Question

Question #96822

Answer 1

The "m" stands for "metastable".

Explanation:

The nuclei in `"Tc-99m"` are in an "excited" state, but they are "forbidden" to make the transition to the more stable `"Tc-99"` by the selection rules for allowed nuclear transitions.

However, there are degrees of "forbidden-ness", and `"Tc-99m"` is able to emit a γ-ray and decay to `"Tc-99"` with a half-life of about 6 h.