The chemical formula for a compound containing one calcium atom for every two iodine atoms is CaI₂.

Here's why:

* Calcium (Ca) is in group 2 of the periodic table, meaning it has a +2 charge when it forms ions.

* Iodine (I) is in group 17 of the periodic table, meaning it has a -1 charge when it forms ions.

To balance the charges, you need two iodine ions (2 x -1 = -2) to counter the +2 charge of one calcium ion.