Type I monatomic ions are metal cations that form only one type of ion. These ions, therefore, will always exist with the same positive charge. For example, calcium ions will only exist with a charge of 2+ and not above or below this value.
In contrast, type II monatomic ions are metal cations that can form more than one positively charged ion. Therefore, the charge of the ions will be dependent on the specific type of ionic compound that it forms. For example, iron may exist with either a 2+ or 3+ charge, expressed as Fe 2+ and Fe 3+, respectively.
Anions, or negatively charged ions, are formed from a single element and have a negative charge. Monatomic anions are only able to form a single negatively charged particle, much like type 1 cations. Examples of these ions include Cl -, S 2- and F -. Monatomic anions and cations may combine to form binary ionic compounds, such as sodium chloride composed of Na+ and Cl -.
Unlike monatomic ions, polyatomic ions are composed of two or more atoms bonded together that have a resulting total net charge. An example of a polyatomic ion is the carbonate ion composed of a carbon and three oxygen molecules with a total net charge of minus two: CO3 2-. Polyatomic ions and monatomic ions may also combine to form ionic compounds. The ionic compound calcium carbonate, CaCO3, is formed from the monatomic ion calcium, Ca 2+, and the polyatomic carbonate ion, CO3 2-.