The goal of this problem is to combine the two ions and create a neutral compound. We want this compound to have an overall charge of zero, meaning that the ionic charges have to balance each other out. This is because a molecule with no overall charge is more stable than a molecule with either positive or negative charge. The ions we are starting with are unstable, and they will be attracted to each other due to their opposite charges. Note that a positively charged ion is called a cation, and an ion with negative charge is called an anion.
In this problem, we are working with calcium as the cation and chlorate as the anion. However, the calcium ion has a charge of +2, which is twice as large as the chlorate ion’s charge of -1. (In chemistry, anytime you see a negative sign written by itself as a superscript on an ion, it means that the charge is -1.) Because of the difference in magnitude of the charges, the calcium ion will attract two chlorate ions (each with a -1 charge) to neutralize its charge of +2.
The overall equation would look like this:
Ca2+ + ClO3- --> Ca(ClO3)2
To check your work, look at the charges in the final compound. The calcium brings a +2 charge to the compound, and each chlorate brings a -1 charge. 2 – 1 – 1 = 0, so the overall molecule is neutral. You no longer need to write out the ionic charges in the final compound.