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IYPT Elements

IYPT 2019 Elements 055: Caesium: Atomic clocks and explosive reactions

Element 55 in our International Year of the Periodic Table series is caesium. Caesium is used in atomic clocks and is the most reactive metal in the periodic table.

Caesium is the element most commonly used in atomic clocks, which provide the most accurate times. Since 1967, the second has been defined in reference to electron transitions in a caesium-133 atom. Caesium atomic clocks in satellites help GPS and mobile phone networks function.

The group 1 metals increase in reactivity as you descend the group, and caesium is the most reactive of these metals – and the most reactive in the periodic table. Because francium is below caesium in group 1, it might seem strange that it doesn’t take the title of most reactive metal… but more on that when we get to francium! In the meantime, Periodic Videos have a video showing the reaction of caesium with water.

Caesium’s reactivity means that the pure metal doesn’t have a huge number of everyday uses. One of its compounds, caesium formate, accounts for most of the use of caesium – it’s used as a drilling fluid for oil well drilling.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.