Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group Melting point 39.30°C, 102.74°F, 312.45 K 
Period Boiling point 688°C, 1270°F, 961 K 
Block Density (g cm−3) 1.53 
Atomic number 37  Relative atomic mass 85.468  
State at 20°C Solid  Key isotopes 85Rb, 87Rb 
Electron configuration [Kr] 5s1  CAS number 7440-17-7 
ChemSpider ID 4512975 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image of an ‘electric eye’ is inspired by the use of rubidium in photocells (sensors that detect light).
A soft metal that ignites in the air and reacts violently with water.
Rubidium is little used outside research. It has been used as a component of photocells, to remove traces of oxygen from vacuum tubes and to make special types of glass.

It is easily ionised so was considered for use in ion engines, but was found to be less effective than caesium. It has also been proposed for use as a working fluid for vapour turbines and in thermoelectric generators.

Rubidium nitrate is sometimes used in fireworks to give them a purple colour.
Biological role
Rubidium has no known biological role and is non-toxic. However, because of its chemical similarity to potassium we absorb it from our food, and the average person has stores of about half a gram.

It is slightly radioactive and so has been used to locate brain tumours, as it collects in tumours but not in normal tissue.
Natural abundance
Rubidium occurs in the minerals pollucite, carnallite, leucite and lepidolite. It is recovered commercially from lepidolite as a by-product of lithium extraction. Potassium minerals and brines also contain rubidium and are another commercial source.
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The lithium potassium mineral lepidolite was discovered in the 1760s and it behaved oddly. When thrown on to glowing coals it frothed and then hardened like glass. Analysis showed it to contain lithium and potassium, but it held a secret: rubidium.

In 1861, Robert Bunsen and Gustav Kirchhoff, of the University of Heidelberg, dissolved the ore in acid and then precipitated the potassium it contained which carried down another heavier alkali metal. By carefully washing this precipitate with boiling water they removed the more soluble potassium component and then confirmed that they really had a new element by examining the atomic spectrum of what remained. This showed two intense ruby red lines never seen before, indicating a new element, which they named after this colour.

A sample of pure rubidium metal was eventually produced in 1928.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 3.03 Covalent radius (Å) 2.15
Electron affinity (kJ mol−1) 46.884 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  85Rb 84.912 72.17
  87Rb 86.909 27.83 4.88 x 1010 β- 


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk Unknown
Crustal abundance (ppm) 90
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) Unknown
Reserve distribution (%) Unknown
Top 3 producers
  • Unknown
Top 3 reserve holders
  • Unknown
Political stability of top producer Unknown
Political stability of top reserve holder Unknown


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
363 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 2.5
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
0.165 - - - - - - - - - -
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Listen to Rubidium Podcast
Transcript :

Chemistry in its element: rubidium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

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Meera Senthilingam

This week, we've got a radio active element that's good at keeping time but also has some fire in its belly. With more on the chemistry of rubidium, here's Tom Bond.

Tom Bond

In a way, the story of rubidium starts in 1859 when the German chemists Robert Bunsen and Gustav Kirchoff invented the spectroscope and in turn opened the door to a new age of chemical analysis. Before that the Bunsen burner had been developed to investigate the coloured flames they saw when combusting various metals and salts. Bunsen and Kirchoff were able to work out that, by using an external light source and a prism, they could separate the wavelengths of emission spectra in these flames, and so the spectroscope was born.

Caesium was their first major discovery using the spectroscope, followed quickly in 1861 by rubidium, which was detected by the red flame produced when they burnt the mineral lepidolite, which contains small amounts of rubidium. Bunsen and Kirchoff realised this colour came from an unknown substance and were then able to purify a small amount of rubidium. Its name is derived from the Latin rubidus, meaning deepest red, which relates to the colour seen after excitation of the single electron in its outer shell.

Rubidium is actually one of our commoner elements and depending on which information source you look at, it is about the 16th most abundant element in the earth's crust, with a concentration somewhere around 90 parts per million. Although it is relatively abundant compared with other elements such as copper, it is not found in a pure state but as a minor fraction in various minerals. Most rubidium is derived as a by product of lepidolite extraction which has the primary goal of producing lithium. Pure rubidium is often obtained by reduction of rubidium chloride using metallic calcium at around 750 ºC and low pressures.

Rubidium is one of the alkaline metals, as group one of the periodic table are otherwise known. The alkali metals have a single electron in their outer shell, which makes them highly reactive with oxygen, water and halogens, and also means that their oxidation state never exceeds +1. As you move down Group 1 of the periodic table the reactivity of the elements increases which is in line with the increasing energy of the outer electron.

While lithium and sodium added to water form part of school chemistry experiments, the extra reactivity of rubidium means the equivalent reaction requires caution and is not for the faint hearted. When a small amount of rubidium is chucked into water, the effect is pretty impressive, and in fact is so violent that the liberated hydrogen can ignite. Rubidium is so reactive that it can catch fire spontaneously in air, meaning it has to be stored under inert conditions.

In terms of their physical properties, the elements of Group 1 are soft metals with low-melting points. Rubidium is no exception to this rule, being silvery-white and melting at 39 ºC. The element has two naturally occurring isotopes. Rubidium-85 is the dominant form, accounting for 72 per cent of the total, while most of the remainder is the radioactive rubidium-87, which has a half-life of 50 billion years. The radioactive isotope decays to form strontium-87. This process gives a way to age rocks, by measuring the isotopes of rubidium and strontium with mass spectrometry, then calculating the ratios of the radioactive forms to their decay products.

Although it is chemically interesting, the element has relatively few commercial applications at present, but the amount of research activity suggests many possibilities exist. One current use is in atomic clocks, though rubidium is considered less accurate than caesium. The rubidium version of the atomic clock employs the transition between two hyperfine energy states of the rubidium-87 isotope. These clocks use microwave radiation which is tuned until it matches the hyperfine transition, at which point the interval between wave crests of the radiation can be used to calibrate time itself.

Rubidium was chosen to investigate the unusual properties of extremely low-temperature fluids, known as Bose-Einstein condensates which have zero viscosity and the ability to spontaneously flow out of their containers. Their existence was predicted in 1925 by Einstein himself, who extended the work of Indian physicist S. N. Bose to suggest bosonic atoms at temperatures close to absolute zero would form their lowest possible energy state, which might allow quantum behaviour to be studied. By the way, bosons are defined as atoms with integer spin, while multiple bosons can occupy the same energy state. It was not until the end of the 20th century that technology advances made cooling elements close to absolute zero feasible. The first pure Bose-Einstein condensate was created using rubidium-87 by a group from the University of Colorado in the US, and for this achievement they earned the 2001 Nobel Prize for physics.

Rubidium is not particularly harmful to humans, and once in the body its ions are rapidly excreted in sweat and urine. Rubidium chloride has been used to study the transport of potassium ions in humans, since rubidium ions are not naturally found in the body and when present they are treated as if they were potassium. In a similar way, because it tends to collect inside cells, especially tumours, the radioactive isotope Rb-82 can be used to locate brain tumours.

The low toxicity of rubidium is confirmed by a study from 1971 which aimed to assess the feasibility of using rubidium chloride as an anti-depressant, since similar effects had been observed in monkeys. After being given 23 grams of rubidium over 75 days, a volunteer showed no harmful side effects. It does though make you wonder whether equivalent clinical studies could take place now. Meanwhile, clinical applications of rubidium in psychiatry have yet to come to fruition. So there we have rubidium, the explosive red element number 37 in the periodic table.

Meera Senthilingam

So this explosive element may have minimal commercial applications but can be used in atomic clocks and has isotopes that can locate brain tumours. Not bad considering it was stumbled upon when analysing the mineral lepidolite. That was Tom Bond with the story of rubidium. Now next week we meet the element that's made our modern lifestyles possible.

John Whitfield

A mixture of powdered tantalum and tantalum oxide is used in mobile phone capacitors, components that store electrical charge and control the flow of current. What makes the element ideal for phones, and for other dinky electronic gadgets, such as handheld game consoles, laptops and digital cameras, is that the metal is extremely good at conducting both heat and electricity, meaning that it can be used in small components that don't crack up under pressure.

Meera Senthilingam

And John Whitfield will be explaining why we have tantalum to thank the next time we play the latest computer games, take hundreds of photos on holiday or when we're downloading this podcast on our laptops. So join John on next week's Chemistry in its Element. Until then, I'm Meera Senthilingam and thank you for listening.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



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