Rubidium (Rb)

Rubidium was the second element found by spectroscopy, a year after Bunsen and Kirchhoff used the same method on caesium: they saw two deep red lines in a mineral water residue that matched nothing known, and named it for the Latin rubidus, deepest red. Rubidium atomic clocks are the workhorses of timekeeping — cheaper and smaller than caesium standards, good to about a second in tens of thousands of years, and sitting in most mobile phone base stations. The first Bose-Einstein condensate, in 1995, was 2,000 rubidium atoms cooled to 170 nanokelvin, and it won the 2001 Nobel Prize in Physics.

Properties

Atomic number

37

Symbol

Rb

Standard atomic weight

85.468 u

Category

Alkali metal

Group

1

Period

5

Block

s-block

Electron configuration

[Kr] 5s¹

Electrons per shell

2, 8, 18, 8, 1

Melting point

312.46 K — 39.3 °C — 102.8 °F

Boiling point

961 K — 687.9 °C — 1270.1 °F

Density

1.53 g/cm³

Electronegativity

0.82 (Pauling)

Oxidation states

+1

Discovered

1861 by Robert Bunsen and Gustav Kirchhoff

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

Noble gas shorthand

[Kr] 5s¹

Shells

K: 2 · L: 8 · M: 18 · N: 8 · O: 1

What it is used for

• Rubidium atomic clocks in phone base stations and GPS receivers
• Laser cooling and Bose-Einstein condensate experiments
• Rubidium-82 for cardiac perfusion PET scans
• Special optical glasses and photocathodes
• Rubidium-strontium dating of very old rock

Hazards

• Ignites on contact with air and reacts explosively with water

Melts at 39.3 °C — a hot day, or a warm hand through the glass ampoule.

Neighbours in the table

Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Rubidium sits in group 1 with hydrogen, lithium, sodium, potassium, cesium, francium.