cesium

American  

[see-zee-uhm] / ˈsi zi əm /

Or caesium

noun

1. a rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Cs; 132.905; 55; 1.9 at 20°C; melts at 28.5°C.

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cesium British  

/ ˈsiːzɪəm /

noun

1. the usual US spelling of caesium

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cesium Scientific  

/ sē′zē-əm /

1. A soft, ductile, silvery-white element of the alkali group. It is liquid at room temperature and is the most reactive of all metals. Cesium is used to make photoelectric cells, electron tubes, and atomic clocks. Atomic number 55; atomic weight 132.905; melting point 28.5°C; boiling point 690°C; specific gravity 1.87; valence 1.

2. See Periodic Table

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Etymology

Origin of cesium

1930–35; < New Latin, special use of Latin caesium, neuter of caesius bluish-grey; see -ium

Example Sentences

Rubidium and cesium, which are used for atomic clocks, can be found at the Utah site, along with scandium, which is essential for the aerospace industry.

From The Wall Street Journal • Dec. 11, 2025

Ionic MT said it discovered high grades of 16 different types of minerals, everything from lithium to alumina, germanium, rubidium, cesium, vanadium and niobium at the site in Utah’s Silicon Ridge.

From The Wall Street Journal • Dec. 11, 2025

The Brookhaven scientists believed cesium nitrate could serve this purpose for lithium metal batteries.

From Science Daily • Jan. 26, 2024

By combining various techniques across two user facilities, the scientists were able to paint a full picture of how the lithium metal battery behaves with the cesium nitrate additive.

From Science Daily • Jan. 26, 2024

And in the early hours of Tuesday morning, the favorable winds that had been sweeping the radioactive gases out to sea had shifted, carrying radioactive cesium inland.

From "Meltdown" by Deirdre Langeland