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Rb

Rubidium

Rubidium (Rb) is an alkali metal with atomic number 37 and an atomic mass of 85.468. Its electron configuration is [Kr] 5s1, which gives it 1 valence electron.

Rubidium key data

Symbol Rb
Atomic number 37
Atomic mass (standard atomic weight) 85.468
Molar mass 85.468 g/mol
Protons 37
Neutrons (most abundant isotope, rubidium-85) 48
Electrons (neutral atom) 37
Valence electrons 1
Category Alkali metal
Period 5
Group 1
Block s-block
Noble gas notation [Kr] 5s1
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1
Electrons per shell 2, 8, 18, 8, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Measured properties of rubidium

From PubChem, the US National Institutes of Health. Public domain. The electron affinity is the measured value for rubidium, from Wikipedia’s electron affinity data page, in place of PubChem’s 0.468 eV.

Melting point 312.46 K (39.31 °C), 87th of 103 by melting point
Boiling point 961 K (687.85 °C), 74th of 93 by boiling point
Density 1.53 g/cm³, 82nd of 96 by density
State at room temperature Solid
Electronegativity 0.82 (Pauling), joint 92nd of 95 by electronegativity
Van der Waals radius 303 pm, 3rd of 99 by atomic radius
First ionisation energy 4.177 eV (403 kJ/mol), 100th of 102 by first ionisation energy
Electron affinity 0.48592 eV (46.884 kJ/mol), 42nd of 57 by electron affinity
Common oxidation states +1
Discovered 1861

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Rubidium electron configuration

The full electron configuration of rubidium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1. With the noble-gas core collapsed it is [Kr] 5s1, and the outermost subshell is 5s1.

Bohr model of rubidium (Rb): a nucleus and five shells holding 2, 8, 18, 8 and 1 electrons from the inside out.
Bohr model of rubidium (Rb). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of rubidium (Rb), [Kr] 5s1, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 5s.
Orbital diagram of rubidium (Rb). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one rubidium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 85.468 / (6.02214076 × 10²³)
  2. = 1.4192 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 85.468
  2. = 7.0461 × 10²¹ atoms

Rubidium on the periodic table

Rubidium is in period 5, group 1, in the s-block.

The other elements of group 1 are hydrogen, lithium, sodium, potassium, caesium and francium.

The rest of period 5

Work with rubidium