Look at what each atom is bound to, as well as its position in the periodic table.
The simplest example would be NaCl.
When NaCl dissolves, it dissociates into the ions of
How do I know that?
NaCl is the representation of the ionic compound. Given that there is no indication of a net charge on the compound, I know that the charge on the Na has to cancel out the charge on the Cl (because otherwise, the compound would have a net charge).
Now I look at the position of each atom to determine what its ion would be. In chemistry, every atom seeks a full octette; i.e. Cl has to gain one electron, whilst Na has to lose one. In an ionic compound exactly that happens. Sodium's electron is taken by Chlorine, hence, giving Chlorine a negative charge (as it has one more electron than it has protons) and Sodium a positive charge.
Your rule should be:
- Look at the charge on the compound (an example of a non-neutral compound would be
- Look at the positions of the atoms in the periodic table and look at how many electrons they have to gain/lose in order to have a full outer shell
- Make sure that when you add up the charges that you determined in step 2. equal to the charge on the entire compound.
If you want me to walk you through a specific example, just comment below and I will reply as soon as possible :)