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
• ATOMIC MASS: 132.905452 u
• ELECTRONEGATIVITY: 0.79 Pauling
• ATOMIC RADIUS: 343 pm
• IONIZATION ENERGY: 3.894 eV
• ELECTRON AFFINITY: 0.472 eV
• MELTING POINT: 301.59 K
• BOILING POINT: 944 K
• DENSITY: 1.93 g/cm³
• ELECTRON CONFIGURATION: [Xe]6s1
• OXIDATION STATES: +1
• STANDARD STATE: Solid
• GROUP BLOCK: Alkali metal
• YEAR DISCOVERED: 1860