Naming Ionic Compounds

Writing the name of an ionic compound follows a very simple protocol.

Write the name of the first ion, then write the name of the second ion.

For example NaNO₃ is called sodium nitrate. Sodium is the first ion and nitrate is the second. You do NOT indicate the number of ions present. So, for example, Li₂SO₄ is called lithium sulfate. It is NOT dilithium sulfate. You will, of course, need to know your polyatomic ions for this.

There are a few issues that can make this seem more complicated.

Issues with the first ion

If the first ion is a metal that has more than one possible charge, you must indicate the charge. For example, the compound CoCl₂ is called cobalt II chloride (the II indicates that the cobalt ion has a charge of 2). This requires you to determine the charge on the metal ion.

So, CoF₂ is cobalt II fluoride and Co₂(SO₄)₃ is cobalt III sulfate.

If the metal has two (and only two) possible charges, there is another way we can indicate the charge. We take the original latin root of the element's name and add either "ic" or "ous" to the end. "ic" is for the higher charge and "ous" indicates the lower charge. So copper I ions can also be called cuprous and copper II ions can be called cupric. This, of course, requires you to recognize the latin roots of element names. Here are a few common ones:

element	root	higher charge	lower charge
Copper	cuprum	cupric	cuprous
cobalt	cobalt	cobaltic	cobaltous
gold	aurum	auric	aurous
nickel	nickel	nickelic	nickelous
tin	stannum	stannic	stannous
lead	plumbum	plumbic	plumbous
iron	ferrum	ferric	ferrous

Issues with the second ion

The second ion is always the negative ion and negative ions have only three possible endings: ate, ite, and ide. The endings "ate" and "ite" are only found on polyatomic ions (sulfate, nitrite, etc)

The "ide" ending is more complicated. It appears at the end of a few polyatomic ions (hydroxide, cyanide) but it also appears at the end of the name for a negative ion made from an element. So, a chlorine atom that has acquired a negative charge is called chloride (Cl^-1).

A different way of saying this is that the ide ending on an element indicates that it is a negative ion. Even more than that,

The "ide" ending indicates that the ion has the charge that makes the atom isoelectronic with the closest noble gas.

So, fluoride is F^-1, oxide is O^-2, nitide is N^-3, sulfide is S^-2, bromide is Br^-1, etc.

Try these to make sure that you understand. The answers are here.