Caesium (Cs)

Caesium is a soft, silvery-golden alkali metal with a melting point of 28.5 °C, defining the SI second.

Core Principles & Analysis

• Robert Bunsen and Gustav Kirchhoff discovered caesium in 1860, the first element found by spectroscopy, naming it for its sky-blue spectral lines.
• The gold-tinged metal melts at 28.5 degrees Celsius, is the most electropositive stable element, and reacts explosively with water.
• The SI second is defined as 9,192,631,770 oscillations of a caesium-133 transition, making the element the heart of atomic clocks.

Specifications & Metrics

• NUM: 55
• SYMBOL: Cs
• CATEGORY: Alkali Metal