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

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Example Sentences

See Examples For:

cesium

To protect the devices from chemical reactions with cesium, the waveguides are coated with a protective layer.

From Science Daily ● Jan. 6, 2026

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

In effect, the meter is defined to be the distance traveled by light in 0.000000003335640952 second, as measured by a cesium clock.

From "A Brief History of Time: And Other Essays" by Stephen Hawking

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