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

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 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

The frequency of the laser creates resonance in the electrons of the cesium atoms.

From Science Daily • May 2, 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