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 28.5°C, 83.3°F, 301.7 K 
Period Boiling point 671°C, 1240°F, 944 K 
Block Density (g cm−3) 1.873 
Atomic number 55  Relative atomic mass 132.905  
State at 20°C Solid  Key isotopes 133Cs 
Electron configuration [Xe] 6s1  CAS number 7440-46-2 
ChemSpider ID 4510778 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 symbol reflects the use of the element in highly accurate atomic clocks.
Caesium is a soft, gold-coloured metal that is quickly attacked by air and reacts explosively in water.
The most common use for caesium compounds is as a drilling fluid. They are also used to make special optical glass, as a catalyst promoter, in vacuum tubes and in radiation monitoring equipment.

One of its most important uses is in the ‘caesium clock’ (atomic clock). These clocks are a vital part of the internetand mobile phone networks, as well as Global Positioning System (GPS) satellites. They give the standard measure of time: the electron resonance frequency of the caesium atom is 9,192,631,770 cycles per second. Some caesium clocks are accurate to 1 second in 15 million years.
Biological role
Caesium has no known biological role. Caesium compounds, such as caesium chloride, are low hazard.
Natural abundance
Caesium is found in the minerals pollucite and lepidolite. Pollucite is found in great quantities at Bernic Lake,Manitoba, Canada and in the USA, and from this source the element can be prepared. However, most commercialproduction is as a by-product of lithium production.
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Caesium was almost discovered by Carl Plattner in 1846 when he investigated the mineral pollucite (caesium aluminium silicate). He could only account for 93% of the elements it contained, but then ran out of material to analyse. (It was later realised that he mistook the caesium for sodium and potassium.)

Caesium was eventually discovered by Gustav Kirchhoff and Robert Bunsen in 1860 at Heidelberg, Germany. They examined mineral water from Durkheim and observed lines in the spectrum which they did not recognise, and that meant a new element was present. They produced around 7 grams of caesium chloride from this source, but were unable to produce a sample of the new metal itself. The credit for that goes to Carl Theodor Setterberg at the University of Bonn who obtained it by the electrolysis of molten caesium cyanide, CsCN.

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) 3.43 Covalent radius (Å) 2.38
Electron affinity (kJ mol−1) 45.505 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 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  133Cs 132.905 100


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 Unknown
Crustal abundance (ppm) 3
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)
242 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 1.6
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
0.394 - - - - - - - - - -
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Listen to Caesium Podcast
Transcript :

Chemistry in its element: caesium


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

(End promo)

Meera Senthilingam

This week, love at first sight. Peter Wothers.

Peter Wothers

I've been asked on a number of occasions what my favourite element is. I used to think either oxygen or hydrogen - both so much fun - but that was until my sample of caesium arrived, when it was love at first sight. Now many people think it's slightly odd having a favourite element, but when they too see my caesium, they understand why it's so special. Who wouldn't be attracted to this beautiful element?

For starters, there are only three metallic elements that are not silver-coloured. Two are well-known and fairly obvious - gold and copper. The third most people would never guess, it's caesium. Apparently, the beautiful gold diminishes if the sample is extremely pure since tiny traces of captured oxygen give it the colour. This is a little disappointing - its colour is quite stunning and I would be sad if it really did disappear when purified.

The next exciting thing about caesium is that my love is not unrequited, it responds to my touch. Strictly speaking, it's the warmth from the hand that melts it, given that its melting point is only 28.4 °C. So just holding its container converts the crystalline solid into liquid gold. Liquid metals are always fascinating - everyone loves mercury; just imagine playing with liquid gold!

But here's the snag that adds to my fascination with this metal - it has a rather fiery temper. In fact, you can't actually touch the metal itself since it spontaneously bursts into flames in the presence of air and reacts explosively with water. Awkward indeed. My caesium is sealed inside a glass tube under an atmosphere of the chemically inert gas argon. So to play with it, you have to hold the glass tube, knowing that if you accidentally crushed it, or dropped it, all hell would break loose.

Caesium gets its name from the Greek for heavenly blue. Not for its eyes (it's only an element!) but less romantically for the appearance of its emission spectrum in the spectroscope. Caesium was discovered in 1860 by Robert Bunsen (he of the burner fame) and physicist Gustav Kirchhoff. The previous year they had invented an instrument known as a spectroscope to help in chemical analysis. When atoms are energetically excited, for instance when a compound is introduced into a flame, electrons can temporarily be promoted to higher energy levels. When they return to their lower energy states, energy is released in the form of light. The spectroscope splits up the light with a prism and reveals a spectrum consisting of series of sharp coloured lines. Each element has its own unique spectrum of lines, like a rainbow barcode. When examining the spectrum of the residue from some spa mineral water, Bunsen and Kirchhoff found a series of lines that did not correspond to any known element. They named the new element caesium because of the distinct blue lines in the spectrum.

It is another electronic transition in caesium that gives us the most accurate clocks on earth. So called caesium atomic clocks are accurate to one second in more than a million years and are used when precision timing is crucial, for instance in tracking the space shuttle.

It is its willingness to lose an electron completely and form a positively charge ion that makes caesium the most reactive metal in the periodic table, and yes I am including its relative francium! All the alkali metals are reactive because they have one outermost electron which is easily removed but on moving down the group, the atoms get larger and larger and this outermost electron gets on average further and further away from the positively charged nucleus. What's more, on moving across the periodic table, from group one with lithium, sodium, potassium etc to group two with beryllium, magnesium, calcium and so on, it becomes increasingly harder to remove the outermost electrons. This means the element for which it is easiest to remove an electron and form a cation, is in the bottom left-hand corner of the periodic table, where caesium is found.

One pseudo-science programme on TV showed the reaction between water and the different group one alkali metals, namely lithium, sodium, potassium, rubidium and caesium. At least that's what they said. Actually, they faked the reaction of rubidium and caesium with water since they thought they were not spectacular enough for TV. They also said that the element beneath caesium in the periodic table, francium, would be even more reactive. They were wrong. It turns out for really heavy elements, the electrons begin to get slightly harder to remove than expected.

In order to understand why, you would need to take into account Einstein's relativistic effects. Theory predicts that the atoms begin to get slightly smaller and that it is actually harder to remove the outermost electron from francium than it is for caesium. Remarkably, this experiment has been carried out and the prediction has been confirmed. This means that despite what you may hear, or might have expected, caesium is the most reactive metal. This is great since francium can only be made in miniscule proportions and then only lasts for a few minutes so you'll never see any. Caesium on the other hand, is readily obtainable, and in its protective environment will last forever. This means we can actually see, hold and play with the most reactive metallic element that nature has given us. It's gorgeous, but watch out, it bites!

Meera Senthilingam

And having seen the melting of this element in action, I must admit it is rather beautiful. That was Cambridge University's Peter Wothers with the chemistry of his favourite element caesium. Is it your favourite yet? Well if not listen next week when we discover an element created by cold fusion.

Eric Scerri

Bohrium is also special in another respect, as the first element to be synthesised by a cold - rather than hot - fusion process between two nuclei. The idea is to make two nuclei collide at low excitation energies and consequently to capitalise on the reduced tendency of such combined atoms to disintegrate. The successful cold fusion synthesis of bohrium was first achieved in 1981 in Darmstadt, Germany, by the fusion of bismuth-209 with chromium-24 to form bohrium-262 with a half-life of about 85 milliseconds. Since then many other isotopes of bohrium have been produced, including the longest lived isotope so far bohrium-270, with a half-life of 61 seconds.

Meera Senthilingam

And join UCLA's Eric Scerri for the chemistry created by this fusion in next week's Chemistry in its element. Until then thank you for listening, I'm Meera Senthilingam.


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.