Ionic compound

For the main-group elements considered here, the group tells us the number of electrons in the atom's outer shell. This lets us predict what happens when ions form in ionic compounds.

An ionic compound contains positive ions and negative ions. Metals usually form positive ions, while many non-metals form negative ions. Some ions, such as ammonium, nitrate and sulfate, contain several atoms bonded together but behave as a single charged unit.

Ionic compounds contain positively and negatively charged ions. Their formula shows the simplest whole-number ratio that makes the compound electrically neutral. For instance, a magnesium ion is Mg²⁺ and a chloride ion is Cl⁻.