Question #96822

The nuclei in $\text{Tc-99m}$ are in an "excited" state, but they are "forbidden" to make the transition to the more stable $\text{Tc-99}$ by the selection rules for allowed nuclear transitions.
However, there are degrees of "forbidden-ness", and $\text{Tc-99m}$ is able to emit a γ-ray and decay to $\text{Tc-99}$ with a half-life of about 6 h.