a meter) layer of a sapphire-like substance. This is what makes aluminum so durable and corrosion resistant. Unless, of course, you run into some mercury, which has a bizzare appetite for the stuff.

You have officially exhausted my supply of aluminum anecdotes.

All images in the public domain and taken from Wikipedia.