Combining Ions

For example, a positive ion with a +1 charge (A⁺) combines with a negative ion with a –1 charge (B^–) like this:

The combination is written all together without showing the charges. The number of ions in the combination is selected so that the total charge for all ions in the combination is zero. Here is another example:

A positive ion with a +2 charge (A²⁺) combines with a negative ion with a –1 charge (B^–) like this:

Here the combination has a two after the negative ion because it takes two of them to balance one of the positive ions. You can think of this combination like this:

If you put all three ions in a box, then the total charge for everything in the box is zero:

Here is another example, a positive ion with a +2 charge (A²⁺) combines with a negative ion with a –3 charge (B^3–) like this:

Here the combination has a three after the positive ion and a two after the negative ion because it takes three A²⁺ ions to balance two B^3– ions. You can think of this combination like this:

If you put all five ions in a box, then the total charge for everything in the box is zero:

In summary, ions must be combined so that the total positive and the total negative charge is equal. This requires that there are enough of each ion to make the total positive and the total negative charge equal to the least common multiple of the two charges. This is summarized in the following table:

So far these examples have used the symbols A and B to stand for any element. Most real ions will be plus or minus 1, 2, or 3. When building chemical formulas using real ions the example formulas in the table above give all of the possible combinations for all the ions with those charges. Fill in the blank places in the table for ions with a plus or minus four charge. First try it yourself, then check with classmates to make sure you have the right idea and get the correct responses in your table.

Naming ionic compounds of two elements is very simple. Just name the metal then give the name of the non-metal while changing the end of the name to –ide. Do not use prefixes! The name of NO₂ is nitrogen dioxide. The name of the ionic compound CaCl₂ is just calcium chloride. Since calcium is always a +2 ion and chloride is a –1 ion this is the only possible combination of these ions. Here are a few more example names:

• Al₂O₃ is aluminum oxide not dialuminum trioxide
• MgBr₂ is magnesium bromide not magnesium dibromide
• Na₂O is sodium oxide not disodium monoxide

Here is a table of ions made up of only one atom: monatomic ions:

There are 18 cations and 12 anions in the table. Together this would give 216 possible combinations! This is too many and ends up being pretty redundant since so many combinations are so similar. Your assignment is to make all possible combinations for just the first two rows in the table. This is a total of just 60 compounds. Your task is to make all possible combinations for Li, Be, B, Na, Mg, Al, Fe, and Pb with N, O, F, P, S, and Cl. Note that both iron (Fe) and lead (Pb) have two ions each! This means that the ion names must include Roman numerals to show the charge of the ion. Get a sheet of paper and make a table like the example below, which gives you three out of the 60 combinations. You may work with a classmate to figure out correct combinations but every student must write their own table.