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 13  Melting point 304°C, 579°F, 577 K 
Period Boiling point 1473°C, 2683°F, 1746 K 
Block Density (g cm−3) 11.8 
Atomic number 81  Relative atomic mass 204.38  
State at 20°C Solid  Key isotopes 205Tl 
Electron configuration [Xe] 4f145d106s26p1  CAS number 7440-28-0 
ChemSpider ID 4514293 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 (from Greek ‘thallos’, meaning ‘a green shoot or twig’), its toxicity and its use in the manufacture of reflective glass.
A soft, silvery-white metal that tarnishes easily.
The use of thallium is limited as it is a toxic element. Thallium sulfate was employed as a rodent killer – it is odourless and tasteless – but household use of this poison has been prohibited in most developed countries.

Most thallium is used by the electronics industry in photoelectric cells. Thallium oxide is used to produce special glass with a high index of refraction, and also low melting glass that becomes fluid at about 125K.

An alloy of mercury containing 8% thallium has a melting point 20°C lower than mercury alone. This can be used in low temperature thermometers and switches.
Biological role
Thallium has no known biological role. It is very toxic and there is evidence that the vapour is both teratogenic (disturbs the development of an embryo or foetus) and carcinogenic. It can displace potassium around the body affecting the central nervous system.
Natural abundance
Thallium is found in several ores. One of these is pyrites, which is used to produce sulfuric acid. Some thallium is obtained from pyrites, but it is mainly obtained as a by-product of copper, zinc and lead refining.

Thallium is also present in manganese nodules found on the ocean floor.
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The discovery of thallium was controversial. William Crookes of the Royal College of Science in London was the first to observe a green line in the spectrum of some impure sulfuric acid, and realised that it meant a new element. He announced his discovery in March 1861 in Chemical News. However, he did very little research into it.

Meanwhile, in 1862, Claude-August Lamy of Lille, France, began to research thallium more thoroughly and even cast a small ingot of the metal itself. The French Academy now credited him its discovery. He sent the ingot to the London International Exhibition of 1862, where it was acclaimed as a new metal and he was awarded a medal. Crookes was furious and so the exhibition committee awarded him a medal as well.

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) 1.96 Covalent radius (Å) 1.44
Electron affinity (kJ mol−1) 36.375 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, 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  203Tl 202.972 29.52
  205Tl 204.974 70.48


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) 0.85
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)
129 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 43
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- 1.59
x 10-5
0.0931 16.9 - - - - - - -
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Listen to Thallium Podcast
Transcript :

Chemistry in its element: thallium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Chris Smith

Hello, this week's element sees us immersed in a murder mystery - Henry Nicholls:

Henry Nicholls

During World War I, Agatha Christie worked in a hospital and then a pharmacy, an experience that could explain the presence of poisons in many of her plots. In The Pale Horse, a thriller published in 1961, the star of the show was thallium, also known as "the poisoner's poison" because many salts of this soft, silvery metal is soluble in water, producing a colourless, odourless and tasteless liquid with a delayed effect on the victim. Here's an excerpt from the dramatic climax in which the novel's narrator Mark Easterbrook solves the mystery of several unexplained deaths.

I slammed back the receiver, then took it off again. I dialed a number and was lucky enough this time to get Lejeune straight away.

"Listen," I said, "is Ginger's hair coming out by the roots in handfuls?"

"Well - as a matter of fact I believe it is. High fever, I suppose."

"Fever my foot," I said. "What Ginger's suffering from, what they've all suffered from, is thallium poisoning. Please God, may we be in time..."

Christie may have got the idea for her plot a few years' earlier in 1957, when the KGB attempted to assassinate Nikolai Khokhlov, a former KGB assassin himself who had defected to the United States. In turn Christie's dramatic and detailed description of the symptoms of thallium poisoning in The Pale Horse is thought to have saved at least two lives and led to the arrest and conviction of a British factory worker who had used thallium to kill his stepmother, two work colleagues and nauseate around 70 others. It is so dangerous because thallium has similar biological properties to potassium ions, hijacking the ubiquitous sodium/potassium membrane pump to smuggle itself into cells throughout the body interfering with the important roles played by potassium.

Thallium is pretty abundant in the earth's crust, found in several selenium-containing minerals. Indeed, it was whilst cooking up one such compound in 1861 that British chemist William Crookes noted that "suddenly a bright green line flashed into view and quickly disappeared." He knew he was onto a new element and called it thallium after the Greek for green shoot or twig - thallos. The following year, he succeeded in isolating small quantities of the element, but nowhere near the quantities obtained by French chemist Claude-Auguste Lamy who was working away independently with a greater bulk of raw material. When, in 1862, Lamy was awarded a medal at the International Exhibition in London For the discovery of a new and abundant source of thallium, Crookes had a fit and it was only with his election to the Royal Society in 1863 - largely on the back of his thallium work - that the cross-channel spat for priority died down. Subsequent work on the chemistry of thallium showed it to have similar properties to several other elements, including silver, mercury and lead. So much so that French chemist Jean-Baptiste Dumas later dubbed it the "ornithorhyncus, or duck-billed platypus of the metals."

The raw material on which both Crookes and Lamy worked came from waste products deposited during the manufacture of sulphuric acid. The commercial production of thallium today is not dissimilar, with the metal mostly recovered as a by-product of smelting iron, zinc or lead sulphides to make sulphur dioxide. The resulting thallium contains the two naturally occurring stable isotopes, with around 30% of it made up of atomic mass 203 and the remaining 70% comprised of atomic mass 205.

Owing to its toxic properties, thallium has been used as a rodenticide, though there are safer ways to kill rats and the use of this chemical in the environment is now banned in many countries. Today, thallium is of greatest use to the electronics industry. In particular, the conductivity of thallium sulphide alters on exposure to infrared light, making it an important compound in photocells. Thallium bromide-iodide crystals have also been used in infrared detectors. The addition of metals like thallium to glass can also reduce its melting point to as low as 150 degrees centigrade. As such low-melting point glasses do not shatter like normal glasses, they are particularly useful for the manufacture of electronic parts. Thallium is also being tested in high-temperature ceramic superconductors.

Alongside the two stable isotopes, there are a further 23 radioisotopes, though most of them with fleeting half lives. One of them, thallium 201, is useful in nuclear medicine. Its injected into the bloodstream and will find its way into all tissues with the help of the sodium/potassium membrane pump. This can then reveal to the clinician any part of the body not bathed in blood or where the membrane transporter is not working properly. In particular, it is used to image the blood flow to heart muscle in patients suspected of coronary artery disease. Thankfully, with a suitably short half-life of just 72.5 hours, Thallium 201 disappears from the body long before it can cause the lethal damage of the more stable isotopes.

In The Pale Horse, Agatha Christie was not as explicit about the treatment for thallium poisoning as she was about its symptoms. "Do they know how to treat thallium poisoning?" asks the narrator Mark Easterbrook when he reaches the hospital where the hair-shedding Ginger has been taken. "You don't often get a case of it," the investigating officer Inspector Lejeune tells him. "But everything possible will be tried." It was, and for those who like their happy endings you'll be pleased to know that Ginger makes a full recovery from the thallium poisoning that had stricken her down.

Chris Smith

That's a relief, she was OK, although you've totally blown the ending Henry! That was science writer Henry Nicholls with the story of Thallium. Next time, to the element that suits someone who doesn't want to blow up the world, maybe just a small bit of it.

Brian Clegg

When it comes to practical uses, this silvery substance is an excellent neutron emitter. This makes it handy for kick-starting nuclear reactors, where a high neutron flow is required to get the chain reaction going. It also means that, in principle, californium would make effective small scale nuclear weapons, requiring as little as five kilograms of californium 251 to achieve critical mass - about half the amount of plutonium required for a bomb.

Chris Smith

That's the story of Californium, which apart from its use potentially as a nuclear weapon is also useful for finding gold and striking oil. And you can join us on next week's Chemistry in its element to find out how. I'm Chris Smith, thank you for listening and goodbye!


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

(End promo)
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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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