Chemical Element: cesium, caesium

(Modern Latin: chemical element; from Latin, caesius, bluish gray; sky blue; metal)

Chemical-Element Information

Symbol: Cs
Atomic number: 55
Year discovered: 1860

Discovered by: Gustav Robert Kirchhoff (1824-1887), a German physicist, and Robert Wilhelm Bunsen (1811-1899), a German chemist.

  • Cesium was discovered spectroscopically by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff in 1860 in mineral water from Durkheim, Germany.
  • Their identification was based upon two bright blue lines of unknown origin in the flame spectra of certain mineral water concentrates.
  • Chemically, it is very similar to the other members of the alkali group (lithium, sodium, potassium, and rubidium); and as such, its earlier discovery was overlooked not merely because of its scarcity but through confusion with potassium.
  • Its principal value is in photoelectric cells where advantage is taken of the fact that cesium, in common with other alkali metals, emits electrons when illuminated by visible light.
  • For the same reason it is widely used in the pickup tubes of television cameras.
  • Cesium is the most reactive of all metals, melting and inflaming spontaneously on exposure to air and decomposing water with explosive violence.
  • Each element on the periodic table is known to vibrate at a distinct frequency.
  • The steady pulse of atomic vibrations can keep time far more accurately than any other kind of clock.
  • Cesium is the element used in atomic clocks. In one second, a cesium atom vibrates more than nine billion times.
  • In 1949, scientists at the U. S. National Bureau of Standards used previous research to build the first atomic clock.
  • Now there is an eighth model of the atomic clock that is being constructed at the National Institute of Standards and Technology in Boulder, Colorado.
  • In the quest for greater time precision, physicists have made a quantum leap forward with the discovery that the oscillations of cesium-133 atoms, subjected to microwave radiation, could be manipulated to allow a time calculation with an error of one second every 1.4 million years.
  • The vibrations in an atom are far more consistent and accurate than any other time-keeping device known to scientists.
  • Atomic clock vibrations are said to be incomprehensibly fast. In fact, the unit of time we know as a “second” was defined in 1967 using the cesium atomic clock.
  • The inherent frequency of cesium atoms became the official international unit of time, now called Universal Coordinated Time (UCT).
  • No longer was time measured on the movements of the earth, a second being a fraction of a day, but by the vibrations of an atom; a second being the time it took a cesium atom to oscillate precisely 9 192 631 770 times.
  • The second quickly became the physical quantity most accurately measured by scientists.
  • Atomic time has replaced “earth time” as the world’s official timekeeper.
  • The first atomic clock was based on the ammonia atom, which quickly was replaced with the much more accurate cesium atom.
  • Research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule.
  • Attention shifted almost immediately to more-promising, atomic-beam devices based on cesium.
  • Just five grams of cesium, a metal with a low melting point, will last about 10 years.
  • Clocks are improving in accuracy at better than a factor of ten every ten years which means that today’s atomic clock is considered “a million times more accurate than the one built in 1949.”
  • GPS, Global Positioning System, technology is based on atomic clock technology. Every GPS unit has an atomic clock on board, and the unit uses satellites and ground stations to orient and determine its precise location.
  • The U.S. Air Force started launching satellites for the GPS in the late 1970’s so they could increase rocket and artillery accuracy, and to improve precise land, sea and air navigation regardless of weather conditions.
  • Twenty-four satellites, some equipped with three atomic clocks, others with four, rocketed into orbit 12 000 miles above the earth where they remain to this day, transmitting their positions earthward with precise timing signals generated by the atomic clocks on board.
  • Far below, GPS receivers can pick up the signals, then electronically calculate latitude, longitude, altitude, speed and compass direction anywhere in the world.
  • Fully operational for military use by 1994, the basic system has since become available to the general public resulting in practical use for supertanker captains and recreational boaters, commercial and private aircraft pilots, surveyors, hikers, and even in automobiles.
  • Without the atomic clocks at each location, the GPS unit would be useless. Extremely accurate time measurements are required to triangulate between satellites, and to determine an exact location.
  • As for telecommunications, people are edging along a continuum of sending more and more data through telecommunications lines, faster and faster.
  • If the channel the data pass through is synchronized in time at both ends, the data can travel much more swiftly. The only way to synchronize these optical fibers with absolute precision is to use atomic clocks.
  • Scientists have long realized that atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies.
  • These resonances are inherently stable over time and space. It is said that an atom of hydrogen or cesium here today is exactly like one a million years ago or in another galaxy.
  • As we continue our “Walk Through Time”, we will see how agencies such as the National Institute of Standards and Technology, the U.S. Naval Observatory, and the International Bureau of Weights and Measures in Paris assist the world in maintaining a single, uniform time system.
  • In fact, it is claimed that within the next few years, new clocks, based on a process that uses supercooled atoms, and achieving an accuracy of just one second lost in 313 million years, will be operational.
  • At the National Institute of Standards and Technology, in Boulder, Colorado, scientists are working on an even more advanced atomic clock, one that functions independently of gravity, a distorting variable in this system of time management.
  • In order to accomplish this, a working model of this clock will operate outside gravity in space.
  • The clock is slated to be placed aboard the Internatioinal Space Station in 2004.
  • Horologists agree that even with a clock that will miss no more than a second every trillion years, unbuilt as yet but conceivable, time will remain elusive and mysterious.
—Some of the information for this section came from the article,
“Taking the Measure of Time”, by Per Ola and Emily D’Aulaire
in the December, 1999, issue of Smithsonian, pages 52-64
(specifically from page 64)

Name in other languages:

British: caesium

French: césium

German: Cäsium

Italian: cesio

Spanish: cesio

Cesium is a nasty chemical. Even its non-radioactive form is highly poisonous.

—Peter D. Zimmerman, professor of science and security;
as seen in "Seize the cesium", International Herald Tribune,
August 2, 2007, page 2.

Information about other elements may be seen at this Chemical Elements List.

A special unit about words that include chemo-, chem- may be seen here.