This is "Unit 5", section 5.3 from the book General Chemistry (v. 1.0).

Has this book helped you? Consider passing it on:
Help Creative Commons
Creative Commons supports free culture from music to education. Their licenses helped make this book available to you.
Help a Public School
DonorsChoose.org helps people like you help teachers fund their classroom projects, from art supplies to books to calculators.

5.3 Polyatomic Ions

Learning Objective

  1. To describe the composition of a chemical compound containing polyatomic ions.

This section concludes the topic of nomenclature for the course. Hopefully you have learned that chemists are very precise and systematic when it comes to names and formulas of chemical compounds. To write names and formulas we apply a step-wise procedure to a few nuggets of memorized trivia. This section is no different in those respects. Unfortunately however this section introduces rather a lot of trivia. The upside is the system is the same. Do the required memorization of names, formulas and charges. The memorization is the hard part. The system is the easy part.

Polyatomic Ions

Polyatomic ionsA group of two or more atoms that has a net electrical charge. are groups of atoms that bear a net electrical charge, although the atoms in a polyatomic ion are held together by the same covalent bonds that hold atoms together in molecules. Just as there are many more kinds of molecules than simple elements, there are many more kinds of polyatomic ions than monatomic ions. Two examples of polyatomic cations are the ammonium (NH4+) and the methylammonium (CH3NH3+) ions. Polyatomic anions are much more numerous than polyatomic cations. The polyatomic anions can be challenging in terms of memorizing the names, formulas and charges. As a starting place it is suggested you begin with the oxoacids you learned in the previous section. From the oxoacids, you can remove hydrogen ions to derive the formulas and charges of the oxoanions. Then it's just a matter of memorizing some new names. See Table 5.3(1).

Table 5.3(1) Names and Formulas of Oxoacids and Their Anions

Name of the Acid

Formula of the Acid

Formula of the Anion

Name of the Anion

acetic acid

HC2H3O2

C2H3O2-

acetate

perchloric acid

HClO4

ClO4-

perchlorate

chloric acid

HClO3

ClO3-

chlorate

chlorous acid

HClO2

ClO2-

chlorite

hypochlorous acid

HClO

ClO-

hypochlorite

nitric acid

HNO3

NO3-

nitrate

nitrous acid

HNO2

NO2-

nitrite

carbonic acid

H2CO3

HCO3-

bicarbonate or hydrogen carbonate

CO32-

carbonate

sulfuric acid

H2SO4

HSO4-

hydrogen sulfate or bisulfate

SO42-

sulfate

sulfurous acid

H2SO3

HSO3-

hydrogen sulfite or bisulfite

SO32-

sulfite

phosphoric acid

H3PO4

H2PO4-

dihydrogen phosphate

HPO42-

hydrogen phosphate

PO43-

phosphate

phosphorous acid

H3PO3

H2PO3-

dihydrogen phosphite

HPO32-

hydrogen phosphite

PO33-

phosphite

See if you can sit down with a blank piece of paper and from memory write down the first two columns which represent the names and formulas of the important oxoacids. Once that part is well in hand, then try adding the remaining columns of Table 5.3(1).

Not all the polyatomic ions are derived from oxoacids. You'll still need to catch the five stray species highlighted in yellow in Table 5.3(2) below. At this point it's brute force memorization, but just watch how often these ions come up in the remainder of the textbook and in your laboratory activities. Soon you'll be a pro!

Table 5.3(2) Common Polyatomic Ions and Their Names

Formula Name of Ion
NH4+ ammonium
CH3NH3+ methylammonium
OH- hydroxide
O22- peroxide
CN- cyanide
SCN- thiocyanate
NO2- nitrite
NO3- nitrate
CO32- carbonate
HCO3- hydrogen carbonate, or bicarbonate
SO32- sulfite
SO42- sulfate
HSO4- hydrogen sulfate, or bisulfate
PO43- phosphate
HPO42- hydrogen phosphate
H2PO4- dihydrogen phosphate
ClO- hypochlorite
ClO2- chlorite
ClO3- chlorate
ClO4- perchlorate
MnO4- permanganate
CrO42- chromate
Cr2O72- dichromate
C2O42- oxalate
HCO2- formate
CH3CO2- acetate
C6H5CO2- benzoate

The method we used to predict the formulas for ionic compounds that contain monatomic ions can also be used for compounds that contain polyatomic ions. The overall charge on the cations must balance the overall charge on the anions in the formula unit. Thus K+ and NO3- ions combine in a 1:1 ratio to form KNO3 (potassium nitrate or saltpeter), a major ingredient in black gunpowder. Similarly, Ca2+ and SO42- form CaSO4 (calcium sulfate), which combines with varying amounts of water to form gypsum and plaster of Paris. The polyatomic ions NH4+ and NO3- form NH4NO3 (ammonium nitrate), which is a widely used fertilizer and, in the wrong hands, an explosive. One example of a compound in which the ions have charges of different magnitudes is calcium phosphate, which is composed of Ca2+ and PO43- ions; it is a major component of bones. The compound is electrically neutral because the ions combine in a ratio of three Ca2+ ions [3(+2) = +6] for every two ions [2(-3) = -6], giving an empirical formula of Ca3(PO4)2; the parentheses around PO4 in the empirical formula indicate that it is a polyatomic ion. Writing the formula for calcium phosphate as Ca3P2O8 gives the correct number of each atom in the formula unit, but it obscures the fact that the compound contains readily identifiable PO43- ions.

Example 5.3-1

Write the formula for the compound formed from each ion pair.

  1. Na+ and HPO42-
  2. potassium cation and cyanide anion
  3. calcium cation and hypochlorite anion

Given: ions

Asked for: formula for ionic compound

Strategy:

A If it is not given, determine the charge on a monatomic ion from its location in the periodic table. Use Table 5.3(1) or Table 5.3(2)to find the charge on a polyatomic ion.

B Use the absolute value of the charge on each ion as the subscript for the other ion. Reduce the subscripts to the smallest whole numbers when writing the formula.

Solution:

  1. B Because HPO42- has a charge of -2 and Na+ has a charge of +1, the empirical formula requires two Na+ ions to balance the charge of the polyatomic ion, giving Na2HPO4. The subscripts are reduced to the lowest numbers, so the formula is Na2HPO4. This compound is sodium hydrogen phosphate, which is used to provide texture in processed cheese, puddings, and instant breakfasts.
  2. A The potassium cation is K+, and the cyanide anion is CN-. B Because the magnitude of the charge on each ion is the same, the formula is KCN. Potassium cyanide is highly toxic, and at one time it was used as rat poison. This use has been discontinued, however, because too many people were being poisoned accidentally.
  3. A The calcium cation is Ca2+, and the hypochlorite anion is ClO-. B Two ClO- ions are needed to balance the charge on one Ca2+ ion, giving Ca(ClO)2. The subscripts cannot be reduced further, so the formula is Ca(ClO)2. This is calcium hypochlorite, the "chlorine" used to purify water in swimming pools.

Exercise

Write the empirical formula for the compound formed from each ion pair.

  1. Ca2+ and H2PO4-
  2. sodium cation and bicarbonate anion
  3. ammonium cation and sulfate anion

Answer:

  1. Ca(H2PO4)2: calcium dihydrogen phosphate is one of the ingredients in baking powder.
  2. NaHCO3: sodium bicarbonate is found in antacids and baking powder; in pure form, it is sold as baking soda.
  3. (NH4)2SO4: ammonium sulfate is a common source of nitrogen in fertilizers.

Summary

An empirical formula gives the relative numbers of atoms of the elements in a compound, reduced to the lowest whole numbers. The formula unit is the absolute grouping represented by the empirical formula of a compound, either ionic or covalent. Empirical formulas are particularly useful for describing the composition of ionic compounds, which do not contain readily identifiable molecules. Some ionic compounds occur as hydrates, which contain specific ratios of loosely bound water molecules called waters of hydration.

Key Takeaway

  • The composition of a compound is represented by an empirical or molecular formula, each consisting of at least one formula unit.

Conceptual Problems

  1. Name each cation.

    1. K+
    2. Al3+
    3. NH4+
    4. Mg2+
    5. Li+

  2. Name each anion.

    1. Br−
    2. CO32−
    3. S2−
    4. NO3−
    5. HCO2−
    6. F−
    7. ClO−
    8. C2O42−

  3. Name each anion.

    1. PO43−
    2. Cl−
    3. SO32−
    4. CH3CO2−
    5. HSO4−
    6. ClO4−
    7. NO2−
    8. O2−

  4. Name each anion.

    1. SO42−
    2. CN−
    3. Cr2O72−
    4. N3−
    5. OH−
    6. I−
    7. O22−

  5. Name each compound.

    1. MgBr2
    2. NH4CN
    3. CaO
    4. KClO3
    5. K3PO4
    6. NH4NO2
    7. NaN3

  6. Name each compound.

    1. NaNO3
    2. Cu3(PO4)2
    3. NaOH
    4. Li4C
    5. CaF2
    6. NH4Br
    7. MgCO3

  7. Name each compound.

    1. RbBr
    2. Mn2(SO4)3
    3. NaClO
    4. (NH4)2SO4
    5. NaBr
    6. KIO3
    7. Na2CrO4

  8. Name each compound.

    1. NH4ClO4
    2. SnCl4
    3. Fe(OH)2
    4. Na2O
    5. MgCl2
    6. K2SO4
    7. RaCl2

  9. Name each compound.

    1. KCN
    2. LiOH
    3. CaCl2
    4. NiSO4
    5. NH4ClO2
    6. LiClO4
    7. La(CN)3

Answer

    1. potassium ion
    2. aluminum ion
    3. ammonium ion
    4. magnesium ion
    5. lithium ion

    1. phosphate ion
    2. chloride ion
    3. sulfite ion
    4. acetate ion
    5. hydrogen sulfate ion (also called bisulfate ion)
    6. perchlorate ion
    7. nitrite ion
    8. oxide ion

    1. magnesium bromide
    2. ammonium cyanide
    3. calcium oxide
    4. potassium chlorate
    5. potassium phosphate
    6. ammonium nitrite
    7. sodium azide

    1. rubidium bromide
    2. manganese(III) sulfate
    3. sodium hypochlorite
    4. ammonium sulfate
    5. sodium bromide
    6. potassium iodate
    7. sodium chromate

    1. potassium cyanide
    2. lithium hydroxide
    3. calcium chloride
    4. nickel(II) sulfate
    5. ammonium chlorate
    6. lithium perchlorate
    7. lanthanum(III) cyanide

Numerical Problems

  1. Write the formula for each compound.

    1. magnesium sulfate, which has 1 magnesium atom, 4 oxygen atoms, and 1 sulfur atom
    2. ethylene glycol (antifreeze), which has 6 hydrogen atoms, 2 carbon atoms, and 2 oxygen atoms
    3. acetic acid, which has 2 oxygen atoms, 2 carbon atoms, and 4 hydrogen atoms
    4. potassium chlorate, which has 1 chlorine atom, 1 potassium atom, and 3 oxygen atoms
    5. sodium hypochlorite pentahydrate, which has 1 chlorine atom, 1 sodium atom, 6 oxygen atoms, and 10 hydrogen atoms

  2. Write the formula for each compound.

    1. cadmium acetate, which has 1 cadmium atom, 4 oxygen atoms, 4 carbon atoms, and 6 hydrogen atoms
    2. barium cyanide, which has 1 barium atom, 2 carbon atoms, and 2 nitrogen atoms
    3. iron(III) phosphate dihydrate, which has 1 iron atom, 1 phosphorus atom, 6 oxygen atoms, and 4 hydrogen atoms
    4. manganese(II) nitrate hexahydrate, which has 1 manganese atom, 12 hydrogen atoms, 12 oxygen atoms, and 2 nitrogen atoms
    5. silver phosphate, which has 1 phosphorus atom, 3 silver atoms, and 4 oxygen atoms

  3. Complete the following table by filling in the formula for the ionic compound formed by each cation-anion pair.

    Ion K+ Fe3+ NH4+ Ba2+
    Cl- KCl
    SO42-
    PO43-
    NO3-
    OH-

  4. Write the empirical formula for the binary compound formed by the most common monatomic ions formed by each pair of elements.

    1. zinc and sulfur
    2. barium and iodine
    3. magnesium and chlorine
    4. silicon and oxygen
    5. sodium and sulfur

  5. Write the empirical formula for the binary compound formed by the most common monatomic ions formed by each pair of elements.

    1. lithium and nitrogen
    2. cesium and chlorine
    3. germanium and oxygen
    4. rubidium and sulfur
    5. arsenic and sodium

  6. Write the empirical formula for each compound.

    1. Na2S2O4
    2. B2H6
    3. C6H12O6
    4. P4O10
    5. KMnO4

  7. Write the empirical formula for each compound.

    1. Al2Cl6
    2. K2Cr2O7
    3. C2H4
    4. (NH2)2CNH
    5. CH3COOH

Answers

    1. MgSO4
    2. C2H6O2
    3. C2H4O2
    4. KClO3
    5. NaOCl·5H2O
  3. Ion K + Fe 3+ NH 4 + Ba 2+
    Cl - KCl FeCl3 NH4Cl BaCl2
    SO 4 2- K2SO4 Fe2(SO4)3 (NH4)2SO4 BaSO4
    PO 4 3- K3PO4 FePO4 (NH4)3PO4 Ba3(PO4)2
    NO 3 - KNO3 Fe(NO3)3 NH4NO3 Ba(NO3)2
    OH - KOH Fe(OH)3 NH4OH Ba(OH)2

    1. Li3N
    2. CsCl
    3. GeO2
    4. Rb2S
    5. Na3As

    1. AlCl3
    2. K2Cr2O7
    3. CH2
    4. CH5N3
    5. CH2O