Here's why:

* Electronic Configuration: Promethium's electronic configuration is [Xe] 4f⁵ 6s².

* Valence Electrons: Valence electrons are the electrons in the outermost shell of an atom. In the case of promethium, the outermost shell is the 6th shell, which contains 2 electrons (6s²).

* f-block Exception: The 4f shell is also considered part of the valence shell for elements in the f-block (like promethium), even though it is not the outermost shell. Promethium has 5 electrons in its 4f shell.

Therefore, promethium has a total of 2 (6s²) + 1 (4f, only one electron is considered valence for chemical bonding) = 3 valence electrons.