Ionic Bonding -Binary

Making an Ionic Compound

Once the number of electrons that will be gained or lost is known, the cation and anion are put together to make a compound. Because ionic compounds are neutral overall, the number of the positives from the cation must balance with the number of negatives from the anion.

If and bond, Li+ and F- would make LiF, because the positive 1 charge balances a negative 1 charge. But if lithium (Li+) and oxygen (O-2) bond, the compound would end up being LiO-. Obviously more positive lithiums are needed to balance out the larger negative of oxygen. It would take 2 lithiums for every 1 oxygen. To show two lithiums are needed, a subscript of “2” is written after the lithium, Li2O (do not write subscripts of 1).

If an ionic compound is made from Aluminum (Al+3) and Sulfur (S-2), the amounts of each element needed would be: Al+3 S-2 Al+3 S-2 S-2 Totals: +6 and -6

So the resulting compound would be Al2S3.

If you look at the subscript on the aluminum, you’ll see it matches the negative number from the sulfur, and the subscript on the sulfur matches the positive number from the aluminum. This trend is used as a shortcut, called the criss-cross method. The number from the metal becomes the subscript of the nonmetal, and the number of the nonmetal becomes the subscript of the metal.

The only time this method has a problem is with numbers that are the same. If Al+3 gets criss-crossed with P-3, at first it would seem the compound should be Al3P3. But with ionic compounds it is important to reduce if possible. So the correct answer is not Al3P3, but AlP.

Note: When writing an ionic compound, always write the metal first.

Exercise 1: In the following grids: 1) Write in the correct charge for each element (remember metals will only be positive and nonmetals will only be negative). 2) Criss-cross the metal and nonmetal in the appropriate box (write the metal first – they’re on the left). Do not write “+” or “–“ signs with the subscripts – just the numbers. Do not write subscripts of “1”. 3) Reduce if possible!

F O N S P

Li (1) (2) (3) Be (4) (5) Al (6) (7) (8)

Na (9) (10)

Naming Binary Ionic Compounds

To name a binary ionic compound, state the name of the metal and then the nonmetal (with a space in-between).

To show that it is a compound that is being discussed and not just two elements, the ending of each compound’s name is changed to “ide”. As it is the nonmetals that are on the end of binary ionic compounds, it is the end of the nonmetal’s name that gets changed. Here is a list of the names:

Nitrogen = Nitride Sulfur = Sulfide Oxygen = Oxide = Chloride Fluorine = Fluoride = Phosphorus = Phosphide = Bromide =

Examples: LiF = CaBr2 = calcium bromide

Notice that the subscripts have no effect on the name of binary ionic compounds.

Exercise 2: For each of the compounds made in the grids in exercise 1, write the name of the compounds.

1. ______2. ______

3. ______4. ______

5. ______6. ______

7. ______8. ______

9. ______10. ______

Writing Binary Ionic Compound Formulas from their Names

When given the name of the ionic compound, it is a simple reversal of the steps to get to its formula. The name will tell you which elements are involved, and then: 1) Write the symbols for the metal and nonmetal. 2) Figure out the charge on each (metals will always be positive, nonmetals will always be negative.) 3) Criss-cross the charges and reduce. Do not write the “+