Actinium-221

Properties and data of the isotope ²²¹Ac.

Content

Actinium-221 is a radioisotope of the chemical element actinium, which, in addition to the element-specific 89 protons, has 132 neutrons in the atomic nucleus, resulting in the mass number 221. The very short-lived, only artificially producible, unstable and therefore radioactive nuclide has no practical significance; dealing with ²²¹Ac is for academic purposes only.

General data

Name of the isotope:Actinium-221; Ac-221Symbol:²²¹Ac or 22189AcMass number A:221 (= number of nucleons)Atomic number Z:89 (= number of protons)Neutrons N:132Isotopic mass:221.01559(5) u (atomic weight of Actinium-221)Nuclide mass:220.9667726 u (calculated nuclear mass without electrons)Mass excess:14.52199 MeVMass defect:1.82460676 u (per nucleus)Nuclear binding energy:1699.6102493 MeV (per nucleus)
7.69054411 MeV (average binding energy per nucleon)Separation energy:S_N = 7.284(57) MeV (first neutron)
S_P = 3.030(57) MeV (first proton)Half-life:52(2) msDecay constant λ:13.329753472307 s^-1Specific activity α:3.632291992690 × 10⁺²² Bq g^-1
981700538565.03 Ci g^-1Spin and parity:
(nuclear angular momentum)(3/2-)Year of discovery:1968

Radioactive Decay

Half-life T_½ = 52(2) ms respectively 5.2 × 10^-2 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
α	²¹⁷Fr	100 %	7.791(57) MeV

Parent Nuclides

Direct parent isotope is: ²²⁵Pa.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 132) and isobaric (same nucleon number A = 221) with Actinium-221. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

OZ	Isotone N = 132	Isobar A = 221
80	²¹²Hg
81	²¹³Tl
82	²¹⁴Pb
83	²¹⁵Bi	²²¹Bi
84	²¹⁶Po	²²¹Po
85	²¹⁷At	²²¹At
86	²¹⁸Rn	²²¹Rn
87	²¹⁹Fr	²²¹Fr
88	²²⁰Ra	²²¹Ra
89	²²¹Ac	²²¹Ac
90	²²²Th	²²¹Th
91	²²³Pa	²²¹Pa
92	²²⁴U	²²¹U
93	²²⁵Np