The so-called “alkali metals” form column one—the first period—of the periodic table. Some of these are familiar, like lithium (#3), sodium (#11), and potassium (#19). We know these things from compounds like lithium carbonate, the medical “lithium” that is used to treat mood disorders. Of course we need sodium chloride (table salt) in our daily diet for its essential role in our physiology. And we eat bananas (or drink OJ) to get potassium salts, another life-sustaining nutrient.

But the actual metals are rare. Outside of chemistry class most folks will never see these things as pure metals. They are typically stored under oil as they oxidize rapidly in the presence of air. I used to toss a chunk of sodium into a bucket of water (outdoors, of course!) for the lovely explosion it made. Lithium, sodium, and potassium are less dense than water and will float. But they will react violently with the water, releasing hydrogen gas which the heat of the reaction will then ignite. Very cool! And the pH of the water surges up, making it alkaline, hence the name “alkali” metals.

Rubidium, #37, has all the same properties as the above except it is denser than water. And it has no known biological function. But it is capable of replacing potassium in organisms and thus can be used as a biomarker.

Rubidium has few industrial uses but has many technical applications in research laboratories. It is used as a laser target, for example, and in atomic clocks. It is named for the ruby color the compounds emit when ignited, and some of the salts are used in fireworks.