Isotopes of sodium
There are 20 isotopes of sodium (11Na), ranging from 17
Na to 39
Na (except for the still-unknown 36Na and 38Na), and two isomers (22m
Na and 24m
Na). 23
Na is the only stable (and the only primordial) isotope. It is considered a monoisotopic element and it has a standard atomic weight of 22.98976928(2). Sodium has two radioactive cosmogenic isotopes (22
Na, with a half-life of 2.6019(6) years; and 24
Na, with a half-life of 14.9560(15) h). With the exception of those two isotopes, all other isotopes have half-lives under a minute, most under a second. The shortest-lived is the unbound 18
Na, with a half-life of 1.3(4)×10−21 seconds (although the half-life of the similarly unbound 17Na is not measured).
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Acute neutron radiation exposure (e.g., from a nuclear criticality accident) converts some of the stable 23
Na (in the form of Na+ ion) in human blood plasma to 24
Na. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed.
22
Na is a positron-emitting isotope with a remarkably long half-life. It is used to create test-objects and point-sources for positron emission tomography.