ce·​si·​um | \ ˈsē-zē-əm also -zhē- How to pronounce cesium (audio) \

Definition of cesium

: a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells — see Chemical Elements Table

Examples of cesium in a Sentence

Recent Examples on the Web

The second is affixed to the motion of the atoms of the element cesium. Brian Resnick, Vox, "The world is about to redefine the kilogram," 15 Nov. 2018 Radiation detectors and dime-sized disks of plutonium and cesium used to make sure the detectors were working properly. Rachel Becker, The Verge, "The US government has trouble keeping track of radioactive material," 17 July 2018 Missing were radiation detectors and small samples of plutonium and cesium used to calibrate them. Dan Goodin, Ars Technica, "Dangerous plutonium stolen from rental car in a hotel parking lot," 16 July 2018 The experts from the Energy Department’s Idaho National Laboratory were sent to San Antonio to retrieve the plutonium and cesium on March 21, 2017, according to a Monday report from The Center for Public Integrity. Ryan Gaydos, Fox News, "Nuclear materials were reportedly stolen in Texas last year -- and they still haven't been found," 16 July 2018 Gases unleashed by the damage exploded, sending plumes of radioactive particles like iodine, cesium, and plutonium into the atmosphere. Vince Beiser, WIRED, "The Robot Assault On Fukushima," 26 Apr. 2018 The particles grab on to ions of cesium, strontium, and other dangerous isotopes in the water, making room for them by spitting out sodium. Vince Beiser, WIRED, "Fukushima’s Other Big Problem: A Million Tons of Radioactive Water," 27 Apr. 2018 The Harvard chemist takes two individual atoms, a sodium and a cesium, each about 10,000 times smaller than a bacterium. Sophia Chen, WIRED, "Chemists Orchestrate the Molecular Union of Two Single Atoms," 27 Apr. 2018 Lasers are used to slow and cool the atoms to near absolute zero, and then more lasers are used to elevate the ball of cesium up the chamber. Jay Bennett, Popular Mechanics, "The Centuries-Long Quest to Measure One Second," 24 Mar. 2017

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'cesium.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of cesium

1861, in the meaning defined above

History and Etymology for cesium

New Latin, from Latin caesius bluish gray

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Time Traveler for cesium

The first known use of cesium was in 1861

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More Definitions for cesium


variants: or chiefly British caesium \ ˈsē-​zē-​əm How to pronounce caesium (audio) \

Medical Definition of cesium

: a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells symbol Cs — see Chemical Elements Table

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More from Merriam-Webster on cesium

Spanish Central: Translation of cesium

Nglish: Translation of cesium for Spanish Speakers

Britannica.com: Encyclopedia article about cesium

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something valued as if it were money

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