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 900°C, 1652°F, 1173 K 
Period Boiling point Unknown 
Block Density (g cm−3) 15.1 
Atomic number 98  Relative atomic mass [251]  
State at 20°C Solid  Key isotopes 249Cf, 252Cf 
Electron configuration [Rn] 5f107s2  CAS number 7440-71-3 
ChemSpider ID 22433 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 on the state flag of California and features a grizzly bear (a symbol of great strength) and a lone star.
Californium is a radioactive metal.
Californium is a very strong neutron emitter. It is used in portable metal detectors, for identifying gold and silver ores, to identify water and oil layers in oil wells and to detect metal fatigue and stress in aeroplanes.
Biological role
Californium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Californium did not exist in weighable amounts until ten years after its discovery. It is prepared, in milligram amounts only, by the neutron bombardment of plutonium-239.
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Californium was first made in 1950 at Berkeley, California, by a team consisting of Stanley Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn Seaborg. They made it by firing helium nuclei (alpha particles) at curium-242. The process yielded the isotope californium-245 which has a half-life of 44 minutes. Curium is intensely radioactive and it had taken the team three years to collect the few milligrams needed for the experiment, and even so only a few micrograms of this were used. Their endeavours produced around 5,000 atoms of californium, but there was enough to show it really was a new element.

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) 1.68
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, 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  249Cf 249.075 - 351 y  α 
        8 x 1010 sf 
  250Cf 250.076 - 13.1 y  α 
        1.7 x 104 sf 
  251Cf 251.080 - 9 x 102 α 
  252Cf 252.082 - 2.65 y  α 
        86 y  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.



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)
Unknown 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)
- - - - - - - - - - -
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Listen to Californium Podcast
Transcript :

Chemistry in its element: californium


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

This week, let's go surfing.

Brian Clegg

What comes to mind when you think of California? Surfing and the Beach Boys? Hollywood and Governor Schwarzenegger? The University of California at Berkeley has ensured that California also has its place in the periodic table with element 98, the tenth of the actinides, californium.

Although it seems perfectly sensible to celebrate the location where it was discovered, californium's name was, in fact, a failure for the team behind its production. Glen T. Seaborg and his co-workers had named americium to parallel the lanthanide above it in the periodic table, europium. They went on to name curium and berkelium in a way that was also derived from the equivalent lanthanide. So, for instance, the actinide berkelium was named after Berkeley because the lanthanide above it, terbium, was named after the Swedish village Ytterby where it was quarried.

When it came to californium, an artificial element first produced in 1950, the equivalent lanthanide would be dysprosium, which comes from the Greek for 'hard to get.' After some head-scratching, Seaborg and his team gave up on the search for an equivalent and just went for the location of the lab. They had already discarded a list of names including cyclotronium and cyclonium, after the device used in producing the first californium, along with the more than a little cheesy radlabium, reflecting the team's origins as part of the radiation laboratory or rad lab.

They did, though, manage a neat bit of rationalization, arguing that they paralleled dysprosium's 'hard to get' meaning because 'the searchers for another element a century ago found it difficult to get to California.' This referred to the state's inaccessibility during the nineteenth century gold rush.

The first isotope of californium produced was californium 245, with a half life of just 44 minutes. The team battered a target of curium with alpha particles using a cyclotron, an early type of particle accelerator still in use today, particularly in medical applications. The cyclotron accelerates charged particles using electrodes that switch rapidly between attracting and repelling as the particles spiral around a circular chamber until they collide with a target. In this case the collision produced californium and a spare neutron.

The most stable of californium's 20 or so produced isotopes is californium 251, which has a half life of 898 years, though many of the isotopes have half-lives measured in minutes. It's most often made now by starting with berkelium 249 and adding neutrons in a nuclear reactor. Although this is a purely artificial element here on earth, it may exist in space as one of the many by-products of supernovas.

When it comes to practical uses, this slivery 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 - but in practice it is so fiddly to produce that even at this scale it is unlikely to be used.

As well as providing the starter for reactors, small amounts of californium have also found their way into a number of devices requiring a flow of neutrons, whether it is specialist detectors or radiotherapy, as a last resort for some cancer treatments where gentler sources have failed.

Perhaps californium's most common application is in moisture gauges used in potential oil wells. These detectors fire fast neutrons through the material to be tested. Hydrogen nuclei, typical of those in water and oil, tend to slow down the neutrons, so a slow neutron detector can be used to search for telltale hydrogen. The neutrons from californium can also be used in prospecting for silver and gold, using a technique called neutron activation analysis which bombards an area to be tested with neutrons and searches for the gamma rays emitted from the bombarded substance, with a characteristic signature.

In the end, though, it's californium's name that remains most significant. Perhaps, to parallel dysprosium's 'hard to get', it should have been lethium, from the latin for 'lying hidden' - but maybe that sounds too like lithium. Shortly after californium was first produced, the name was the subject of a running joke between its discoverers and the New Yorker magazine.

The magazine observed that the discoverers had missed a trick. It commented that 'California's busy scientists will undoubtedly come up with another atom or two one of these days, and the university might well have anticipated that. Now it has lost forever the chance of immortalizing itself in the atomic tables with some such sequence as universitium (97), offium (98), californium (99), berkelium (100).' Spelling out 'University of California, Berkeley,' across the table.

The discoverers fired back that the problem with calling elements 97 and 98 universitium and offium was the appalling possibility that some New Yorker could discover 99 and 100 and name them newium and yorkium. The New Yorker staff claimed already to be at work on these elements. but as yet all the journalists had achieved was to think up the names.

As it is, we can never be quite sure if 'californium' refers to the state or the university - and it is hard to produce - so in these respects, at least, californium parallels dysprosium as an element that's 'hard to get'.

Chris Smith

Well in that case, if we're naming things after other things that are hard to get hold of, how about a taxi in rush hourium or, worse still perhaps, what about a James Blunt CD you can tolerateium? That would be my suggestion. That was Brian Clegg, with this week's element Californium. Next time, it's over to Sarah Staniland.

Sarah Staniland

I always find the question 'what's your favourite element' a difficult one. There are several front runners for vastly varying reasons; however, always a top contender has to be cobalt because it excels in several important character traits: Cobalt has amazing beauty and strength, as well as great cooperation.

Chris Smith

I thought she was talking about me for a minute there. That's Sarah Staniland from Leeds University who will be here next week with the story of cobalt. Do try and join us. Thanks for listening, I'm Chris Smith, 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.