Rubidium – facts and properties
- Symbol
- Rb
- Atomic number
- 37
- Atomic mass
- 85.468 u
- Period
- 5
- Group
- 1
- Category
- alkali metal
- Electron configuration
- [Kr] 5s¹
- Electronegativity
- 0.82
- Atomic radius
- 265 pm
- Valence electrons
- 1
- State at standard conditions
- solid
- Melting point
- 39.31 °C
- Boiling point
- 688 °C
- Density
- 1.53 g/cm³
- Discovered
- 1861
Rubidium is a soft, silvery-white alkali metal with atomic number 37. It reacts violently with water and can ignite by itself in air. It is named after two dark red lines in its spectrum, which led Robert Bunsen and Gustav Kirchhoff to it in 1861. Hardly anyone ever meets rubidium, yet its atoms keep atomic clocks ticking, and one of its natural isotopes works as a clock for the age of rocks.
Position in the periodic table
Occurrence and extraction
Rubidium never occurs as a free metal. In the Earth's crust it is not an especially rare element, but it is spread very thinly and forms almost no minerals of its own. It usually accompanies the chemically similar potassium and is present in traces in micas, feldspars and salt deposits. Somewhat more is found in lepidolite, a lithium-bearing mica, and in pollucite, a caesium mineral.
Rubidium is therefore obtained as a by-product when lithium and caesium are extracted from such minerals. It is needed only in small quantities and is expensive. The metal is usually made by reducing rubidium chloride with calcium under vacuum.
The discovery is a classic of spectroscopy. In Heidelberg, Bunsen and Kirchhoff had just learned to identify elements by their flame spectra. In 1861 they noticed another pair of lines in the red part of the spectrum of lepidolite. They named the new element after the Latin rubidus, dark red.
Properties and reactions
Rubidium is in period 5 and group 1, an alkali metal below potassium. Its electron configuration is [Kr] 5s¹, with 1 valence electron. It gives that electron up very easily, so the electronegativity of 0.82 is low and the atomic radius of 265 pm is large. The metal has a density of 1.53 g/cm³, so it is denser than water, and it is so soft that a knife cuts it. It melts at 39.31 °C, only just above body temperature, and boils at 688 °C.
Rubidium is among the most reactive metals ever handled in laboratories. In air it tarnishes at once and can catch fire spontaneously. With water it reacts even more violently than potassium, forming rubidium hydroxide and hydrogen, which usually ignites, and the result is an explosion. The metal is therefore stored under protective gas or oil. In a flame it gives a red-violet colour, similar to potassium.
In compounds rubidium almost always appears as a singly charged positive ion. The ion resembles the potassium ion so closely that the body partly mistakes one for the other, yet rubidium has no known function in the organism.
Uses in everyday life and technology
Rubidium plays almost no part in consumer products. Its main uses are in research and measurement.
The best-known example is the atomic clock. Rubidium atoms absorb microwaves of one exactly defined frequency, and that frequency serves as the clock's beat. Such clocks are smaller and cheaper than the caesium clocks that define the second, yet still keep very accurate time. They are found in mobile networks, where base stations have to be synchronised, and in navigation satellites.
Quantum physics needs rubidium too. Its atoms can be slowed and trapped very well with laser light. In 1995 researchers in the United States used rubidium-87 to create the first Bose-Einstein condensate, a state in which many atoms just above absolute zero behave like a single matter wave. The 2001 Nobel Prize in Physics went to work in this field.
The isotope rubidium-87 has a special role. It is weakly radioactive and turns into strontium-87 with a half-life of roughly fifty billion years. If you measure the ratio of the two isotopes in a rock, you can calculate the rock's age.
In medicine the short-lived isotope rubidium-82 is used for PET scans of the heart. Metallic rubidium does not belong in a school; alkali metals this reactive are for specialist laboratories.
Quick question
Which element has the larger atomic radius?
FAQ
How does rubidium react with water?
Very violently. Rubidium attacks water more fiercely than sodium or potassium. Rubidium hydroxide and hydrogen form, and the hydrogen ignites from the heat of the reaction. The metal can explode and throw water around. That is why rubidium is kept under protective gas or oil and used only in specialist laboratories.
What is rubidium used for?
Mainly for atomic clocks that time mobile networks and navigation satellites, and for research with cold atoms. A natural isotope, rubidium-87, also makes it possible to date rocks. In everyday life you will not come across the element.
Why is it called rubidium?
Rubidium shows two dark red lines in its flame spectrum. Bunsen and Kirchhoff therefore named it after the Latin rubidus, meaning dark red. It was the second element they found with the new method of spectral analysis.