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 1572°C, 2862°F, 1845 K 
Period Boiling point 4000°C, 7232°F, 4273 K 
Block Density (g cm−3) 15.4 
Atomic number 91  Relative atomic mass 231.036  
State at 20°C Solid  Key isotopes 231Pa 
Electron configuration [Rn] 5f26d17s2  CAS number 7440-13-3 
ChemSpider ID 22387 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 icon is based on the Japanese monogram for ‘ichi’ – number one. This reflects the origin of the element’s name from the Greek ‘protos’, meaning first.
A silvery, radioactive metal.
Protactinium is little used outside of research.
Biological role
Protactinium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Small amounts of protactinium are found naturally in uranium ores. It is also found in spent fuel rods from nuclear reactors, from which it is extracted.
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Mendeleev said there should be an element between thorium and uranium, but it evaded detection. Then, in 1900, William Crookes separated an intensely radioactive material from uranium, but did not identify it. In 1913, Kasimir Fajans and Otto Göhring showed that this new element decayed by beta-emission and it existed only fleetingly. We now know it is a member of the sequence of elements through which uranium decays. It was the isotope protactinium-234, which has a half-life of 6 hours 42 minutes.

A longer-lived isotope was separated from the uranium ore pitchblende (uranium oxide, U3O8) in 1918 by Lise Meitner at the Kaiser-Wilhelm Institute in Berlin. This was the longer-lived isotope protactinium-231, also coming from uranium, and its half-life is 32,500 years.

In 1934, Aristid von Grosse reduced protactinium oxide to protactinium metal by decomposing its iodide (PaF5) on a heated filament.

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) 2.43 Covalent radius (Å) 1.84
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 5, 4
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  231Pa 231.036 100 3.25 x 104 α 
        > 2 x 1017 sf 


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) 0.0000014
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)
- - - - - - 3.44
x 10-10
x 10-8
x 10-6
0.000174 0.00306
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Listen to Protactinium Podcast
Transcript :

Chemistry in its element: protactinium


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've got an element whose origin and location in the periodic table seems to be causing some complications. To tell us more about the mysteries of protactinium here's Eric Scerri:

Eric Scerri

In 1871 the discoverer of the periodic table, Dimitri Mendeleev, made the following prediction among several others. I quote:

"Between thorium and uranium we can further expect an element with an atomic weight of about 235. This element should form a highest oxide X2O5, like niobium and tantalum to which it should be analogous."

The modern atomic weight for the predicted element, or protactinium as it is now known, is close to 231. Even though this seems reasonably accurate, Mendeleev was somewhat unlucky regarding this atomic weight since he was not to know that protactinium is a member of only four 'pair reversals' in the entire periodic table. This situation occurs when two elements need to be reversed contrary to their atomic weights in order to order them correctly.

When atomic number was discovered it turned out to be a better ordering principle than atomic weight for the elements in the periodic table. All four known pair reversals, including the one concerning protactinium and thorium were suddenly resolved. Even though protactinium has a lower atomic weight than thorium it should be placed after thorium because of its greater atomic number.

But it appears that Mendeleev's brief predictions were broadly fulfilled since the element does indeed show an analogy with tantalum in forming Pa2O5 as its highest and most stable oxide. Nevertheless protactinium also shows a strong horizontal analogy with thorium and uranium in also displaying the +4 oxidation state, something that Mendeleev does not seem to have anticipated. Finally, as Mendeleev also correctly predicted protactinium occurs with uranium, and more specifically in the ore called pitchblende.

But whereas uranium and thorium were isolated in 1789 and 1828 respectively, it was not until the twentieth century before protactinium, was first discovered. Of course it depends on what one really means by the discovery of an element. Does it mean somebody realizing that a mineral contains a new element, or does it mean the first time an element is actually isolated? Depending on what choice is made the discovery of protactinium can be assigned to different scientists. And in the case of protactinium there is an even further complication.

In the year 1900 the English scientist and inventor Sir William Crookes pointed out that a new radioactive substance was present in some ores of uranium and he called this substance uranium-X. Uranium-X turned out to be two different substances later called UX-1 and UX-2 of which the second, UX-2 was first isolated by the Polish chemist Kasimir Fajans in 1913. This was a very short-lived isotope of 234Pa with a half-life of just over one minute. Fajans called the new element brevium after its short half-life.

In 1917 the German physicist Lise Meitner isolated a more stable isotope of the element, 231Pa with a vastly longer half-life of about 33,000 years. At this point Fajans withdrew the name brevium since the custom was to name an element according to longest-lived isotope. Meitner than chose the somewhat awkward sounding name of protoactinium which was eventually abbreviated to protactinium. The name she chose refers to the fact that this element is the progenitor of element 89 or actinium, which is formed when protactinium decays via the loss of an alpha particle.

In the very same year, the same isotope, 231Pa, was independently isolated by Frederick Soddy, who had first coined the term isotope, and his colleague John Cranston when they were working together in Glasgow.

Protactinium is a highly radioactive and highly toxic element with yet no commercial applications but nevertheless of some scientific interest. For example, a measurement of the ratio of 231Pa and 230Th in ocean sediments allows scientists to reconstruct the movements of bodies of North Atlantic water that took place during the melting of the last ice-age.

In 1961 the Atomic Energy Authority in Britain produced a concentrated mass of 125 grams of protactinium after processing 60 tons of radioactive waste. For many years this has remained as the only significant supply of protactinium which has provided samples of the element to labs around the world.

Meera Senthilingam

So a highly radio active and toxic element whose origin and isolation had scientists mystified. That was Eric Scerri from the University of California, Los Angeles, with the perplexing tale of protactinium. Now next week an element that's definitely got one up on the rest when it comes to history.

Brian Clegg

Perhaps iridium's best-known claim to fame is as a clue in a piece of a 65 million-year-old Crime Scene Investigation. The concentration of iridium in meteorites is considerably higher than in rocks on the Earth. One class of meteorite, called chondritic (meaning they have a granular structure) still has the original levels of iridium that were present when the solar system was formed.

Meera Senthilingam

And join Brian Clegg to find out how Iridium can inform us about life on earth 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.