A soft, silvery-white metallic chemical element that is highly reactive; the element with atomic number 55 on the periodic table.
From Latin caesius (blue-gray) because the element produces a blue line in its atomic spectrum. Named in 1860 by spectroscopists who observed this characteristic blue coloration.
Caesium was the first element discovered using spectroscopy—scientists literally identified it by its color signature in light, which is how it got its name from the Latin word for blue-gray, making it a gorgeous example of elements revealing their secrets through light.
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