Krypton-78

Properties and data of the isotope ⁷⁸Kr.

Contents

General Data

Radioactive Decay

Parent Nuclides

Isotones und Isobares

Literature

Krypton-78 is a primordial stable isotope of the chemical element krypton, which has 42 neutrons in its atomic nucleus in addition to the element-specific 36 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 78.

The first detection of the nuclide ⁷⁸Kr using a mass spectrograph was reported in 1920 [1].

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

General data

Name of the isotope:Krypton-78; Kr-78Symbol:⁷⁸Kr or 7836KrMass number A:78 (= number of nucleons)Atomic number Z:36 (= number of protons)Neutrons N:42Nucleon pairing (Z - N):even - evenNuclear ratio (N/Z ratio):1.1666666666667 (= neutron-proton ratio)Neutron excess (N-Z):6Isotopic mass:77.920365(5) u (atomic weight of Krypton-78)Nuclide mass:77.9006169 u (calculated nuclear mass without electrons)Mass excess:-74.17952 MeVMass defect:0.725262624 u (per nucleus)Nuclear binding energy:675.57778268 MeV (per nucleus)
8.66125362 MeV (average binding energy per nucleon)Separation energy:S_N = 12.0801(20) MeV (first neutron)
S_P = 8.2324(28) MeV (first proton)Half-life:stableSpin and parity:
(nuclear angular momentum)0+Charge radius:4.2038(33) femtometer fmYear of discovery:1920

Radioactive Decay

The nucleus of the krypton-78 isotope is considered stable. Theoretically, Kr-78 could decay radioactively to selenium-78 after electron capture (EE) via a double beta decay (2νECEC) [2]; however, with a calculated half-life of 9.2(13) × 10²¹ years, the probability is so low that it would be practically irrelevant.

Half-life T_½ = stable.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
2β⁺ ?

Parent Nuclides

Direct parent isotopes are: ⁷⁸Rb, ⁷⁸Br.

Occurrence

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

	Atomic Mass m_a	Quantity	Half-life	Spin
Krypton Isotopic mixture	83.798 u	100 %
Isotope ⁸⁰Kr	79.916378(5) u	2.286(10) %	stable	0+
Isotope ⁸²Kr	81.913483(6) u	11.593(31) %	stable	0+
Isotope ⁸⁴Kr	83.91149773(3) u	56.987(15) %	stable	0+
Isotope ⁸⁶Kr	85.91061063(3) u	17.279(41) %	stable	0+
Isotope ⁷⁸Kr	77.920365(5) u	0.355(3) %	stable	0+
Isotope ⁸³Kr	82.914126517(10) u	11.500(19) %	stable	9/2+

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 42) and isobaric (same nucleon number A = 78) with Krypton-78. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 42	Isobar A = 78
21	⁶³Sc
22	⁶⁴Ti
23	⁶⁵V
24	⁶⁶Cr
25	⁶⁷Mn
26	⁶⁸Fe
27	⁶⁹Co
28	⁷⁰Ni	⁷⁸Ni
29	⁷¹Cu	⁷⁸Cu
30	⁷²Zn	⁷⁸Zn
31	⁷³Ga	⁷⁸Ga
32	⁷⁴Ge	⁷⁸Ge
33	⁷⁵As	⁷⁸As
34	⁷⁶Se	⁷⁸Se
35	⁷⁷Br	⁷⁸Br
36	⁷⁸Kr	⁷⁸Kr
37	⁷⁹Rb	⁷⁸Rb
38	⁸⁰Sr	⁷⁸Sr
39	⁸¹Y	⁷⁸Y
40	⁸²Zr	⁷⁸Zr
41	⁸³Nb
42	⁸⁴Mo
43	⁸⁵Tc
44	⁸⁶Ru

External data and identifiers

Adopted Levels, Gammas:NuDat 78Kr

Literature and References

[1] - F. W. Aston:
Isotopes and Atomic Weights.
In: Nature, 105, (1920), DOI 10.1038/105617a0.

[2] - Deepak Patel, Praveen C. Srivastava:
Investigation of 2vECEC in ¹³²Ba and revisited for ⁷⁸Kr.
In: arXiv:2511.00862, (2025), DOI 10.48550/arXiv.2511.00862.