Naming Ionic Compounds

There are three different types of ions that could be involved in an ionic bond and the rules for naming changes with the type of ion. (Please note,

First, a simple bond between a metal that is NOT a transition metal and a nonmetal. The charges of these elements is always the same, so we don't have to do anything fancy with the names. We simply write the name of the positive ion first. Then, we write the negative ion and change the ending to -ide. Simple! For example, NaCl would be sodium chloride; MgBr2 would be magnesium bromide. There is no need to mention the subscript 2 because it only tells us that we need 2 Br- ions to balance out the Mg+2 ion.

Second, bonds involving transition metals and nonmetals can be a little trickier. This is because many of the transition metals can have different charges. Iron can have a +2 or +3 charge! That means iron can bond with oxygen in two ways. It can be FeO or Fe2O3! So, how are you supposed to clarify that it the name? Simple, we write the charge of iron in parenthesis. FeO would be iron (II) oxide (notice the negative ion still ends in -ide) and Fe2O3 would be iron (III) oxide.

How do you figure out which iron is which using the formula? How about a little reverse formula writing :) Now, FeO has iron and oxygen bonded in a 1:1 ratio; in class we said that is the ratio is 1:1 that means the charges are equal and opposite. We know that oxygen always forms a -2 ion (because it is in family 6 and gains two electrons); that means that iron has to be a +2 ion (opposite and equal). On the other hand, Fe2O3 is a 2:3 ratio, so we'll do the criss-cross method in reverse.

Fe₂O₃

Fe⁺³    O^-2

Since this iron had to have a +3 charge, we write it as iron (III) oxide.

Third, the bond can involve polyatomic ions, which are atoms bonded together and acting as a single ion. Any compound involving polyatomic ions is named with the positive ion first and the negative ion second (as all are and as the formulas are written). BUT, the name of the polyatomic ion is NEVER changed. If the formula is MgSO4 then the name is magnesium sulfate. Mg+2 is the positive ion of magnesium and SO4 -2 is sulfate. There is no change to the ending of the negative ion because it is a polyatomic ion. We haven't had any of these for you to name in class and you aren't expected to memorize the names of polyatomic ions.  

Writing Formulas for Ionic Compounds

Chemical formulas are written to show the ratio of elements in a compound. Ionic bonds are between positive metal ions and negative nonmetal ions. When writing the formula for an ionic compound, the positive ion is written first and the negative ion second. Ions must bond to each other in a ratio that creates a neutral compound. This means your positive and negative charges must balance out.

To get the charges to balance out, we add subscripts to show the number of each type of ion in the compound. If the charges are already equal, but opposite, they will be in a 1:1 ratio. So, Na+1 and Cl -1 combine to form NaCl. Al+3 and N-3 combine to form AlN. However, if the charges are NOT equal (they will still be opposite because like charges won't attract), we use the criss-cross method. For the criss-cross method, we take the number for one element's charge and cross it over to become the other element's subscript.

Image from here

Creating ionic bonds using polyatomic ions would be similar. However, we would have the charge of the entire polyatomic ion cancel out with the other charge. If more the subscript for the polyatomic ion would be more than 1, we would put parenthesis around the polyatomic ion and then write the new subscript. You cannot change anything about the polyatomic ion. It must stay together as if it were a single ion because that's how polyatomic ions work.

Image from here