What does the notation $^{197\mathrm{m}2}\ce{Pb}$ mean? Specifically, the '$\mathrm{m}$2' part.

I've found this and it appears to have something to do with charge distribution. The original notation was found here.


1 Answer 1


As per this Wikipedia page on isotopes:

The letter $m$ is sometimes appended after the mass number to indicate a nuclear isomer, a metastable or energetically-excited nuclear state (as opposed to the lowest-energy ground state), for example $\ce{^{180\mathrm{m}}_73Ta}$ (tantalum-180m).

Note that the lower subscript to the left of the chemical symbol is the atomic number of the element (in the example above, 73 is the atomic number for tantalum) and is omitted in your example.

However, your question specifically is about the "m2" part. I'm quoting the Wikipedia page on nuclear isomers here:

For isotopes with more than one metastable isomer, "indices" are placed after the designation, and the labeling becomes m1, m2, m3, and so on.

In your case, 197 is the mass number for this lead isotope, and m2 is the second excited metastable state, with a half-life of 1.15 $\mu$s.


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