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 Melting point 3033°C, 5491°F, 3306 K 
Period Boiling point 5008°C, 9046°F, 5281 K 
Block Density (g cm−3) 22.5872 
Atomic number 76  Relative atomic mass 190.23  
State at 20°C Solid  Key isotopes 192Os 
Electron configuration [Xe] 4f145d66s2  CAS number 7440-04-2 
ChemSpider ID 22379 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 suggests the use of the element in making high-quality pen nibs.
A shiny, silver metal that resists corrosion. It is the densest of all the elements and is twice as dense as lead.
Osmium has only a few uses. It is used to produce very hard alloys for fountain pen tips, instrument pivots, needles and electrical contacts. It is also used in the chemical industry as a catalyst.
Biological role
Osmium has no known biological role. The metal is not toxic, but its oxide is volatile and very toxic, causing lung, skin and eye damage.
Natural abundance
Osmium occurs uncombined in nature and also in the mineral osmiridium (an alloy with iridium). Most osmium is obtained commercially from the wastes of nickel refining.
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In 1803, Smithson Tennant added platinum to dilute aqua regia, which is a mixture of nitric and hydrochloric acids, and observed that not all the metal went into solution. Earlier experimenters had assumed that the residue was graphite, but he suspected it was something else, and he began to investigate it. By a combination of acid and alkali treatments he eventually separated it into two new metal elements, which he named iridium and osmium, naming the latter on account of the strong odour it gave off. Its name is derived from osme the Greek word for smell. Although it was recognised as a new metal, little use was made of it because it was rare and difficult to work with, although it was extremely hard wearing and for several years it was used for pen nibs and gramophone needles.

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) 2.16 Covalent radius (Å) 1.36
Electron affinity (kJ mol−1) 106.1 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 8, 6, 4, 3, 2, 0, -2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  184Os 183.952 0.02
  186Os 185.954 1.59 2 x 1015 α 
  187Os 186.956 1.96
  188Os 187.956 13.24
  189Os 188.958 16.15
  190Os 189.958 26.26
  192Os 191.961 40.78


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) 0.000037
Recycling rate (%) >30
Substitutability High
Production concentration (%) 60
Reserve distribution (%) 95
Top 3 producers
  • 1) South Africa
  • 2) Russia
  • 3) Zimbabwe
Top 3 reserve holders
  • 1) South Africa
  • 2) Russia
  • 3) USA
Political stability of top producer 44.3
Political stability of top reserve holder 44.3


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)
130 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)
- - - - - - - 1.85
x 10-10
x 10-8
x 10-6
x 10-5
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Listen to Osmium Podcast
Transcript :

Chemistry in its element: osmium


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 the illuminating story of the chemical that christened a light bulb company and helps us to find fingerprints but in the wrong hands can stink to high heaven. To tell the story of the densest element we know here's science broadcaster Quentin Cooper.

Quentin Cooper

Given the whole periodic table to choose from, how to pick a particular element to talk about rather than any other? They've all got their charms and quirks - well, except maybe bismuth.I've never had much time for bismuth - but the deal was I had to single out one. And then it came to me. A real light-bulb moment. Osmium. Under-appreciated under-exploited osmium - if any element needs a change of PR this is the one. It's brittle, prone to ponginess and arguably the dunce of the periodic table. Even the man who discovered osmium treated it rather sniffily. Perhaps in part that's because Smithson Tennant, an English chemist, was also the first to establish that diamond is a form of this was never going to match up to that glittering career highlight. What also didn't help was that his discovery of osmium around 1803 came as part of a job lot - he isolated another element, alongside it: also a metal it was hard and yellowy-white and some of its compounds had a kind of rainbow sheen when they caught the light so he gave it a nice shiny name - iridium as in iridescent. No such luck for the bluish-silver substance he found at the same time : it reeked - or at least some of its compounds did. Tennant described the "pungent and penetrating smell" as one of the new element's "most distinguishing characters". So he called it osmium - osme being the Greek for odour. Not very nice.but at least apposite: as a powder even at room temperature it gives off osmium tetroxide, which is so corrosively pungent it can damge the eyes, lungs and skin..although strangely that doesn't prevent it sometimes being used - with extreme care - to help detect fingerprints..

So osmium is not just an element, it's a smellement, and it's also way beyond lead and gold and platinum as probably the most immensely dense of the whole bunch. I say probably because it depends how you measure it - and while some rate it as densest others argue it's just pipped by the very thing it was discovered with, iridium. Down the decades as tests have been refined, the right to wear the dense-is cap has repeatedly shifted between the two.. making the only safe option to declare them joint-winners of the prestigious title of densest element in the periodic table. Given the two share both discovery and a date with density it's perhaps no surprise to find they also rub along in nature and occur as an alloy, wittily known as osmiridium - something you'll find in upmarket fountain pen nibs and odd bits of surgical equipment. Osmium itself also plays a part in some catalysts, and is used for staining specimens in microscopy.

None of these is what you might call a bulk application - which may account for why it's estimated that the current annual amount of osmium now produced right around the world weighs less than a large tiger. Or about 100 kilograms if you prefer conventional units. Time was, though, when osmium was considerably more sought after. Not because of its density or smelly compounds, but because of its high melting point. Very high - over 3 thousand degrees C. After Thomas Edison produced the first commercial electric light in 1879, the race was on to improve on his design - for starters there was the filament - the bit that glows to produce the light but, crucially, doesn't melt. There had to be something better than Edison's use of bamboo - I mean, bamboo, really.what was he thinking? Lots of possibilities were explored, but - largely thanks to the work of the Austrian chemist Carl Auer Von Welsbach - the top two elements-as-filaments ended up being osmium and tungsten.which has an even higher melting point. These days it's tungsten that's the clear favourite, but in 1906 when a name was needed for a new German company making these improved lights, they simply went with a verbal alloy of the two. Os from Osmium and ram from Wolfram - the German name for Tungsten.hence Osram - now one of the largest lighting manufacturers in the world.and hence my bright light-bulb moment when it came to picking osmium.

Chris Smith

Quentin Cooper who was turning the spotlight for us this week onto Osmium. Thanks Quentin. Next time we're meeting the metal that can sooth this burning issue.

Andrea Sella

A few weeks ago I had a stupid accident in the lab; I wont go into the details; I am not terribly proud about what happened. But the result is I suffered from some superficial burns on my face and neck. I was seen to by a specialist nurse who nodded at me and then handed me tub of ointment. 'Its flammacerium', she said, 'apply it twice a day'. 'Flama what', I replied, 'cerium', she said. I was delighted. 'Cerium, it can not be serious, it's my favorite element'.

Chris Smith

And that's Andrea Sella who will be introducing the chemical that can quite literally get right under your skin but at the same time clean up car emissions and also polish the mirrors of telescopes. That's the science of cerium in next week's Chemistry in it's element, I hope you can join us. I'm Chris Smith, thank you for listening, see you next time.


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.