C&EN: IT'S ELEMENTAL: THE PERIODIC TABLE

RUBIDIUM

STEPHEN K. RITTER, C&EN WASHINGTON

Rubidium has the most pleasant-sounding name of all the elements, at least in my opinion. It leaves a warm and satisfying feeling when you say it, like the feeling one gets when tasting a good merlot. I'm not sure that any other element name comes close, except maybe beryllium. But honestly, rubidium is not one of the more popular or well-known elements.

The word rubidium comes from the Latin rubidus, meaning dark red, which coincidentally is the same color as merlot. The name stems from the color of the two prominent lines in the red region of the element's spectrum. Rubidium has a rich chemistry, although most of it is the same as lithium, sodium, and potassium. So in descending order, that makes rubidium fourth place among the alkali metals. And, as in the Olympics, no one remembers who finishes in fourth place. Maybe there is a connection here as to why people usually stop at potassium when they make their way down the first column of the periodic table.

Actually, the real reason rubidium's popularity suffers is that the element has no known major biological or mineral roles and few commercial applications. This is related to its occurrence in Earth's continental crust. Rubidium was originally thought to be rare, but actually it's the 22nd most abundant element. At 90 ppm in the crust, this places it, impressively, just after chromium at 102 ppm and ahead of nickel at 84 ppm. For further comparison, copper comes in at only 60 ppm. The drawback to this claim to fame is that rubidium isn't found in large quantities in any one spot.

RUBIDIUM AT A GLANCE

Name: From the Latin rubidius, deepest red, the color of its spectral lines.

Atomic mass: 85.47.

History: Discovered in 1861 by German chemists Robert Bunsen and Gustav Kirchoff while studying the mineral lepidolite.

Occurrence: Soft, silvery white metal. Rubidium can be liquid at ambient temperature if at or above 40 °C.

Appearance: Colorless, odorless gas at room temperature; pale blue as a liquid and a solid; faintly blue, brackish odor as gaseous ozone.

Behavior: One of the most electropositive and alkaline elements. It ignites spontaneously in air and reacts violently with water, setting fire to the liberated hydrogen. It colors flame yellowish-violet. Rubidium can be toxic by ingestion.

Uses: Used in cathode-ray tubes and as a "getter" for vacuum systems.

The richest sources of rubidium are a few aluminum silicate minerals, where the element typically is found with cesium and potassium. Rubidium salts also are found dissolved in mineral water. One disappointment is that, despite having a similar Latin root, rubidium is not part of the chemical makeup of a ruby. Ruby, a type of corundum, is an aluminum oxide doped with chromium.

To accentuate the positive, there are some interesting things to say about rubidium. For example, one exciting fact about the element is that it's easily excitable. The large atomic radius of 248 pm and shielding from the nucleus by inner-shell electrons means that rubidium's 5s electron is barely being held in place. Rubidium's first ionization enthalpy of 403 kJ per mol is second only to 376 kJ per mol for cesium. Thus, rubidium and cesium are two of the most reactive elements.

Rubidium is a silvery white metal that, unlike M&M candies that "melt in your mouth, not in your hand," will start to melt in your hand (38.9 ºC) and explode in your mouth. Rubidium bursts into flame in air to form a set of oxides and reacts vigorously with water to form RbOH, one of the strongest known bases. It has two natural isotopes, ⁸⁵Rb (72.2%) and ⁸⁷Rb (27.8%). The latter isotope is radioactive, with a long half-life of 4.9 3 10¹⁰ years.

As for applications, rubidium is used in a few electronic devices, as a frequency reference in atomic clocks, and to estimate the age of rocks. The element's future is full of potential if it could be more easily isolated. For example, rubidium compounds are being studied for medical uses, such as a potential antidepressant akin to lithium. A rubidium ionic crystal, RbAg₄I₅, has high room-temperature conductivity and could be used in thin films for batteries. Another prospect is Rb₃C₆₀, which is a potential superconducting material.

That brings us to the coup de gr^ace of this essay: what I did on my summer vacation. My family went to Spain to visit relatives on my wife's side. In Ourense, Galicia, in northwest Spain, we came across rubidium at a set of hot springs.

The springs are located next to the Miño River, which flows through downtown Ourense. In the first century, the Romans established a town around the springs and built a strategic bridge over the river. The mineral water has been used since that time for bathing and drinking, and we witnessed some of the locals coming to fill jugs with the water to take home. In the late 1800s, a doctor analyzed the water, and his results are listed on a plaque next to one of the springs' fountains. Among the ions are 0.16 mg per L rubidium, 102.2 mg per L sodium, and 11.2 mg per L calcium. These values are typical for the concentration of various ions in mineral waters worldwide.


HOT TO TROT A 2.5-inch sealed ampule contains 1 g of rubidium that has been partially melted by heat from a photographic lamp.

Stephen K. Ritter is a senior editor for C&EN's Science, Technology & Education group. He likes finding chemistry in unexpected places.

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