Rubidium: Element and geochemistry

History

Rubidium was first identified spectroscopically by Bunsen and Kirchhoff in 1861 during analysis of alkalis from lepidolite, from two deep red lines (Latin – rubidus, ‘deepest red’) not previously identified with any other known element at the time. It was isolated by Bunsen within 2 years of its identification. Natural radioactivity of Rb was reported by N.R. Campbell and A. Wood in 1906.

Physical properties

Rubidium: symbol Rb, atomic number 37, atomic mass 85.4678, electron configuration [Kr] 5s¹, valence Rb⁺¹, melting point 39.48°C, boiling point 688°C. The metal is silvery white, extremely soft (0.3, Mohs scale), reacts violently with water and ignites spontaneously in air.

Rubidium has two naturally occurring isotopes, ⁸⁵Rb (72.15%) and ⁸⁷Rb (27.85%); these proportions appear to be constant in terrestrial material. ⁸⁷Rb is radioactive (reported independently in 1937 by Hahn and Mattauch), and decays via β⁻ to ⁸⁷Sr (half-life 4.88 × 10¹⁰ years λ = 1.42 × 10⁻¹¹/year); this...