Here's why:

* Promethium is a radioactive metal. It's not a mineral that can be tested for hardness using the Mohs scale. The Mohs scale is used to measure the scratch resistance of minerals.

* Promethium is highly reactive. It's not found in its pure form in nature, and it's difficult to handle due to its radioactivity.

Therefore, attempting to assign a Mohs hardness value to promethium is not practical or meaningful.