Rubidium (Rb)
Rubidium (Rb) is an alkali metal with atomic number 37 and a standard atomic weight of 85.468. At room temperature it is a solid that melts at 39 °C and boils at 688 °C.
Rubidium key facts
Rubidium at a glance
Identity
| Symbol | Rb |
|---|---|
| Atomic number (Z) | 37 |
| Atomic weight | 85.468 ± 0.001 (CIAAW 2024) |
| Category | Alkali metal |
| Group | 1 — alkali metals (with hydrogen) |
| Period | 5 |
| Block | s-block |
| CAS number | 7440-17-7 |
Atomic properties
| Electron configuration | [Kr] 5s1 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1 |
|---|---|
| Electrons per shell | 2, 8, 18, 8, 1 |
| Oxidation states | +1 Less common: -1 |
| Electronegativity (Pauling) | 0.82 |
| First ionisation energy | 4.177 eV (403 kJ/mol) 2nd: 27.29 eV · 3rd: 39.247 eV |
| Electron affinity | 0.486 eV |
| Atomic radius (empirical) | 235 pm |
| Covalent radius | 210 pm |
| Van der Waals radius | 303 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 39.3 °C · 102.7 °F · 312.45 K |
| Boiling point | 688 °C · 1,270.4 °F · 961.15 K |
| Density | 1.53 g/cm³ |
| Specific heat capacity | 0.363 J/(g·K) |
| Thermal conductivity | 58.2 W/(m·K) |
| Crystal structure | Body-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 85Rb 72.17%, 87Rb 27.83% (radioactive) |
| Stable isotopes | 2 |
| Known isotopes | 34 |
| Abundance in Earth's crust | 90 mg/kg |
| Abundance in seawater | 0.12 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Rubidium electron configuration
Rubidium has 37 electrons. Written in short form, its ground-state configuration is [Kr] 5s1 — the electrons of the noble gas krypton, plus 5s1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1.
As a member of group 1 it has 1 valence electron in its outer shell (shell 5), which largely decides its chemistry.
Discovery and name
1861 — Robert Bunsen and Gustav Kirchhoff · Germany.
Latin rubidus, deep red — for its spectral lines.
Known for
An extremely reactive alkali metal. Rubidium atomic clocks provide precise timing in telecoms and GPS satellites.
Isotopes of rubidium
Naturally occurring rubidium is a mix of 2 isotopes. Each has 37 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 85Rb | 37 | 48 | 72.17% | stable |
| 87Rb | 37 | 50 | 27.83% | radioactive · half-life 49.7 billion years |
34 isotopes of rubidium are known in our data set (half-lives as published in nuclear data tables).
Rubidium compared with group 1
The other elements in group 1 (alkali metals (with hydrogen)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Hydrogen | 1 | 1.0080 | -259 °C | -253 °C | 0.0899 g/L | 2.2 |
| Lithium | 3 | 6.94 | 180 °C | 1,342 °C | 0.534 g/cm³ | 0.98 |
| Sodium | 11 | 22.990 | 98 °C | 883 °C | 0.97 g/cm³ | 0.93 |
| Potassium | 19 | 39.098 | 64 °C | 759 °C | 0.89 g/cm³ | 0.82 |
| Rubidium | 37 | 85.468 | 39 °C | 688 °C | 1.53 g/cm³ | 0.82 |
| Cesium | 55 | 132.91 | 28 °C | 671 °C | 1.873 g/cm³ | 0.79 |
| Francium | 87 | [223] | 21 °C (est.) | — | 1.87 g/cm³ (pred.) | 0.7 |