Lesson 15: Making Compounds with Polyatomic Ions
(see pages 179-180)
Forming compounds with polyatomic ions is very similar to the way monatomic ions form compounds. You must
remember that the polyatomic ions (although made of more than one individual element) function as one unit. The
charge given to the polyatomic ion is for the whole ion and not any individual portion. Let's look at some examples of
how compounds made with polyatomic ions form.
Sodium hydroxide is a compound commonly known as lye which is used to unclog drains and to make lye soap. The
ions which make sodium hydroxide are the cation sodium and the anion hydroxide.
Sodium
Hydroxide
Na+1
(OH)-1
To make the compound sodium hydroxide, we would need to use one sodium ion and one hydroxide ion. In doing so,
the sum of the charges equals zero. Recall, also, that it is customary to write the cation first, followed by the anion.
NaOH
Proper way to write
sodium hydroxide
[Na+1-1(OH)-11] Remember don't write it like this! (see lesson 14)
Note that one sodium cation
can bond with one hydroxide anion
to form sodium hydroxide
Let's look at some more examples. Calcium carbonate is the chemical which makes up the shells of many ocean
creatures. Another name for fossilized calcium carbonate is limestone. The constituents of calcium carbonate are the
calcium cation and the carbonate anion.
Calcium
Ca+2
Carbonate
(CO3)-2
In order to make the compound calcium carbonate, the charges of the ions must add up to zero. In this case, using
one atom of each ion will result in a total charge of zero.
CaCO3
[Ca+21(CO3)-21] Remember ...
Here is another example. Hydrogen peroxide is a well-known antiseptic solution used to "foam out" impurities in
shallow wounds. The hydrogen peroxide compound is composed of the hydrogen cation and the peroxide anion.
Hydrogen
H+1
Peroxide
(O2)-2
If we take one hydrogen cation and one peroxide ion, the sum of the charges does not add up to zero. We must take a
multiple of one or both of the ions in order for the sum of the charges to equal zero. In this case, taking two hydrogen
cations (each with a + I charge) and one peroxide anion (with its -2 charge) will result in a total charge of zero.
[H+12(O2)-21] Remember ...
H 2 O2
Note that it takes two hydrogen
cations for each peroxide anion to
create the stable hydrogen peroxide
compound.
And here is a final example. The chemical formula for rust is iron (III) oxide. The iron (III) oxide compound is
composed of the iron (III) cation and the oxide anion. Recall that ions with a Roman Numeral as part of their name, tell
you the charge of the cation. In this case iron (III) is Fe+3.
Iron (III)
Fe+3
Oxide
O-2
Forming this compound is a little more challenging. In order to get our charges to add to zero, we must take two iron
(III) cations and three oxide cations. (2 times (+3)
Fe2O3
plus 3 times ( -2) = 0)
[Fe+32O-23] Remember ...
Note that it takes two iron (III) cations
along with three oxide anions to form
rust!
If you are given Fe2O3 and you are asked what is the charge (or oxidation state) of the iron cation, here is how you
determine the charge on the iron cation:
1) The oxygen anion always has a charge of -2. So three oxygen anions have a charge of -6
[3 times (-2) = -6]
2) If the anion side (oxygen side) has a charge of -6, then the cation side (iron side) must have a charge of +6 for a
neutral compound [-6 plus (+6) = 0]
3) Since the cation side (the iron side) has a charge of +6, each of the two iron cations must have a charge of +3.
[2 times (+3) = +6]
Write the chemical formula for each compound. Refer to the Master Ion List (Lesson 14) or your text if necessary.
Compound Name
Chemical Formula
Compound Name
I. Sodium chloride
21. Barium hydroxide
2. Potassium nitrate
22. Calcium nitrite
3. Lithium chloride
23. Cesium sulfate
4. Barium sulfite
24. Nickel nitrate
5. Calcium carbonate
25. Calcium oxide
6. Strontium oxide
26. Sodium bromide
7. Tin (IV) sulfate
27. Silver chromate
8. Hydrogen chlorate
28. Copper (II) fluoride
9. Potassium perchlorate
29. Potassium permanganate
10. Aluminum acetate
30. Lithium carbonate
II. Magnesium sulfite
31. Nickel cyanide
12. Mercury (II) bromide
32. Lead (II) hypochlorite
13. Sodium dichromate
33. Zinc chlorite
14. Strontium iodide
34. Hydrogen peroxide
15. Ammonium dichromate
35. Copper (I) sulfite
16. Potassium chloride
36. Hydronium sulfide
17. Chromium perchlorate
37. Nickel phosphate
18. Calcium cyanide
38. Mercury (II) carbonate
19. Barium chlorite
39. Tin (IV) sulfate
20. Ammonium chloride
40. Potassium hydroxide
Chemical Formula