Astatine-209

Properties and data of the isotope ²⁰⁹At.

Content

General data

Name of the isotope:Astatine-209; At-209Symbol:²⁰⁹At or 20985AtMass number A:209 (= number of nucleons)Atomic number Z:85 (= number of protons)Neutrons N:124Isotopic mass:208.986169(5) u (atomic weight of Astatine-209)Nuclide mass:208.9395453 u (calculated nuclear mass without electrons)Mass excess:-12.88349 MeVMass defect:1.753408304 u (per nucleus)Nuclear binding energy:1633.28931473 MeV (per nucleus)
7.81478141 MeV (average binding energy per nucleon)Separation energy:S_N = 8.485(10) MeV (first neutron)
S_P = 2.704(5) MeV (first proton)Half-life:5.42(5) hDecay constant λ:3.55241482451 × 10^-5 s^-1Specific activity α:1.028516464719 × 10⁺¹⁷ Bq g^-1
2779774.2289727 Ci g^-1Spin and parity:
(nuclear angular momentum)9/2-Year of discovery:1951

Radioactive Decay

Half-life T_½ = 5.42(5) h (hours) respectively 1.9512 × 10⁴ seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
α	²⁰⁵Bi	4.1(5) %	5.7569(20) MeV
EE/β⁺	²⁰⁹Po	95.9(5) %	3.482(5) MeV

Parent Nuclides

Direct parent isotopes are: ²¹³Fr, ²⁰⁹Rn.

Nuclear Isomers

Nuclear isomers or excited states with the activation energy in keV related to the ground state.

Nuclear Isomer	Excitation Energy	Half-life	Spin
^209mAt	2429.32(22) keV	0.916(10) μs	29/2+

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 124) and isobaric (same nucleon number A = 209) with Astatine-209. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

OZ	Isotone N = 124	Isobar A = 209
75	¹⁹⁹Re
76	²⁰⁰Os
77	²⁰¹Ir
78	²⁰²Pt
79	²⁰³Au	²⁰⁹Au
80	²⁰⁴Hg	²⁰⁹Hg
81	²⁰⁵Tl	²⁰⁹Tl
82	²⁰⁶Pb	²⁰⁹Pb
83	²⁰⁷Bi	²⁰⁹Bi
84	²⁰⁸Po	²⁰⁹Po
85	²⁰⁹At	²⁰⁹At
86	²¹⁰Rn	²⁰⁹Rn
87	²¹¹Fr	²⁰⁹Fr
88	²¹²Ra	²⁰⁹Ra
89	²¹³Ac	²⁰⁹Ac
90	²¹⁴Th	²⁰⁹Th
91	²¹⁵Pa
92	²¹⁶U