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 1545°C, 2813°F, 1818 K 
Period Boiling point 1950°C, 3542°F, 2223 K 
Block Density (g cm−3) 9.32 
Atomic number 69  Relative atomic mass 168.934  
State at 20°C Solid  Key isotopes 169Tm 
Electron configuration [Xe] 4f136s2  CAS number 7440-30-4 
ChemSpider ID 22400 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 reflects the origin of the element’s name, and suggests a distant region to the far north (ultima Thule).
A bright, silvery metal.
When irradiated in a nuclear reactor, thulium produces an isotope that emits x-rays. A ‘button’ of this isotope is used to make a lightweight, portable x-ray machine for medical use. Thulium is used in lasers with surgical applications.
Biological role
Thulium has no known biological role. It is non-toxic.
Natural abundance
Thulium is found principally in the mineral monazite, which contains about 20 parts per million. It is extracted by ion exchange and solvent extraction. The metal is obtained by reducing the anhydrous fluoride with calcium, or reducing the oxide with lanthanum.
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Thulium was first isolated in 1879 as its oxide by Per Teodor Cleve at the University of Uppsala, Sweden. The discoveries of the many rare earth elements (aka lanthanoid) began with yttrium in 1794. This was contaminated with these chemically similar elements. Indeed the early chemists were unaware they were there. In 1843, erbium and terbium were extracted from yttrium, and then, in 1874, Cleve looked more closely at erbium and realised that it must contain yet other elements because he observed that its atomic weight varied slightly depending on the source from which it came. He extracted thulium from it in 1879.

In 1911, the American chemist Theodore William Richards performed 15,000 recrystallisations of thulium bromate in order to obtain an absolutley pure sample of the element and so determine exactly its atomic weight.

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) 2.27 Covalent radius (Å) 1.77
Electron affinity (kJ mol−1) 99.283 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, 2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  169Tm 168.934 100


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 9.5
Crustal abundance (ppm) 0.3
Recycling rate (%) <10
Substitutability High
Production concentration (%) 97
Reserve distribution (%) 50
Top 3 producers
  • 1) China
  • 2) Russia
  • 3) Malaysia
Top 3 reserve holders
  • 1) China
  • 2) CIS Countries (inc. Russia)
  • 3) USA
Political stability of top producer 24.1
Political stability of top reserve holder 24.1


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)
160 Young's modulus (GPa) 74
Shear modulus (GPa) 30.5 Bulk modulus (GPa) 44.5
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- 6.03
x 10-10
x 10-5
0.0561 5.22 130 - - - - -
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Listen to Thulium Podcast
Transcript :

Chemistry in its element: thulium


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's element takes us into the unknown, entering dark, mysterious lands.

Brian Clegg

In medieval times, when maps were bedecked with strange and exotic unknowns, where the corners might be inscribed 'Here be monsters', the most distant place that could be conceived, lying beyond the borders of the known world, was labelled 'Ultima Thule'. Thule is sometimes pronounced Tooli, though it looks as if it should be Thool, which frankly sounds much more suitably dark and mysterious. Originally this was the classical name for a mysterious land, six day's sail to the north of Britain, thought by the Greek historian Polybius to be the most northerly part of the world. 'Ultima Thule' took things one stage further - it was the farthest part of Thule.

When thulium was named by Per Teodor Cleve in 1879, it was down to a slight misunderstanding of the meaning of thule. Cleve would eventually discover a total of four elements, and won the Davy Medal from the Royal Society for his work on the rare earth metals, but here he wasn't entirely accurate. He wrote 'For the oxide placed between ytterbia and erbia. I propose the name of thullium derived from Thule, the ancient name of Scandinavia.' Not only had he misplaced Thule, he couldn't even spell it right, putting two L's in the name - but today we spell thulium, like Thule, with a single L.

Sitting towards the end of the lanthanides, the floating strip of elements on the periodic table that squeezes between barium and lutetium, thulium has atomic number 69. It's one of the rare earths, elements that are largely misnamed as they are quite common. The name reflects the rarity of the original ore in which they were found - but in thulium's case it's not such a bad title as this soft, silvery metal is one of the rarest of the rare earths, and more valuable than platinum.

The initial discovery of the element was something of an accident. Traces of erbium and terbium had been found when ytrrium was first discovered, though it wasn't initially realized that they were new elements in their own right. Cleve was examining the erbium oxide separated from the mix and found that this too was corrupted. It had a small amount of an unknown substance which gave a slight variation to the atomic weight. The ever finer separation of the contents of this productive ore would eventually yield the oxides of two further elements - hol-mium and finally thulium.

For a long time thulium was a Cinderella substance. There was nothing you could do with thulium that couldn't be done better and cheaper with one of the other elements. It looked likely that it would be consigned to the dustbin of useless chemical substances. It's notable that one science writer has said of thulium 'the most surprising thing about it is there's nothing surprising about it.' But that's a little unfair.

Thulium isn't exactly mass market, but about 50 tonnes of it is mined each year, broadly in three bands of ores - Australia and China, the US and Brazil, and India and Sri Lanka. And that's not an effort that would be put in for nothing.

The only natural isotope of thulium, usually found as an oxide is thulium 169. This is stable, but thulium 170 with a half life of 128 days, produced by bombarding thulium in a nuclear reactor, has proved a good portable source of x-rays. It was first suggested for this role in the 1950s and has frequently turned up since in small scale devices, such as those used in dentist's surgeries. As a low energy source, it's relatively safe, making it a good bet for low tech applications that also find it cropping up in engineering, where the x-rays can be used to hunt for cracks in components.

Less common, but still valuable, is thulium's role in doping a special type of garnet, yttrium aluminium garnet or YAG. The crystal is used as the active medium in a laser with a wavelength of around 2,000 nanometres, which is ideal for laser surgery, so once again thulium comes to our medical aid.

Thulium might not have many uses, but it did contribute to the Nobel Prize of American chemist Theodore William Richards. If ever a Nobel Prize was awarded for sheer dogged hard work, then it was the one won by Richards in 1914. The Nobel citation must be one of the least exciting ever made. It was 'in recognition of his accurate determination of the atomic weight of a large number of chemical elements.' But this reflects for thulium alone a total of 15,000 recrystalisation experiments before Richards had a pure enough sample of thulium bromate to be able to fix its atomic weight to his satisfaction (168.93421 to be precise).

When Per Teodor Cleve named thulium he was working at the University of Uppsala in Sweden, the oldest of the Nordic universities. He wanted to celebrate historic Scandinavian culture - and even if he didn't quite position the mythological land correctly, for Cleve his new discovery would remain the Ultima Thule.

Meera Senthilingam

Taking us into distant lands there with the element that comes to our medical aid in lasers and small scale x-rays. That was Science Writer Brian Clegg with the chemistry of Thulium. Now next week, an element that can be manipulated to give us what we want.

Andrea Sella

Dark grey in colour and with a very glossy glass-like sheen, it looks like a metal but is in fact quite a poor conductor of electricity, and there in many ways, lies the secret of its ultimate success. By deliberately introducing impurities like boron or phosphorus one can subtly change the electrical behaviour of the element. Such tricks lie at the heart of the functioning of the silicon chips that allow you to listen to this podcast. In less than 50 years silicon has gone from being an intriguing curiosity to being one of the most fundamental elements in our lives.

Meera Senthilingam

And to find out more about how crucial silicon is in our everyday lives, join Andrea Sella 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.


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