Helium-3

Properties and data of the isotope ³He.

Contents

Helium-3 is a stable isotope of the chemical element helium. The atomic nucleus of the nuclide consists of the element-specific 2 protons and a neutron, which results in the mass number 3. He-3 is - besides hydrogen-1 - the only stable isotope of an element with more protons than neutrons.

³He occurs naturally on Earth, but only in very small amounts (0.0002%); the other stable isotope is Helium-4 (with a fraction of 99.9998%).

See also: List of individual Helium isotopes (and general data sources).

General data

Name of the isotope:Helium-3; He-3Other names:HelionSymbol:³He or 32HeMass number A:3 (= number of nucleons)Atomic number Z:2 (= number of protons)Neutrons N:1Nucleon pairing (Z - N):even - oddNuclear ratio (N/Z ratio):0.5 (= neutron-proton ratio)Neutron excess (N-Z):-1Isotopic mass:3.01602932(2) u (atomic weight of Helium-3)Nuclide mass:3.0149322 u (calculated nuclear mass without electrons)Mass excess:14.93122 MeVMass defect:0.0082856599999999 u (per nucleus)Nuclear binding energy:7.71804258 MeV (per nucleus)
2.57268086 MeV (average binding energy per nucleon)Separation energy:
S_P = 5.493474497(22) MeV (first proton)Cross section:σ(n.γ) = 5300 barn (thermal neutron capture cross-section)Half-life:stableSpin and parity:
(nuclear angular momentum)1/2+Magnetic dipole moment:μ(μ_N) = - 2.127625306(25)Charge radius:1.9661(30) femtometer fmMirror nucleus:Hydrogen-3Year of discovery:1934

Parent Nuclides

Direct parent isotopes are: ³H, ⁴Li.

Occurrence

Comparison of the natural Helium isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):

	Atomic Mass m_a	Quantity	Half-life	Spin
Helium Isotopic mixture	4.002602 u	100 %
Isotope ³He	3.01602932(2) u	0.0002(2) %	stable	1/2+
Isotope ⁴He	4.0026032541(4) u	99.9998(2) %	stable	0+

NMR data

Nuclear magnetic properties and parameters of the NMR active Nuclide ³He

Isotope:³He-NMRQuantity:0.0002(2) %Spin:1/2+Nuclearmagnetic moment
μ/μ_N:- 2.127625306(25)Gyromagnetic ratio γ:-20.3801587 · 10⁷ rad T^-1 s^-1
-32.43603 MHz T^-1Nuclear g-factor:g_l = -4.255250612Resonance frequency:v₀ = 32.4380 MHz at 1 TRelative Sensitivity:[related to ¹H = 1.000]:
0.44220 (H₀ = const.)
0.7619 (v₀ = const.)
0.000000606 (at natural abundance)
relative receptivity as compared to ¹³C:
0.00356 (at natural abundance)
2510 (enriched)Reference compound:
(conditions)Helium gas.Frequency ratio:Ξ(³He) = 76.179437 % [based on the reference substance]Chemical shift range:ca. 58 ppm [-50 to 8]

The gyromagnetic ratio of the doubly positively charged (electron-free) ³He²⁺ atom (helion) is a fundamental natural constant that has the following values:

Shielded Helion	γ´_h = 2.037894569(24) · 10⁸ s^-1 T ^-1
Shielded Helion in MHz/T	γ´_h = 32.43409942(38) MHz T ^{- 1}

The relative standard uncertainty is 1.2 · 10^-8.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 1) and isobaric (same nucleon number A = 3) with Helium-3. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.