Caesium (Cs): Properties & Uses

Caesium element

Caesium is the chemical element that has literally redefined a time. All thanks to Caesium, the entire world now keeps the time so accurately that it has forced us to reconsider what time really is; although, today new elements are discovered that are far better than caesium but it introduced a bizarre bug into timekeeping. To know more about this, first, let’s have a close look towards the properties and then uses of the chemical element Caesium.

Caesium (element that redefined time)

Caesium (An Element that Redefined Time)


It is a chemical element with the symbol ‘Cs’ and atomic number 55. Also, it is the first element to be discovered with a spectroscope. In 1967, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the Second (unit of time) and the metre (unit of distance) Since then Caesium has been widely used in highly accurate atomic clocks.


The Molten State of Caesium in an Ampoule

Physical Properties

It is a soft, silvery-golden alkali metal. It is very ductile, a pale metal which darkens in the presence of trace amounts of oxygen. In the presence of mineral oil (which is used to keep it, during transport), it loses metallic lustre and takes on duller, grey appearance. It is one of only five elemental metals that are liquid at or near room temperature with a melting point of  28.5 °C. It has only one stable isotope, Caesium-133. The metal has a rather low boiling point, 641 °C (1,186 °F), the lowest of all metals other than mercury. Its compounds burn with a blue or violet colour. The golden colour of caesium comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. For lithium, through rubidium, this frequency is in the ultraviolet, but for caesium, it enters the blue-violet end of the spectrum. Thus caesium transmits and partially absorbs violet light preferentially while other colours (having lower frequency) are reflected; hence, it appears yellowish.

Caesium metal

Caesium metal

Chemical Properties

Caesium metal is highly reactive. It ignites spontaneously in air and reacts explosively with water even at low temperatures. It reacts with ice at temperatures as low as −116 °C. Because of this high reactivity, caesium metal is classified as a hazardous material. It is stored and shipped in dry, saturated hydrocarbons such as mineral oil. It can be handled only under inert gas, such as argon. However, a caesium-water explosion is often less powerful than a sodium-water explosion with a similar amount of sodium. Caesium can be stored in vacuum-sealed borosilicate glass ampoules. It is the most electropositive element. The caesium-ion is also larger and less hard than those of lighter alkali metals.


Most caesium compounds contain the element as the cation Cs+, which binds ionically to a wide variety of anions. Salts of Cs+ are usually colourless unless the anion itself is coloured. The phosphate, acetate, carbonate, halides, oxide, nitrate, and sulfate salts are water-soluble. Caesium hydroxide (CsOH) is hygroscopic (the phenomenon of attracting and holding water molecules) and strongly basic. It forms alloys with other alkali metals, gold, and mercury. It forms well-defined intermetallic compounds with antimony, gallium, indium, and thorium, which are photosensitive. Cs+ forms complexes with Lewis bases in solution. Because of its large size, Cs+ usually adopts coordination numbers greater than 6. This difference is apparent in the 8-coordination of CsCl. It crystallizes in the simple cubic crystal system.

caesium chloride crystal structure

Caesium Chloride Crystal Structure


Caesium has 39 known isotopes, ranging in mass number from 112 to 151 but only stable isotope is Caesium-133 with 78 neutrons. Its radioactive Caesium-135 isotope has a very long half-life of about 2.3 million years, the longest of all radioactive isotopes of caesium. It is one of the long-lived fission products of uranium produced in nuclear reactors. Almost all caesium produced from nuclear fission comes from the beta decay of originally more neutron-rich fission products. During nuclear weapon testing in the 195os, Caesium-137 was released into the atmosphere and returned to the surface of the earth. It is a ready marker of the movement of soil and sediment from those times.


It is a relatively rare element. It is found in few minerals. The only economically important ore for caesium is pollucite Cs(AlSi 2O 6), which is found in a few places around the world in zoned pegmatites, associated with the more commercially important lithium minerals, lepidolite and petalite. The world’s most significant and richest known source of caesium is the Tanco Mine at Bernic Lake in Manitoba, Canada, estimated to contain 350,000 metric tons of pollucite ore, representing more than two-thirds of the world’s reserve base. At the present rate of world mine production of 5 to 10 metric tons per year, reserves will last for thousands of years.

Pollucite mineral

Pollucite mineral


1) Atomic Clocks

The first accurate caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory (NPL) in the UK. Caesium clocks raise the standards for the measurement of time exponentially. These clocks measure frequency with an error of 2 to 3 parts in 10^14, which corresponding to an accuracy of 2 nanoseconds per day or 1 second in 1.4 million years. The caesium standard is the primary standard for standards-compliant time and frequency measurements. Caesium clocks regulate the timing of cell phone networks and the Internet.

Caesium atomic clocks

Caesium atomic clocks

Definition of a Second (Time)

Caesium has a far higher resonant frequency 9,192,631,770 Hz, to be precise. This is one reason Essen used the element to make the first of the next generation of clocks – the “atomic” clocks. The principle behind this caesium clock is that when you hit the right frequency the caesium get excited and outermost electron gets excited and jumps into a wider orbit. This is known as “transition”. As the electron moves out into the wider orbit it absorbs energy, and as it jumps back in, it releases in the form of light, fluorescing very slightly. It means you can tell when you’ve hit the sweet spot of 9,192,631,770 Hz. The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Caesium transition as defination of Second (time)

Caesium transition as the definition of Second (time)

We now have new ways of measuring time. At NPL they are experimenting with the elements strontium and ytterbium that operate at far, far higher frequencies – right up in the optical rather than the microwave spectrum. But the accuracy of caesium clocks has introduced a potentially disastrous glitch into the world’s timekeeping.


Without the caesium clock, for example, satellite navigation would be impossible. GPS satellites carry synchronised caesium clocks that enable them collectively to triangulate your position and work out where on earth you are and the practical applications do not end here. For example electricity grids. As wind and solar energy become more widespread, the grid will need to time accurately its reactions to unexpected moves in the wind or passing clouds. If this gets wrong, you end up with blackouts. The switch to atomic time was for good reason. The rotation of the earth was not such a reliable measure of time. No day or year is exactly the same length.

Atomic clocks

2) Petroleum Exploration

The present-day use of non-radiative caesium is, in caesium formate, drilling fluid (often used while drilling oil and natural gas wells and on exploration drilling rigs) for the extractive oil industry. The function of drilling fluid is to lubricate drill bits, to bring rock cuttings to the surface and to maintain pressure on the formation during drilling of the well. The high density of caesium formate brine coupled with the relatively gentle nature of most caesium compounds, reduces the requirement for toxic high-density suspended solids in the drilling fluids. It is relatively environment friendly also. Furthermore, it is biodegradable and may be recycled, which is important in view of its high cost.

Petroleum exploration

Petroleum exploration

3) Electric Powers and Electronics

It is important for its photoemissive properties, converting light to electron flow. It is used in photoelectric cells because caesium based cathodes have a low threshold voltage for the emission of electrons. Devices using caesium include photomultiplier tubes, video camera tubes. Germanium, rubidium, selenium, silicon, tellurium, and several other elements can be substituted for caesium in photosensitive materials. Caesium iodide (CsI), bromide (CsBr) and caesium fluoride (CsF) crystals are employed for scintillators in scintillation counters widely used in mineral exploration. Caesium vapour is used in many common magnetometers (a device used to measure magnetism.) Caesium has a great affinity for oxygen and is used as a “getter” in vacuum tubes. Other uses of the metal include high-energy lasers, vapour glow lamps, and vapour rectifiers.


4) Chemical Uses

It has relatively few chemical applications. Doping with caesium compounds enhances the effectiveness of several metal-ion catalysts for chemical synthesis. It is also used in the catalytic conversion of sulfur dioxide into sulfur trioxide in the production of sulfuric acid. Caesium fluoride is an important base and an anhydrous source of fluoride ion. Caesium salts sometimes replace potassium or sodium salts in organic syntheses, such as cyclization, esterification, and polymerization.

Caesium fluoride

Caesium fluoride crystals

5) Isotope Applications

Caesium-137 radioisotope has a half-life of roughly 30 years and its availability from the nuclear fuel cycle and having Barium-137 as the stable product are its advantages and; hence, it has been used in agriculture, cancer treatment, and the sterilization of food, sewage sludge, and surgical equipment. Radioactive isotopes of caesium in radiation devices were used in the medical field to treat certain types of cancer. Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, levelling, and thickness gauges. It has also been used in well logging devices for measuring the electron density of the rock formations.

Use of radioactive Caesium-137 isotope to treat Cancer

Use of Caesium-137 isotope to treat Cancer

6) Other Uses

Caesium and rubidium have been added as a carbonate to glass because they reduce electrical conductivity and improve the stability and durability of fibre optics and night vision devices. Caesium fluoride or caesium aluminium fluoride are used in fluxes formulated for brazing aluminium alloys that contain magnesium. Caesium salts have been used as antishock reagents following the administration of arsenical drugs. Because of their effect on heart rhythms, however, they are less likely to be used than potassium or rubidium salts. They have also been used to treat epilepsy.

Night vision devices

Night Vision Devices



Add Comment