85Rb

85Rb is the stable isotope of the chemical element rubidium with mass number 85. In naturally occurring rubidium, about 72 percent is 85Rb and about 28 percent is 87Rb. The nucleus contains 37 protons and 48 neutrons.

Chemically and physically, 85Rb behaves as rubidium does: it is a soft, silvery-white alkali metal that is highly reactive with water and air. The element forms various oxides and hydrates and is typically stored under oil or in inert atmospheres. The metal has a melting point of 39.3°C and a boiling point of 688°C. Isotope mass differences have negligible effect on chemical behavior.

85Rb is stable; its half-life is effectively infinite. In contrast, 87Rb is radioactive with a half-life of about 4.88 × 10^10 years and decays to 87Sr; this decay underpins rubidium–strontium dating in geology.

Occurrence and uses: Rubidium occurs in trace amounts in Earth's crust and is commercially extracted from minerals and brines. In science, both 85Rb and 87Rb are used in atomic physics research and laser cooling experiments; 87Rb is particularly important in atomic clocks and magnetometers, while 85Rb has also been used in various trapping experiments.