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 1750°C, 3182°F, 2023 K 
Period Boiling point 4785°C, 8645°F, 5058 K 
Block Density (g cm−3) 11.7 
Atomic number 90  Relative atomic mass 232.038  
State at 20°C Solid  Key isotopes 230Th, 232Th 
Electron configuration [Rn] 6d27s2  CAS number 7440-29-1 
ChemSpider ID 22399 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 imagery used here is that associated with Thor, the Norse god connected with thunder. It includes Thor’s hammer (Mjolnir).
A weakly radioactive, silvery metal.
Thorium is an important alloying agent in magnesium, as it imparts greater strength and creep resistance at high temperatures. Thorium oxide is used as an industrial catalyst.

Thorium can be used as a source of nuclear power. It is about three times as abundant as uranium and about as abundant as lead, and there is probably more energy available from thorium than from both uranium and fossil fuels. India and China are in the process of developing nuclear power plants with thorium reactors, but this is still a very new technology.

Thorium dioxide was formerly added to glass during manufacture to increase the refractive index, producing thoriated glass for use in high-quality camera lenses.
Biological role
Thorium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Thorium is found as the minerals thorite, uranothorite and thorianite. It is also found in monazite, which is the most important commercial source. Several methods are used to produce the metal, such as reducing thorium oxide with calcium or electrolysis of the fluoride.
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In 1829, Jöns Jakob Berzelius of the Royal Karolinska Institute, Stockholm extracted thorium from a rock specimen sent to him by an amateur mineralogist who had discovered it near Brevig and realised that it had not previously been reported. The mineral turned out to be thorium silicate, and it is now known as thorite. Berzelius even produced a sample of metallic thorium by heating thorium fluoride with potassium, and confirmed it as a new metal.

The radioactivity of thorium was first demonstrated in 1898 by Gerhard Schmidt and confirmed by Marie Curie. Thorium, like uranium, survives on Earth because it has isotopes with long half-lives, such as the predominant one, thorium-232, whose half life is 14 billion years.

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) 2.45 Covalent radius (Å) 1.90
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 4
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  230Th 230.033 - 7.56 x 104 α 
        > 2 x 1018 sf 
  232Th 232.038 100 1.4 x 1010 α 
        1.2 x 1021 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 7.6
Crustal abundance (ppm) 5.6
Recycling rate (%) Unknown
Substitutability High
Production concentration (%) 80
Reserve distribution (%) 31
Top 3 producers
  • 1) India
  • 2) Brazil
  • 3) Malaysia
Top 3 reserve holders
  • 1) USA
  • 2) Australia
  • 3) India
Political stability of top producer 10.8
Political stability of top reserve holder 56.6


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)
118 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)
- - - - - 3.33
x 10-11
x 10-8
x 10-6
0.000154 0.00401 0.061
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Listen to Thorium Podcast
Transcript :

Chemistry in its element - thorium


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, the no risk, no fear discovery of elements. Here's Lars Öhrström:

Lars Öhrström

Frequently after more spectacular chemistry demonstrations, the scientist on stage will warn the audience 'not to try this at home'. One person who certainly did not listen to such warnings was Swedish chemist Jöns Jacob Berzelius. Instead, he and his co-workers performed many groundbreaking experiments in the kitchen of his flat in the corner of Nybrogatan and Riddargatan in Stockholm. In 1815, for example, Berzelius isolated a new element from a mineral sent to him from the Swedish mining town of Falun and named it thorium after the Scandinavian god of thunder, Thor.

Only to realise a few years later that he was wrong and what he though was a new element was in fact yttrium phosphate.

However, in 1828, by then long since world famous and credited with discovering three other elements, he received a strange mineral sample from the reverend Hans Esmark in Norway. In his new laboratory at the Swedish Royal Academy of Sciences, Berzelius isolated yet another element, and because he liked the name or because of a superficial resemblance of the minerals, this element is what we now call thorium, with the symbol Th.

While Berzelius did figure out many of the chemical properties of this new element, one crucial characteristic escaped him, its radioactivity. This should not surprise us though, as the phenomenon of radioactivity was not discovered until long after his death by Henri Becquerel in 1896. Today, its radioactivity seems logical as when we look at the periodic table, we find thorium, element 90, just after actinium in the last row of the periodic table known as the actinides, comprising of famous radioactive elements such as uranium and plutonium.

In the years after its discovery, thorium rested mostly undisturbed on the laboratory shelves until called to duty to light up the streets and homes of the world's metropoles. This was because of another of its remarkable properties: its oxide ThO2 has the highest melting point of all known oxides. Thus in the fierce heat in the flame of burning gas it would not melt, but glow intensively with a bright white light, making thorium oxide incandescent gas mantles the obvious choice for gaslight devices all over the world.

The importance of gaslight is now forgotten, but arguably this was a greater advance than the invention of the electric light, because for first time in history abundant light was available after sunset. Initially, other metal oxides were used, but besides problems with the melting points, the colour of the light they gave off was not quite right, and so in 1891 Austrian chemist Auer von Welsbach came up with the thorium solution after a first failed attempt with a magnesium, lanthanum and yttrium product in 1885.

Now, you may think that this was in fact a poisoned gift and that the upper classes of the late 19th century, after years of radioactive exposure from decaying thorium atoms, suffered from radiation related illnesses. But thankfully this wasn't so. Thorium decays by emitting alpha particles, and these alpha particles, or helium two plus ions, as they should really be called, do not travel very far and are easily stopped by the glass cover of a gas lantern and even the human skin.

In fact, thorium oxide mantles are still in use today, and you may even have come into contact with them yourself in camping lanterns. They are completely harmless unless you eat them, or inhale the powder from pulverized mantles. However, as the manufacture requires large amounts of thorium oxide, it is preferred to avoid it, and normally, most gas mantles sold in outdoors equipment shops today are advertised as 'thorium free'. But the next time you stock up for your camping expedition, by all means, bring your Geiger counter!

So, short from eating it, there are no particular worries in handling such tiny amounts of thorium oxide. However, eating it was just the point when using the x-ray contrast agent thorotrast, a state-of-the-art diagnostic aid in the 1930s and 1940s, depending on thorium's excellent ability to absorb x-rays.

Undoubtedly, the superior x-ray photographs generated this way saved many lives, so the risk of developing cancer some 20 years later was probably worth taking in serious cases. Thankfully, though, less dangerous contrast agents were soon developed.

Thorium thus spent its first sixty years in obscurity, then had fifty years in the limelight.

Thorium may be three times more abundant on Earth than uranium, it is difficult to estimate, and can also be used in nuclear reactors. In addition, thorium and uranium deposits do not necessarily occur at the same places, thus countries with large potential uranium resources may well have very little thorium and vice-versa.

The proponents of this so called thorium fuel cycle also claim it has important technical advantages, but it seems hopes for "burning" weapon grade plutonium or producing waste with reduced risks of nuclear arms proliferation are largely unfounded. On the contrary, the high melting point of the oxide is a drawback in this application as it makes the preparation of the fuel more difficult.

So, although a number of nuclear reactors worldwide have been run on thorium-based fuels the last decades, and some have even been connected to the electrical grid, it may yet be a long time until our houses and streets are again lit up with thorium based technology.

Meera Senthilingam

So time will tell if Thorium makes its comeback (with minimal exposure risks, that is). That was Lars Öhrström from the Chalmers tekniska högskola in Sweden, with the radioactive chemistry of Thorium.

Now next week, an element that lived up to its predictions

David Lindsay

In 1879, Lars Nilson isolated the oxide of a new metal from the minerals gadolinite and euxenite. Nilson was a student of the legendary Jacob Berzelius, himself discoverer of many elements. Nilson named this oxide scandia, after Scandinavia. The discovery of this element was especially notable, as, seven years previously, Mendeleev had used his periodic table to predict the existence of ten as yet unknown elements, and for four of these, he predicted in great detail the properties they should have. One of these four, Mendeleev predicted, should have properties very similar to boron, and he named this new element "ekaboron", meaning "like boron". The metal of this new oxide, scandia, was indeed found to have similar properties to this "ekaboron", thus demonstrating the power of Mendeleev's construct.

Meera Senthilingam

And join Reading University's David Lindsay to find out what these properties of scandium were that resembled boron so closely, as well as its applications, in next week's Chemistry in its element. Until then, I'm Meera Senthilingham 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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