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 16  Melting point 254°C, 489°F, 527 K 
Period Boiling point 962°C, 1764°F, 1235 K 
Block Density (g cm−3) 9.20 
Atomic number 84  Relative atomic mass [209]  
State at 20°C Solid  Key isotopes 209Po, 210Po 
Electron configuration [Xe] 4f145d106s26p4  CAS number 7440-08-6 
ChemSpider ID 4886482 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
An image based on Luna E-1, the first spacecraft of the Soviet ‘Luna’ programme. Later Luna spacecraft carried ‘Lunokhod’ rovers to the moon. These were the first rovers to explore the moon’s surface and were powered by polonium.
A silvery-grey, radioactive semi-metal.
Polonium is an alpha-emitter, and is used as an alpha-particle source in the form of a thin film on a stainless steel disc. These are used in antistatic devices and for research purposes.

A single gram of polonium will reach a temperature of 500°C as a result of the alpha radiation emitted. This makes it useful as a source of heat for space equipment.

It can be mixed or alloyed with beryllium to provide a source of neutrons.
Biological role
Polonium has no known biological role. It is highly toxic due to its radioactivity.
Natural abundance
Polonium is a very rare natural element. It is found in uranium ores but it is uneconomical to extract it. It is obtained by bombarding bismuth-209 with neutrons to give bismuth-210, which then decays to form polonium. All the commercially produced polonium in the world is made in Russia.
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Uranium ores contain minute traces of polonium at levels of parts per billion. Despite this, in 1898 Marie Curie and husband Pierre Curie extracted some from pitchblende (uranium oxide, U3O8) after months of painstaking work. The existence of this element had been forecast by the Mendeleev who could see from his periodic table that there might well be the element that followed bismuth and he predicted it would have an atomic weight of 212. The Curies had extracted the isotope polonium-209 and which has a half-life of 103 years.

Before the advent of nuclear reactors, the only source of polonium was uranium ore but that did not prevent its being separated and used in anti-static devices. These relied on the alpha particles that polonium emits to neutralise electric charge.

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) 1.97 Covalent radius (Å) 1.42
Electron affinity (kJ mol−1) 183.3 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 6, 4, 2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  209Po 208.982 - 128 y  α 
  210Po 209.983 - 138.4 d  α 


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) 0.0000000002
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)
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 Polonium Podcast
Transcript :

Chemistry in its element: polonium


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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Chris Smith

Hello, this week in Chemistry in its element the story of a substance that was named to snub Russia, power space probes keeps paper static free and has even been used as a murder weapon in London. To reveal the secrets of Marie Curie's element, and that's polonium, here's Johnny Ball

Johnny Ball

Polonium, (element 84), was discovered in 1898 and named after Poland, the homeland of Marie Curie (Ne Sklodowska) who found it with her husband Pierre Curie. This loyalty was a direct affront to Russia who had dominated Poland for so long. The only way she could become educated whilst a teenager, was by risking imprisonment by the Russians by attending secret underground schools, which had to change locations every couple of days. It was only by escaping to Paris, following her older brother and sister, that she was able to forge a career. She was so poor in the early years in Paris, that she sometimes fainted through lack of food. Still she worked tirelessly.

In 1894 she met Pierre, who had made a name for himself in discovering piezoelectricity and was one of her lecturers. They married in July 1895. She wore a black dress as it would be serviceable for her work in the laboratory. They did not exchange rings, but bought each other a bicycle, on which they honeymooned.

X rays had been discovered by Roentgen (Nov 95) and uranium radiation by Becquerel (Feb 96) in Paris. Working with him (98), Marie coined the phrase "radioactivity" and decided to make this here object of study, because no one else was doing it. They realised that radiation was coming from the very atoms and that this was a sign of the atoms breaking up. Only by studying the break up of atoms through radiation, were scientists able to clearly understand how atoms are made up. For this the Curies and Becquerel shared the Nobel Prize for Physics in 1903.

The discovery of polonium (July 98) was no mean task. Pitchbende, a uranium bearing ore, seemed to be far to radio active than could be accounted for by the uranium. The couple got the waste ore free, after the uranium had been removed. They sieved and sorted by hand, ounce by ounce, through tons of pitchblende before tiny amounts of polonium were discovered. With the polonium extracted, there was clearly something far more radioactive left behind and soon they had isolated the much more important element radium in December 1898. Radium was so named as it glowed in the dark.

Pierre died in a tragic accident in 1906. In driving rain he seemed to walk in front of a large horse-drawn wagon, and a wheel shattered his head. Some think  the pain he was in as a result of radiation burns and sickness may have caused his lack of awareness. Marie was devastated, but her work continued. For discovering polonium and radium, she received the Nobel Prize for Chemistry in 1911, becoming the only woman ever to receive two such prizes.

However, there was still more success due for the family. Her daughter Irene also became a scientist, and in 1934, Marie saw Irene and her husband Frederick Joliot-Curie produce the first ever artificial radioactive element. This led to our modern ability to manipulate almost every element for our specific scientific needs. Irene and Frederick also received the Nobel Prize in 1935, but sadly Marie had now died.

Natural polonium, Po-210, is still very rare and forms no more than 100 billions of a gram per ton of uranium ore. Because it is so rare, polonium is made by first making bismuth (also found in pitchblende). Bismuth-209 is found and then artificially changed to bismuth-210 which then decays to form polonium-210. This process requires a nuclear reactor, so it is not an easy element to source.

It was a shocking discovery that the former Russian agent Alexander Litvinenko was poisoned with this very radioactive element. The alpha particles it emits are so weakly penetrating it could easily have been carried in a simple sealed container, and would have to be ingested, for example in a cup of tea, to do any serious harm. However, once inside the body, as it continued to disintegrate, it would become fatal.

Polonium has a position in the periodic table that could make it a metal, a metalloid or a nonmetal. It is classed as a metal as its electrical conductivity decreases as its temperature rises. Because of this property it is used in industry to eliminate dangerous static electricity in making paper or sheet metal.

Because of its short half life, its decay generates considerable heat (141 W per gram of metal). It can be used as a convenient and very light heat source to generate reliable thermoelectric power in space satellites and lunar stations, as no moving parts are involved.

Chris Smith

Johnny Ball lifting the lid on the radioactive element polonium discovered by Marie Curie and her husband Pierre. Next time on Chemistry in its element we remain radioactive much like the substance itself with earth scientist Ian Farnan.

Ian Farnan

Anyone familiar with the iconic image of the mushroom cloud understands the tremendous explosive power of a correctly controlled detonation of plutonium. The energy density is mind-boggling: a sphere of metal 10 cm in diameter and weighing just 8 kg is enough to produce an explosion at least as big as the one that devastated Nagasaki in 1945.

Chris Smith

Ian Farnan with what promises to be an explosive edition of Chemistry in its element next week. I'm Chris Smith, thank you for listening and see you next time.


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.