Binary Ionic Compounds

Binary Ionic Compounds tutorial

An ionic compound is a compound that is formed between a metal and a non-metal. (Metalloids can also be used in ionic compounds, sometimes as cations and sometimes as anions, depending on the properties of the specific element.). In ionic compounds the metal will always be a cation and the non-metal will always be an anion. Please note, the negative oxidation numbers we wrote on top of Groups IVA, VA, VIA & VIIA on our periodic tables refer only to the non-metals and the metalloids. The metals in these columns have different oxidation numbers.

When forming ionic compounds the goal is to balance the number of positive charges with the number of negative charges. More specifically, you want to ensure that the number of electrons that the cations are giving up is equal to the number of electrons the anions need so that both have full outer energy level.

Binary ionic compounds contain only two elements, one is the cation and the other is the anion. It is important to remember that when writing binary ionic compounds THE CATION MUST ALWAYS BE WRITTEN FIRST. The rest of the rules will be outlined in the following example.

Part I. How to Write a Binary Ionic Compound Formula

In this first example we will use barium and sulfur.

Description of Action	Action
1. Write the symbol of the cation with its charge.	1. Ba2+
2. To the right of the cation, write the anion and its charge.	2. Ba2+ S2-
3. Cross each element's oxidation number to the lower right side of the other element's symbol.	3. Ba2- S2+
4. Remove all (+) signs, (-) signs and ones.	4. Ba2S2
5. Reduce, if necessary. You can only reduce when the subscripts of all the symbols have a common denominator.	5. Since Ba and S both have a 2 for a subscript, it can be reduced to 1. And, since ones are not written, the answer is: BaS

Important: When writing an element with its oxidation number, the oxidation number should be written as a superscript (higher than the symbol). Upon crossing oxidation numbers you must write the number as a subscript (lower than the symbol). In both cases the number is always written to the right of the element symbol to which it applies.

Part II. Naming Binary Ionic Compounds

On your periodic table, cross out the suffixes for the non-metals and metalloids that form anions. For each of the halogens, cross out -ine; for selenium and tellurium, cross out -ium; for carbon, arsenic, and sulfur, cross out the last two letters of each name; and for oxygen, nitrogen and phosphorus, cross out the last four letters of each element's name. What remains will be referred to as the anion's root name. We will use this root name and the new suffix -ide when naming ionic compounds.

For our example, we will name the compound we made earlier using barium and sulfur, BaS.

Description of Action	Action
1. Write the name of the cation.	1. barium
2. To the right of the cation name, write anion's root name.	2. barium sulf
3. Add the suffix -ide to the end of the anion.	3. barium sulfide

Important: Lower case lettering should be used when naming an ionic compound. Also note that our answer is two separate words. The first is the cation and the second is the root of the anion and the suffix -ide.