The compound formed by mixing a barium ion (Ba²⁺) and a hydroxide ion (OH⁻) is barium hydroxide (Ba(OH)₂).

Here's why:

* Ionic Compounds: Barium hydroxide is an ionic compound, meaning it's formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).

* Balancing Charges: Barium has a +2 charge, while hydroxide has a -1 charge. To achieve neutrality, you need two hydroxide ions for every barium ion.

* Chemical Formula: This results in the chemical formula Ba(OH)₂.