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| Standard atomic weight Ar°(Na) | ||||||||||||||||||||||||||
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There are 20 isotopes of sodium (11Na), ranging from 17
Na to 39
Na (except for the still-unknown 36Na and 38Na),[4] and five isomers (two for 22
Na, and one each for 24
Na, 26
Na, and 32
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;[nb 1] 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).
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.
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