Here's how to determine the formula for the ionic compound formed between calcium ions (Ca²⁺) and hydroxide ions (OH⁻):

1. Determine the charges:

* Calcium ion: Ca²⁺ (has a +2 charge)

* Hydroxide ion: OH⁻ (has a -1 charge)

2. Find the least common multiple (LCM) of the charges:

* The LCM of 2 and 1 is 2.

3. Determine the subscripts:

* To achieve a +2 charge from calcium, we need one calcium ion (1 x +2 = +2).

* To achieve a -2 charge from hydroxide, we need two hydroxide ions (2 x -1 = -2).

4. Write the formula:

* Since the charges balance, the formula for the ionic compound is Ca(OH)₂. This is known as calcium hydroxide.