cesium

Meanings

1. 1
   noun

   the soft, highly reactive alkali-metal chemical element with atomic number 55 and symbol Cs

   Cesium-133 provides the frequency standard used to define the second.

The American spelling of caesium, coined from Latin caesius ("sky blue") after the element's bright blue spectral lines. Robert Bunsen and Gustav Kirchhoff discovered it by spectroscopy in 1860.

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