The formula for the compound formed by barium ion (Ba²⁺) and hydroxide ion (OH⁻) is Ba(OH)₂.

Here's how to determine the formula:

* Identify the charges: Barium has a +2 charge (Ba²⁺) and hydroxide has a -1 charge (OH⁻).

* Balance the charges: To create a neutral compound, you need two hydroxide ions (2 x -1 = -2) to balance the +2 charge of the barium ion.

* Write the formula: This gives you the formula Ba(OH)₂.