Here's why:

* Electron Configuration: Bismuth has an electron configuration of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p³.

* Valence Electrons: Bismuth has 5 valence electrons in its outermost shell.

* Tendency to Lose Electrons: Bismuth tends to lose its 3 valence electrons to achieve a stable, full outer shell configuration, similar to the noble gas Xenon.

Therefore, the stable ion of bismuth is Bi³⁺.