Cesium

Cesium (or caesium) was the first element discovered using spectroscopy in 1860 by Robert Bunsen and Gustav Kirchhoff. They identified it by the beautiful sky-blue spectral lines it produced, and named it from the Latin 'caesius,' meaning sky blue. Cesium is the softest metal — you can squish it like clay — and it is so reactive that it explodes on contact with water, even ice!

Cesium atomic clocks are the most accurate timekeeping devices ever made. The official definition of one second is based on cesium-133 atoms vibrating exactly 9,192,631,770 times. GPS satellites, the internet, and financial trading systems all rely on cesium clocks for precise timing. Without cesium, your phone's clock would gradually drift and GPS navigation would be useless.

Cesium's symbol 'Cs' is a useful two-letter element in Spellement. While 'cs' is uncommon at the start of English words, partial emphasis allows using 'c' or 's' individually — both among the most frequent consonants in English, providing excellent word-building flexibility.