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DictionarycesiumAlkali metal
noun

cesium

SEE-zee-um/ˈsiːziəm/

A soft alkali metal element with atomic number 55.

The laboratory keeps the soft cesium sample in a sealed container.

A sealed glass ampoule holding a small, soft-looking silvery cesium sample beside a compact laboratory atomic-clock instrument, connecting the element with its timekeeping use without adding people.
Etymology
cesLatinblue-gray
+iumModern Latinmetallic element

The “blue-gray” root refers to the two prominent blue lines in cesium’s spectrum. Those lines enabled its identification, so the name records how the element was discovered.

In context

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More examples

  • The periodic table assigns cesium atomic number fifty-five.
  • The instrument measures the spectrum of cesium.
  • The display compares cesium with other alkali metals.

Similar words

rubidium
A different alkali metal directly above cesium in the periodic table.
potassium
Another alkali metal with distinct physical and chemical properties.