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; -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

Because a vanadium peroxide molecule is negatively charged, it needed alkali cations for charge balance, Nyman said, and the researchers used potassium, rubidium and cesium alkali cations for this study.

From Science Daily • Feb. 12, 2024

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

From Science Daily • Jan. 26, 2024

In a reactor, many of the uranium 235 nuclei split apart, transforming into radioactive isotopes such as iodine 131 and cesium 137.

From "Meltdown" by Deirdre Langeland