Rubidium-87
Properties and data of the isotope 87Rb.
Rubidium-87 is a very long-lived radioisotope of the chemical element rubidium, which has 50 neutrons in its atomic nucleus in addition to the element-specific 37 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 87.
The radioactive nuclide 87Rb was first reported in 1921. The discovery was made using a so-called Cavendish mass spectrograph [1].
See also: List of individual Rubidium isotopes (and general data sources).
General data
Name of the isotope:Rubidium-87; Rb-87Symbol:87Rb or 8737RbMass number A:87 (= number of nucleons)Atomic number Z:37 (= number of protons)Neutrons N:50Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.3513513513514 (= neutron-proton ratio)Neutron excess (N-Z):13Isotopic mass:86.909180529(6) u (atomic weight of Rubidium-87)Nuclide mass:86.8888839 u (calculated nuclear mass without electrons)Mass excess:-84.59779 MeVMass defect:0.81359145499999 u (per nucleus)Nuclear binding energy:757.85555878 MeV (per nucleus)
8.71098343 MeV (average binding energy per nucleon)Separation energy:SN = 9.92212(20) MeV (first neutron)
SP = 8.621097(5) MeV (first proton)Half-life:4.97(3) × 1010 aDecay constant λ:4.422445253458 × 10-19 s-1Specific activity α:3096.8125885681 Bq g-1
8.369763752886 × 10-8 Ci g-1Spin and parity:
(nuclear angular momentum)3/2-Magnetic dipole moment:μ(μN) = +2.751324(74)Charge radius:4.1989(21) femtometer fmYear of discovery:1921
Radioactive Decay
Rubidium-87 is a primordial radioisotope with a half-life of 49,700,000,000 years, which means that the terrestrial 87Rb deposits were already present when the earth was formed. The radioactive nuclide decays into the ground state of the stable isotope strontium-87, emitting an electron (β--particle) with an energy of 0.08167(36) MeV and an antineutrino (0.1671(8) MeV).
Half-life T½ = 4.97(3) × 1010 a respectively 1.5673392 × 1018 seconds s.
Decay
mode | Daughter | Probability | Decay energy | γ energy
(intensity) |
|---|
| β- | 87Sr | 100 % | 0.282275(6) MeV | |
Parent Nuclides
Direct parent isotopes are: 87Kr, 87mSr.
Occurrence
Comparison of the natural Rubidium isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):
| | Atomic Mass ma | Quantity | Half-life | Spin |
|---|
Rubidium
Isotopic mixture | 85.4678 u | 100 % | | |
| Isotope 85Rb | 84.911789736(5) u | 72.17(2) % | stable | 5/2- |
| Isotope 87Rb | 86.909180529(6) u | 27.83(2) % | 4.97(3) × 1010 a | 3/2- |
NMR data
Nuclear magnetic properties and parameters of the NMR active Nuclide 87Rb
Isotope:87Rb-NMRQuantity:27.83(2) %Spin:3/2-Nuclearmagnetic moment
μ/μN:+2.751324(74)Gyromagnetic ratio γ:8.7851 · 107 rad T-1 s-1
13.98192 MHz T-1Nuclear g-factor:gl = 1.834216Quadrupole moment Q:+0.1335(5) barn (100 fm2)Line width parameter (factor):l = 237.63 fm4Resonance frequency:v0 = 13.9814 MHz at 1 TRelative Sensitivity:[related to 1H = 1.000]:
0.17704 (H0 = const.)
1.6419 (v0 = const.)
0.0493 (at natural abundance)
relative receptivity as compared to 13C:
290 (at natural abundance)
1042 (enriched)Reference compound:
(conditions)Solution of 0.01M rubidium chloride (RbCl) in deuterium oxide (D2O).Frequency ratio:Ξ(87Rb) = 32.720454 % [based on the reference substance]Chemical shift range:ca. 110 ppm [-80 to +30] The two naturally occurring rubidium isotopes are quadrupolar and therefore NMR active. In contrast to rubidium-85, the 87Rb nucleus produces less broad signals in the NMR spectrum and is more sensitive.
Isotones and Isobars
The following table shows the atomic nuclei that are isotonic (same neutron number N = 50) and isobaric (same nucleon number A = 87) with Rubidium-87. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
| Z | Isotone N = 50 | Isobar A = 87 |
|---|
| 28 | 78Ni | |
| 29 | 79Cu | |
| 30 | 80Zn | |
| 31 | 81Ga | 87Ga |
| 32 | 82Ge | 87Ge |
| 33 | 83As | 87As |
| 34 | 84Se | 87Se |
| 35 | 85Br | 87Br |
| 36 | 86Kr | 87Kr |
| 37 | 87Rb | 87Rb |
| 38 | 88Sr | 87Sr |
| 39 | 89Y | 87Y |
| 40 | 90Zr | 87Zr |
| 41 | 91Nb | 87Nb |
| 42 | 92Mo | 87Mo |
| 43 | 93Tc | 87Tc |
| 44 | 94Ru | 87Ru |
| 45 | 95Rh | |
| 46 | 96Pd | |
| 47 | 97Ag | |
| 48 | 98Cd | |
| 49 | 99In | |
| 50 | 100Sn | |
Data changes (history)
- A more precise value for the nuclear magnetic moment was adopted here (2023, [2]); previously: μ = +2.75129(8).
External data and identifiers
CAS:13982-13-3InChI Key:IGLNJRXAVVLDKE-NJFSPNSNSA-NSMILES:[87Rb]PubChem:ID 6335802Adopted Levels, Gammas:NuDat 87Rb
Literature and References
[1] - F. W. ASTON:
The Constitution of the Alkali Metals.
In: Nature, 107, 72, (1921), DOI 10.1038/107072b0.
[2] - Yu. I. Neronov, A. N. Pronin:
Study of NMR Signals of Rubidium in Aqueous Solutions and Determination of the Magnetic Moments of Rb-85 and Rb-87 Nuclei.
In: Technical Physics, 68(3), (2023), DOI 10.1134/S106378422390070X.
Last update: 2026-01-24
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