Seaborgium-257

Properties and data of the isotope ²⁵⁷Sg.

Contents

Seaborgium-257 is a radioisotope of the chemical element seaborgium, which has 151 neutrons in its atomic nucleus in addition to the element-specific 106 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 257. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²⁵⁷Sg is exclusively for academic purposes.

The discovery of the supershear nuclide was reported in 2025; seaborgium-257 was observed as one of the products formed in the reaction ²⁰⁶Pb(⁵²Cr,n) [1]. The half-life was determined to be T_1/2 = 12.5+3.7-2.3 ms.

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

General data

Name of the isotope:Seaborgium-257; Sg-257Symbol:²⁵⁷Sg or 257106SgMass number A:257 (= number of nucleons)Atomic number Z:106 (= number of protons)Neutrons N:151Isotopic mass:257 u (atomic weight of Seaborgium-257)Nuclide mass:256.9418598 u (calculated nuclear mass without electrons)Mass excess:0 MeVMass defect:2.137855708 u (per nucleus)Nuclear binding energy:1991.39976487 MeV (per nucleus)
7.74863722 MeV (average binding energy per nucleon)Half-life:12.6 msDecay constant λ:55.011680996821 s^-1Specific activity α:1.289058723364 × 10⁺²³ Bq g^-1
3483942495580.5 Ci g^-1Spin and parity:
(nuclear angular momentum)(9/2-)Year of discovery:2025

Radioactive Decay

Half-life T_½ = 12.6 ms respectively 1.26 × 10^-2 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
SZ	div	90 %
α	²⁵³Rf	10 %

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 151) and isobaric (same nucleon number A = 257) with Seaborgium-257. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 151	Isobar A = 257
92	²⁴³U
93	²⁴⁴Np
94	²⁴⁵Pu
95	²⁴⁶Am
96	²⁴⁷Cm
97	²⁴⁸Bk
98	²⁴⁹Cf
99	²⁵⁰Es	²⁵⁷Es
100	²⁵¹Fm	²⁵⁷Fm
101	²⁵²Md	²⁵⁷Md
102	²⁵³No	²⁵⁷No
103	²⁵⁴Lr	²⁵⁷Lr
104	²⁵⁵Rf	²⁵⁷Rf
105	²⁵⁶Db	²⁵⁷Db
106	²⁵⁷Sg	²⁵⁷Sg

Literature and References

[1] - P. Mosat, J. Khuyagbaatar, J. Ballof et al.:
Probing the Shell Effects on Fission: The New Superheavy Nucleus ²⁵⁷Sg.
In: Physical Review Letters, 134, 232501, (2025), DOI 10.1103/s7hr-y7zq.