Caesium

Caesium is a soft, silvery-gold alkali metal with atomic number 55. Its highly reactive atoms underpin the SI definition of the second and are used in photoelectric devices.

Alkali metal: An element in Group 1 of the periodic table, characterized by one outer electron and high chemical reactivity. Caesium’s chemistry follows the group’s characteristic tendency to lose one outer electron.
Caesium atomic clock: An atomic clock that measures time using the microwave transition of caesium-133. This is the defining precision application of caesium’s stable hyperfine frequency.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Caesium occupies Group 1 and Period 6, between rubidium and barium.
Robert Bunsen: A German chemist who developed analytical methods and co-discovered caesium and rubidium. Bunsen and Gustav Kirchhoff identified caesium from its spectral lines.
Francium: A highly radioactive alkali metal with atomic number 87 and no stable isotopes. Francium lies below caesium in Group 1 but is too rare and unstable for practical clocks.
Electron configuration: The arrangement of electrons among an atom’s orbitals and energy levels. Caesium’s outer electron occupies the 6s orbital, shaping its chemical and spectroscopic behavior.
SI base units: The seven fundamental units of the International System of Units. The second is defined by fixing the frequency of a caesium-133 transition.
Atomic number: The number of protons in an atom’s nucleus, which identifies its chemical element. Caesium’s 55 protons distinguish it from every other element.
Gustav Kirchhoff: A German physicist whose work established foundational laws of electrical circuits and spectroscopy. His spectroscopic collaboration with Bunsen revealed caesium in mineral water.
Rubidium atomic clock: An atomic clock that uses a rubidium transition as its frequency reference. Rubidium clocks offer a compact alternative to caesium frequency standards.

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