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 Actinides  Melting point 1050°C, 1922°F, 1323 K 
Period Boiling point 3200°C, 5792°F, 3473 K 
Block Density (g cm−3) 10 
Atomic number 89  Relative atomic mass [227]  
State at 20°C Solid  Key isotopes 227Ac 
Electron configuration [Rn] 6d17s2  CAS number 7440-34-8 
ChemSpider ID 22404 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 Greek symbol ‘alpha’ and metallic ‘rays’ are representative of the element as a source of alpha radiation, and also the origin of its name.
Actinium is a soft, silvery-white radioactive metal. It glows blue in the dark because its intense radioactivity excites the air around it.
Actinium is a very powerful source of alpha rays, but is rarely used outside research.
Biological role
Actinium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Actinium used for research purposes is made by the neutron bombardment of radium-226. Actinium also occurs naturally in uranium ores.
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This element was discovered in 1899 by André Debierne at Paris. He extracted it from the uranium ore pitchblende (uranium oxide, U3O8) in which it occurs in trace amounts. In 1902, Friedrich Otto Giesel independently extracted it from the same mineral and, unaware it was already known, gave it the named emanium.

Actinium extracted from uranium ores is the isotope actinium-227 which has half-life of 21.7 years. It occurs naturally as one of the sequence of isotopes that originate with the radioactive decay of uranium-235. A tonne of pitchblende contains around 150 mg of actinium.

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) 2.47 Covalent radius (Å) 2.01
Electron affinity (kJ mol−1) 33.77 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
  227Ac 227.028 - 21.77 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.

Supply risk

Relative supply risk Unknown
Crustal abundance (ppm) 0.00000000055
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)
120 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 Actinium Podcast
Transcript :

Chemistry in its element: actinium


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, a glowing element that significantly changed the periodic table.

Richard Corfield

When I was a little boy my father used to tell a story about a acquaintance of his who kept a lump of rock in his desk. His party trick - after a few drinks - was to draw the curtains, touch the pebble to the forehead of a volunteer, turn out the light and lead the hilarity as the victim's face blazed with a ghostly blue light. Eventually my father's acquaintance died and his executor started to dispose of his possessions. Finding the lump of rock in his desk, and noticing the sourceless dull blue glow that surrounded it he sought advice. Within hours the house had been sealed off and men in white environment suits with tongs and a lead box were relocating the magic pebble to the Atomic Energy Research Establishment at Harwell in Oxfordshire, the secret hub of Britain's nuclear research and development industry from the end of the Second World War to the 1990s.

The pebble was, of course, pitchblende; the naturally occurring mineral that Pierre and Marie Curie had used as the source of the radioactive elements that they discovered in the closing years of the 19th century. Pitchblende does not just contain actinium (the topic of this podcast), it also contains radium, radon and polonium; the latter, if we are to believe recent news reports, the Russian assassin's toxin of choice. Actinium, like radium and polonium, emits an ethereal blue radiance which contributes to pitchblende's luminescent properties. Although, radium, radon and polonium were observed first, of all the components of pitchblende actinium was the first to be isolated.

Actinium was discovered by Andre-Louis Debierne, a friend of Marie and Pierre Curie who worked with them on isolating the radioactive elements in pitchblende. Although he published descriptions of the element in 1899 and then again in 1900 there is some doubt as to whether his techniques had actually allowed the element to be properly identified. What is clear however, is that the German chemist Friedrich Oskar Giesel was also investigating actinium and, by 1904, had unambiguously isolated it. Because of the glow that emanated from it he named his new element emanium. Giesel was an admirer and loyal supporter of the Curie's and consequently was not interested in disputing the priority of discovery of a radioactive element that had come out of a lab whose work he admired hugely. Hence when it became clear that Debierne and Giesel were working on the same element Giesel was content to allow the Frenchman's claim to priority stand, and so today the element is still known by the name Debierne gave it - actinium.

Whoever discovered it, actinium has an important place in the history of chemistry. It was the first of the non-primordial elements to be discovered. Primordial elements are those that have existed in their current state since before the Earth was formed. In other words their half-life is greater than about 108 years. All stable elements are primordial, as are many radioactive elements. Chemically, actinium, which in its native form is a silvery metal, has similar characteristics to that of the other rare earth elements such as lanthanum.

Actinium has thirty-six isotopes all of which are radioactive. 227Ac, the isotope which comprises all naturally occurring actinium has the longest half-life at 21,773 years. All the remaining radioactive isotopes have half-lives of the less than ten hours, the majority having half-lives of less than a minute.

227Ac is about a hundred and fifty times as radioactive as radium making a valuable as the neutron source of energy. Although actinium is found in trace of amounts in uranium ore, more commonly it is synthesised in milligram amounts by the neutron irradiation of radium-226 in a nuclear reactor.

Actinium gives its name to a block of fifteen elements that lie between actinium and lawrencium in the periodic table with atomic numbers 89 through 103. These actinides - or actinoides as they are more correctly known these days - gain their name from the first element in the series, actinium, itself named after the Greek word for ray thus reflecting the element's - already mentioned - visible radioactivity.

The actinoides were the first major addition to be made to Mendeleev's periodic table. American physicist Glenn T Seaborg was experiencing unexpected difficulty isolating the elements americium and curium during his work with the Manhattan Project during the second world war.

He found himself wondering if these elements more properly belonged to a different series from the transition metals, which would explain the differing chemical properties of the new elements he was synthesising in the nuclear reactor at Berkeley University in California.

In 1945, Seaborg formally proposed the actinides and in so doing created the most significant change to the periodic table since Mendeleev's creation of it in 1869.

Early in his career, Seaborg was a pioneer in the study of nuclear medicine and developed numerous isotopes of elements with important applications in the diagnosis and treatment of diseases, most notably 131Iodine which is used in the treatment of thyroid disease. Actinium also has a role to play in nuclear medicine. 225Ac can be used as the active agent in Targeted Alpha Therapy (TAT) a technique for inhibiting the growth of secondary cancers by direct irradiation with nuclear material, in this case 213Bi derived from 225Ac.

And so an element discovered in the same mineral - pitchblende - which kick-started the whole science of nuclear chemistry, today stands at the crossroads of one of the most challenging of all medical disciplines - finding a cure for cancer. The irony is that pitchblende inflicted a dreadful toll on those who worked with it in the early years of the study of radioactivity. Marie Curie suffered terrible radiation burns from handling it, and eventually, in later life, contracted radiation-induced aplastic anaemia from which she died. Even today Marie Curie's papers from the summit of her career in the 1890s - including her cookbook - are still considered too dangerous to handle, and are kept in lead-lined boxes

Meera Senthilingam

So whilst offering hope for treating the deadly effects of cancer, the element itself had deadly effects on its founders and therefore must be handled with care. That was science writer Richard Corfield with the radio active chemistry of actinium. Now next week we go beyond the actinides.

Simon Cotton

When the last member of the actinide series, element 103 or Lawrencium, was discovered, I was at school doing my A levels. The isotope found had a mass of 258 and it didn't hang about for long - it had a half-life of just 3.8 seconds. This was not unexpected as half lives had been getting shorter right along the actinide series. This discovery prompted the scientific community to start asking, are there any elements waiting to be made beyond lawrencium, and if so, where would they fit in the periodic table?

Meera Senthilingam

Join Simon Cotton to find out how element 104, rutherfordium was discovered and how its place in the periodic table was found, 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.



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