Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

POTASSIUM AT A GLANCE

Name: From the English potash. The symbol for potassium comes from the Latinkalium,alkali.

Atomic mass: 39.10.

History: Isolated in 1807 by Sir Humphry Davy.

Occurrence: The seventh most abundant metal, but it is never found free in nature. Potassium is an essential constituent for plant growth and is found in most soils. It occurs in many minerals mined in Germany, Spain, Canada, the U.S., and Italy.

Appearance: Soft, silvery metal.

Behavior: One of the most reactive and electropositive metals. Apart from lithium, it is the least dense metal known. Potassium oxidizes very rapidly in air and must be stored under argon or under a suitable mineral oil. It reacts with water to form potassium hydroxide, hydrogen gas, and heat, and the heat usually ignites the hydrogen. Potassium and its salts impart a lilac color to flames.

Uses: The metal is rarely used, but its compounds are important components of fertilizers, match heads, glass, soaps, and detergents. The potassium-40 isotope is used to date rocks.

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.

TIMEKEEPER The tiny time-standard cell from the heart of a rubidium atomic clock.

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.

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.