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

See Examples For:

cesium

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

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

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

From The Wall Street Journal ● Dec. 11, 2025

The system can be adapted to detect other similar contaminants in water, including cadmium, copper, lithium, barium, cesium, and radium, Ranno says.

From Science Daily ● May 14, 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