Here's how to break it down:

* Calcium (Ca): Calcium is an alkaline earth metal with a +2 charge.

* Perchlorate (ClO₄⁻): Perchlorate is a polyatomic ion with a -1 charge.

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