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 1135°C, 2075°F, 1408 K 
Period Boiling point 4131°C, 7468°F, 4404 K 
Block Density (g cm−3) 19.1 
Atomic number 92  Relative atomic mass 238.029  
State at 20°C Solid  Key isotopes 234U, 235U, 238
Electron configuration [Rn] 5f36d17s2  CAS number 7440-61-1 
ChemSpider ID 22425 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 is based around the common astrological symbol for the planet Uranus.
A radioactive, silvery metal.
Uranium is a very important element because it provides us with nuclear fuel used to generate electricity in nuclear power stations. It is also the major material from which other synthetic transuranium elements are made.

Naturally occurring uranium consists of 99% uranium-238 and 1% uranium-235. Uranium-235 is the only naturally occurring fissionable fuel (a fuel that can sustain a chain reaction). Uranium fuel used in nuclear reactors is enriched with uranium-235. The chain reaction is carefully controlled using neutron-absorbing materials. The heat generated by the fuel is used to create steam to turn turbines and generate electrical power.

In a breeder reactor uranium-238 captures neutrons and undergoes negative beta decay to become plutonium-239. This synthetic, fissionable element can also sustain a chain reaction.

Uranium is also used by the military to power nuclear submarines and in nuclear weapons.

Depleted uranium is uranium that has much less uranium-235 than natural uranium. It is considerably less radioactive than natural uranium. It is a dense metal that can be used as ballast for ships and counterweights for aircraft. It is also used in ammunition and armour.
Biological role
Uranium has no known biological role. It is a toxic metal.
Natural abundance
Uranium occurs naturally in several minerals such as uranite (pitchblende), brannerite and carnotite. It is also found in phosphate rock and monazite sands. World production of uranium is about 41,000 tonnes per year.

Extracted uranium is converted to the purified oxide, known as yellow-cake. Uranium metal can be prepared by reducing uranium halides with Group 1 or Group 2 metals, or by reducing uranium oxides with calcium or aluminium.
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In the Middle Ages, the mineral pitchblende (uranium oxide, U3O8) sometimes turned up in silver mines, and in 1789 Martin Heinrich Klaproth of Berlin investigated it. He dissolved it in nitric acid and precipitated a yellow compound when the solution was neutralised. He realised it was the oxide of a new element and tried to produce the metal itself by heating the precipitate with charcoal, but failed.

It fell to Eugène Peligot in Paris to isolate the first sample of uranium metal which he did in 1841, by heating uranium tetrachloride with potassium.

The discovery that uranium was radioactive came only in 1896 when Henri Becquerel in Paris left a sample of uranium on top of an unexposed photographic plate. It caused this to become cloudy and he deduced that uranium was giving off invisible rays. Radioactivity had been discovered.

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) 2.41 Covalent radius (Å) 1.83
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 6, 5, 4, 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  233U 233.040 - 1.590 x 105 α 
        > 2.7 x 1017 sf 
  234U 234.041 0.0054 2.453 x 105 α 
        1.5 x 1016 sf 
  235U 235.044 0.7204 7.03 x 108 α 
        1.0 x 1019 sf 
  236U 236.046 - 2.342 x 107 α 
        2.5 x 1016 sf 
  238U 238.051 99.2742 8.2 x 1015 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 5.7
Crustal abundance (ppm) 1.3
Recycling rate (%) Unknown
Substitutability High
Production concentration (%) 33
Reserve distribution (%) 31
Top 3 producers
  • 1) Kazakhstan
  • 2) Canada
  • 3) Australia
Top 3 reserve holders
  • 1) Australia
  • 2) Kazakhstan
  • 3) Canada
Political stability of top producer 61.8
Political stability of top reserve holder 74.5


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)
116 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)
- - - - 9.47
x 10-10
x 10-6
x 10-6
0.000263 0.00678 0.0933 0.803
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Listen to Uranium Podcast
Transcript :

Chemistry in its element: uranium


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

For Chemistry in its element this week, can you guess what connects boat keels, armour piercing weaponry, beautiful coloured glass that you can track down with a geiger counter and more oxidation states than a chemist can shake a glass rod at. If not, here's Polly Arnold with the answer.

Polly Arnold

Uranium is certainly one of the most famous, or perhaps I should say infamous, elements. It is the heaviest naturally occurring element. It is actually more abundant in the earth's crust than silver. It is one of eight elements named in honour of celestial objects, but you might not think that uranium deserves to be named after the planet Uranus. The lustrous black powder that the chemist Klaproth isolated from the mineral pitchblende in 1789 - just eight years after Uranus was discovered - was in fact an oxide of uranium. Not until fifty two years later did Eugène Melchior Peligot reduced uranium tetrachloride with potassium, and from these harsher conditions obtained the pure silvery white metal at last. Samples of the metal tarnish rapidly in air, but if the metal is finely divided, it will burst into flames.

Uranium sits amongst the actinides, the second shell of metals to fill their f-orbitals with valence electrons, making them large and weighty.

Chemically, uranium is fascinating. Its nucleus is so full of protons and neutrons that it draws its core electron shells in close. This means relativistic effects come into play that affect the electron orbital energies. The inner core s electrons move faster, and are drawn in to the heavy nucleus, shielding it better. So the outer valence orbitals are more shielded and expanded, and can form hybrid molecular orbitals that generated arguments over the precise ordering of bonding energies in the uranyl ion until as recently as this century.

This means that a variety of orbitals can now be combined to make bonds, and from this, some very interesting compounds. In the absence of air, uranium can display a wide range of oxidation states, unlike the lanthanides just above it, and it forms many deeply coloured complexes in its lower oxidation states. The uranium tetrachloride that Peligot reduced is a beautiful grass-green colour, while the triiodide is midnight-blue. Because of this, some regard it as a 'big transition metal'. Most of these compounds are hard to make and characterise as they react so quickly with air and water, but there is still scope for big breakthroughs in this area of chemistry.

The ramifications of relativistic effects on the energies of the bonding electrons has generated much excitement for us synthetic chemists, but unfortunately many headaches for experimental and computational chemists who are trying to understand how better to deal with our nuclear waste legacy.

In the environment, uranium invariably exists as a dioxide salt called the uranyl ion, in which it is tightly sandwiched between two oxygen atoms, in its highest oxidation state. Uranyl salts are notoriously unreactive at the oxygen atoms, and about half of all known uranium compounds contain this dioxo motif. One of the most interesting facets of this area of uranium chemistry has emerged in the last couple of years: A few research groups have found ways to stabilise the singly reduced uranyl ion, a fragment which was traditionally regarded as too unstable to isolate. This ion is now beginning to show reactivity at its oxygen atoms, and may be able to teach us much about uranium's more radioactive and more reactive man-made sisters, neptunium and plutonium - these are also present in nuclear waste, but difficult to work with in greater than milligram quantities.

Outside the chemistry lab, uranium is best known for its role as a nuclear fuel. It has been at the forefront of many chemists' consciousness over recent months due to the international debate on the role that nuclear power can play in a future as a low-carbon energy source, and whether our new generations of safer and efficient power stations are human-proof.

To make the fuel that is used to power reactors to generate electricity, naturally occurring uranium, which is almost all U-238, is enriched with the isotope U-235 which is normally only present in about 0.7 %.

The leftovers, called depleted uranium, or DU, have a much-reduced U-235 content of only about 0.2 %. This is 40 % less radioactive than natural uranium, and the material that we use to make compounds from in the lab.

Because it is so dense, DU is also used in shielding, in the keels of boats and more controversially, in the noses of armour-piercing weapons. The metal has the desirable ability to self-sharpen as it pierces a target, rather than mushrooming upon impact the way conventional tungsten carbide tipped weapons do.

Critics of DU weaponry claim it can accumulate around battlefields. Because uranium is primarily an alpha-emitter, its radioactivity only really becomes a problem if it gets inside the body, where it can accumulate in the kidneys, causing damage. However, uranium is also a heavy metal, and its chemical toxicity is of greater importance - it is approximately as toxic as lead or mercury.

But uranium doesn't deserve it's image as one of the periodic table's nasties. Much of the internal heat of the earth is considered to be due to the decay of natural uranium and thorium deposits. Perhaps those looking to improve the public image of nuclear power should demand the relabelling of geothermal ground-source heat pumps as nuclear?

The reputation of this element would also be significantly better if only uranium glass was the element's most publicly known face. In the same way that lead salts are added to glass to make sparkling crystal glassware, uranyl salts give a very beautiful and translucent yellow-green colour to glass, although glassmakers have experimented to produce a wide range of gem-like colours. An archaeological dig near Naples in 1912 unearthed a small green mosaic tile dated back to 79 AD, which was reported to contain uranium, but these claims have not been verified. However in the early-19th and early 20th century it was used widely in containers and wine-glasses. If you think that you own a piece, you can check with a Geiger counter, or by looking for the characteristic green fluorescence of the uranium when held under a UV-lamp. Pieces are generally regarded as safe to drink from, but you are advised not to drill holes in them, or wear them. Fair enough.

Chris Smith

Or inadvertently eating it too, presumably. That was Edinburgh University chemist Polly Arnold explaining the softer side of the armour piercing element Uranium. Next week Andrea Sella will be introducing us to some crystals with intriguing properties.

Andrea Sella

"It's amazing stuff. You HAVE to see this." He pulled out of his pocket a sample vial containing some stunning pink crystals that glinted alluringly. "Wow!" I said - you can always impress a chemist with nice crystalline products. "It gets better." he said mysteriously. He beckoned me into a hallway. "Look" he said. As the crystals caught the light from the new fluorescent lights hanging from the ceiling, the pink colour seemed to deepen and brighten up. "Wow!" I said again. We moved the crystals back into the sunlight and the colour faded again, and moving the crystals back and forth they glowed and dimmed in magical fashion.

Chris Smith

But what did they contain? Well, the answer's Erbium and you can hear all about it in next week's Chemistry in its element. 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

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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.