Cesium

About Cesium

Cesium is the alkali metal taken to its limit. Of every stable element, it has the lowest first ionization energy (375.7 kJ/mol) and the lowest electronegativity (0.79 on the Pauling scale), which is just the quantitative way of saying its lone 6s electron is barely held on. Drop a chunk in water and the reaction is energetic enough to shatter the glass; the same loosely-bound electron is what makes cesium photocathodes the standard for low-work-function photoemission. Bunsen and Kirchhoff pulled it out of Dürkheim mineral water in 1860 — the first element discovered by spectroscopy, named for the two sky-blue lines they saw in the flame. The defining modern application is the cesium-133 atomic clock: the SI second is now defined as exactly 9,192,631,770 cycles of the hyperfine transition between the F=3 and F=4 ground-state levels of ¹³³Cs, and primary frequency standards based on cesium-fountain clocks at NIST and BIPM hold the rest of the timekeeping world to sub-nanosecond agreement. Cesium formate brines (density up to 2.3 g/cm³) are also the densest non-toxic drilling fluids on the market.