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

In contrast, the open structure of light cages lets cesium atoms diffuse into the core much more quickly, cutting the filling process down to just a few days without sacrificing optical performance.

From Science Daily • Jan. 6, 2026

The U.S. currently relies on other countries to secure rubidium, cesium and scandium.

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

But the scientists' analyses were not yet complete; they also had to check for stabilization of the lithium metal anode 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