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.
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