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## Combining Ions

Ionic compounds can be made of one kind of metal and one kind of non-metal. Metals make positively charged ions by losing electrons. Non-metals make negatively charged ions by gaining electrons. Positive ions must combine with negative ions in order to make a neutral compound.

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

one A+ and one B make the combination AB

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:

one A2+ and two B make the combination AB2

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:

A2+ plus B plus B

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

 A2+ B–       B–

Here is another example, a positive ion with a +2 charge (A2+) combines with a negative ion with a –3 charge (B3–) like this:

three A2+ and two B3– make the combination A3B2

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 B3– ions. You can think of this combination like this:

A2+ plus A2+ plus A2+ plus B3– plus B3–

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

 A2+   A2+   A2+ B3–       B3–

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:

 B– B2– B3– B4– A+ AB A2B A3B A2+ AB2 AB A3B2 A3+ AB3 A2B3 AB A4+

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

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Here is a table of ions made up of only one atom, so-called monatomic ions:

 Group 1 Group 2 Group 13 Group 14 Group 15 Group 16 Group 17 Li+ Be2+ B3+ — N3– O2– F– Na+ Mg2+ Al3+ Si4+ P3– S2– Cl– K+ Ca2+ Ga3+ Ge4+ As3– Se2– Br– Rb+ Sr2+ In3+ Sb3– Te2– I– Cs+ Ba2+ — —

Work together in teams of three or four to create all of the possible correct combinations of ions from this table. Write their formulas and names in a nice neat table. Each group member must make his or her own copy of this table. An example has been included below to get you started but create your table on your own paper. Alternatively, use a computer to type up your table. Don’t forget to use the subscript function for all the numbers that are in your formulas. Get help from a classmate if you don’t remember how to do this.

 Positive Ion Negative Ion Formula for Combination Name Li+ F– LiF lithium fluoride Li+ O2– Li2O lithium oxide Li+ N3– Li3N lithium nitride

As an incentive, I will excuse one zero on a homework assignment if you can find all of the possible combinations and write correct formulas and names for all of them. If no one finds all of them in the time that you have then I will award the team with the largest number of correct combinations with correct formulas and names an extra five points on the quiz for this unit.

You will receive a homework grade for completing this assignment. If you have at least 25 correct combinations with formulas and names then you will earn 100. For any number less than 25 you will earn a proportionally lower grade.

There are twelve negative ions so there are twelve possible combinations for each and every one of the sixteen positive ions. There are therefore 192 combinations that can be made.

This homework belongs with the Chemical Nomenclature group activity.