Oxygen-21

Properties and data of the isotope ²¹O.

Content

General data

Name of the isotope:Oxygen-21; O-21Symbol:²¹O or 218OMass number A:21 (= number of nucleons)Atomic number Z:8 (= number of protons)Neutrons N:13Isotopic mass:21.008655(13) u (atomic weight of Oxygen-21)Nuclide mass:21.0042664 u (calculated nuclear mass without electrons)Mass excess:8.06208 MeVMass defect:0.166589164 u (per nucleus)Nuclear binding energy:155.17680673 MeV (per nucleus)
7.38937175 MeV (average binding energy per nucleon)Separation energy:S_N = 3.805(12) MeV (first neutron)
S_P = 20.99(8) MeV (first proton)Half-life:3.42(10) sDecay constant λ:0.20267461419881 s^-1Specific activity α:5.812071787802 × 10⁺²¹ Bq g^-1
157083021291.97 Ci g^-1Spin and parity:
(nuclear angular momentum)(5/2+)Mirror nucleus:Aluminium-21Year of discovery:1968

Radioactive Decay

Half-life T_½ = 3.42(10) s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
β^-	²¹F	100 %	8.1096 MeV

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 13) and isobaric (same nucleon number A = 21) with Oxygen-21. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

OZ	Isotone N = 13	Isobar A = 21
5	¹⁸B	²¹B
6	¹⁹C	²¹C
7	²⁰N	²¹N
8	²¹O	²¹O
9	²²F	²¹F
10	²³Ne	²¹Ne
11	²⁴Na	²¹Na
12	²⁵Mg	²¹Mg
13	²⁶Al	²¹Al
14	²⁷Si
15	²⁸P
16	²⁹S
17	³⁰Cl
18	³¹Ar
19	³²K