Here's why:

* Electron Configuration: Barium (Ba) has an electron configuration of [Xe] 6s². This means it has two electrons in its outermost shell.

* Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration, usually with eight electrons in their outermost shell (the octet rule).

* Ionic Bonding: Barium is a metal, and metals tend to lose electrons to form positive ions (cations). To achieve a stable octet, barium readily loses its two outer shell electrons.

Therefore, the most common ion of barium is Ba²⁺.