* Barium's Electron Configuration: Barium (Ba) has an atomic number of 56. Its electron configuration is [Xe] 6s².

* Noble Gas Configuration: The nearest noble gas to barium is Xenon (Xe). Xenon has a full outer shell of electrons ([Xe] is its electron configuration).

* Losing Electrons: To achieve a noble gas configuration, barium needs to lose its two 6s electrons.

Therefore, a barium atom must lose 2 electrons to form an ion with a noble-gas configuration. This ion would be Ba²⁺.