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 Lanthanides  Melting point 1042°C, 1908°F, 1315 K 
Period Boiling point 3000°C, 5432°F, 3273 K 
Block Density (g cm−3) 7.26 
Atomic number 61  Relative atomic mass [145]  
State at 20°C Solid  Key isotopes 145Pm, 147Pm 
Electron configuration [Xe] 4f56s2  CAS number 7440-12-2 
ChemSpider ID 22386 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 is based on a scene from an Ancient Greek vase. It depicts the god Atlas witnessing Zeus’ punishment of Prometheus. Prometheus was chained to a rock on a mountain top. Every day an eagle tore at his body and ate his liver, and every night the liver grew back. Because Prometheus was immortal, he could not die, but he suffered endlessly.
A radioactive metal.
Most promethium is used only in research. A little promethium is used in specialised atomic batteries. These are roughly the size of a drawing pin and are used for pacemakers, guided missiles and radios. The radioactive decay of promethium is used to make a phosphor give off light and this light is converted into electricity by a solar cell.

Promethium can also be used as a source of x-rays and radioactivity in measuring instruments.
Biological role
Promethium has no known biological role.
Natural abundance
Promethium’s longest-lived isotope has a half-life of only 18 years. For this reason it is not found naturally on Earth. It has been found that a star in the Andromeda galaxy is manufacturing promethium, but it is not known how.

Promethium can be produced by irradiating neodymium and praseodymium with neutrons, deuterons and alpha particles. It can also be prepared by ion exchange of nuclear reactor fuel processing wastes.
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In 1902, Bohuslav Branner speculated that there should be an element in the periodic table between neodymium and samarium. He was not to know that all its isotopes were radioactive and had long disappeared. Attempts were made to discover it and several claims were made, but clearly all were false. However, minute amounts of promethium do occur in uranium ores as a result of nuclear fission, but in amounts of less than a microgram per million tonnes of ore.

In 1939, the 60-inch cyclotron at the University of California was used to make promethium, but it was not proven. Finally element 61 was produced in 1945 by Jacob .A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at Oak Ridge, Tennessee. They used ion-exchange chromatography to separate it from the fission products of uranium fuel taken from a nuclear reactor.

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) 2.38 Covalent radius (Å) 1.86
Electron affinity (kJ mol−1) Unknown 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 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  145Pm 144.913 - 17.7 y  EC 
  147Pm 146.915 - 2.623 y  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.



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)
Unknown Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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Listen to Promethium Podcast
Transcript :

Chemistry in its element: promethium


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

(End promo)

Meera Senthilingam

This week, we enter the world of Greek mythology to reveal the great powers of the element promethium.

Brian Clegg

Of all the figures in Greek myth, Prometheus has to be one of the most significant for science. This Titan brought fire to mankind. For that gift he was punished by having his liver pecked out by an eagle every day. Such was the reward for being an early technologist.

In other legends Prometheus gave us maths and science, agriculture and medicine - or even created humans in the first place. This uncertainty of just what Prometheus was responsible for is echoed in the uncertainty of who discovered the element promethium, number 61 in the periodic table.

We know who named it. That was Grace Coryell, the wife of Charles Coryell who with colleagues Jacob Marinsky and Lawrence Glendenin produced promethium at the Oak Ridge National Laboratory, near Knoxville, Tennessee, in 1945. Mrs Coryell allegedly felt they were, like Prometheus, stealing fire from the gods - presumably a reference to the atomic bomb programme, rather than anything significant about promethium itself. But this wasn't the first reference to element 61.

As far back as 1902 there were suspicions that such an element should exist. Promethium sits in the lanthanides, the floating bar of elements that squeezes between barium and lutetium. The rare earth elements either side of it, neodymium and samarium, seemed not to have the right relationship in their chemical properties to be neighbours. It was as if there were a gap between, and Czech chemist John Bohuslav Branner suspected that a missing element occupied that gap.

This suspicion was reinforced by Henry Moseley, the English physicist who gave structure to the concept of atomic number, realizing that it reflected the number of protons in an atom's nucleus. What had, until then, been a rather arbitrary numbering system was given a specific meaning - and in 1914, Moseley realized that there was a missing element in number 61.

Before Coryell's team isolated promethium there were at least two others in the 1920s who claimed to have found element 61. An Italian team found something they named florentium after their city, while an American group in Illinois came up with illinium. Both these findings were announced in 1926, with the Americans publishing first, promptly followed by the Italians, who claimed priority because they had results locked away in a safe dating back two years. But in practice neither of these findings could be duplicated, and apart from a failed 1938 attempt at Ohio State University, the discovery remained unclaimed until the 1945 isolation of promethium. It was a by-product of uranium fuel in one of the early reactors being used to produce plutonium for the atomic bomb. Coryell's team intended to call it clintonium, after the Clinton Laboratories where they worked, until Mrs Coryell persuaded them that the classical name was better.

One of the reasons promethium was so elusive for a relatively low atomic number element is that it doesn't have a stable state - it's one of only two elements below 83 that only has radioactive isotopes, the other being technetium. The most stable form of promethium has a half life of just 17.7 years - that's promethium 145 - so it's hardly surprising that it proved difficult to pin down, though it does occur naturally in tiny quantities in the ore pitchblende when uranium 238 splits spontaneously. The amounts produced are so small - around a trillionth of a gram from a tonne of ore - that promethium was unlikely to be discovered this way. However it would be wrong to say that promethium is negligible in nature. It has been detected using spectroscopes, devices that analyze materials from the light they give off, on the star HR465 in the constellation Andromeda. No one is quite sure why this star is pumping out what must be considerable quantities of promethium.

This grey metallic element gives off beta particles - electrons from the nucleus - as it decays. These can cause radioactive damage in their own right, but prometheum is probably most dangerous because those beta particles generate X-rays when they hit heavy nuclei, making a sample of promethium bathe its surroundings in a constant low dosage X-ray beam.

It was initially used to replace radium in luminous dials when it was realized that radium was too dangerous. Promethium chloride was mixed with phosphors that glow yellowy-green or blue when radiation hits them. However, as the dangers of the element's radioactive properties became apparent, this too was dropped from the domestic glow-in-the-dark market, only used now in specialist applications.

The obvious use of promethium is for portable X-ray devices, though this isn't an application that has been properly developed yet. Instead the element's beta radiation has been used in industry to measure the thickness of materials, and the isotope promethium 147 has been used in nuclear batteries. These are long life power sources that make use of the beta radiation (which is, after all, made up of electrons, the source of an electrical current) to generate power. Such batteries, often less than a centimetre across, can keep in action for around five years, twice promethium 147's half life. They have been used in everything from missiles to pacemakers.

In its early days, nuclear power seemed to promise vast amounts of cheap, portable energy. Science fiction of the period featured walnut-sized generators that could run a household, all driven by nuclear fission. In nuclear batteries, promethium comes about as close as we've ever got to a portable nuclear powered energy source. So in that small way at least, it lives up to the titan it was named after - Prometheus, the bringer of fire.

Meera Senthilingam

So a provider of portable and long life power named after the bringer of fire. That was science writer Brian Clegg bringing us the powerful and mythological chemistry of promethium. Now next week an element that shoots us off into outer space, quite literally.

Richard Corfield

Despite its rarity and hazards it seems appropriate that an element first synthesised during a global conflict that saw the development of the vehicles that would one day take us to the Moon and beyond is now so pivotal to space exploration, providing our robotic pioneers not only with power but also the ability to analyse extraterrestrial materials as well.

Meera Senthilingam

Trying to guess what wondrous element this is? Join Richard Corfield to find out the discovery, chemistry and applications of the element curium in 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.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.