Calcium and hydroxide ions combine in a 1:2 ratio to form calcium hydroxide (Ca(OH)₂).

Here's why:

* Calcium (Ca) has a +2 charge. This means it loses two electrons to become stable.

* Hydroxide (OH) has a -1 charge. This means it gains one electron to become stable.

To balance the charges and create a neutral compound, you need two hydroxide ions for every one calcium ion. This results in the formula Ca(OH)₂.