Metal-Ammonia Solutions

For over a hundred years, chemists have known about the unusual properties of solutions formed when alkali metals -- the elements in the far left column of the periodic table -- dissolve in liquid ammonia (NH3). "Since the early 1800s," says Klein, "the idea that ammonia could dissolve an alkali metal and that the solution changes color has been well known. But it's only in the last 30 years that it's been the subject of more intense study."

At low concentrations, a metal-ammonia solution is blue, and it behaves like an electrolyte -- the metal atoms lose one electron and become positively charged ions, and the free electrons in solution act like negative ions. There is electrical conductivity, similar to salt in water or battery acid, but the conductivity is more like an insulator than a metal.

At high concentrations, around 10 percent metal and higher, the solution changes to a coppery-bronze color. Along with the color change comes a shift to the high electrical conductivity of a liquid metal. How does the change occur? What exactly happens to shift the electronic state from insulator to metal? Experiments have measured the change, and theorists have been able to speculate about how it happens. Until recently, however, the computing power wasn't available to provide detailed, quantitative understanding.

"How would you build these images of what's going on," says Klein, "without computers? The problem just wasn't suited to be tackled. It's a complex problem, involving ions in solution and electrons and the electronic states changing from localized to delocalized -- this is very tough to describe by analytic theory."

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