Polyatomic Ions and Their Compounds

Polyatomic Ions  A polyatomic ion is an electrically charged group of two or more chemically bonded atoms that functions as a single ion.  The atoms of the ion are bonded together with covalent bonds, but the group of atoms as a whole has a charge.  Polyatomic ions combine with other monatomic or polyatomic ions to form ionic compounds according to the same rules as monatomic ions.

Nitrate (NO 3 - )

Sulfate ion (SO 4 2- )

Writing the Formula for a Compound Containing a Polyatomic Ion  Just as in binary compounds, cations and anions must be combined such that the total positive charges equal the total negative charges.  Determine the ions and their charges. The charges of monatomic ions may be determined from their position on the periodic table or, in the case of transition elements, from the Roman numeral in the name of the compound. The charges of the polyatomic ions are given on the chart on page 178.  Combine the ions in such a way that the overall compound is neutral.

Example: Sodium phosphate  Na is found in the first column. Its charge is therefore +1.  The phosphate ion is PO  To make a neutral compound, there must be three Na + for every one PO Therefore, the formula is Na 3 PO 4.

Example: Copper (II) hydroxide  The copper ion is Cu +2 (The Roman numeral indicates this).  The hydroxide ion is OH - (From the chart on page 178).  There must be two OH - ions for every one Cu +2.  Therefore, the formula must be Cu(OH) 2.  Note the parentheses that enclose the polyatomic ion. Parentheses are used to show that everything inside the parentheses are doubled by the subscript 2. Cu(OH) 2 has 2 O and 2 H. To write CuOH 2 would mean that the compound has 1 O and 2 H.

Example: Ammonium sulfate  The cation is the polyatomic ion ammonium, NH 4 +.  The anion is the sulfate ion, SO  Two NH 4 + are needed for every one SO 4 2-,  Therefore, the formula is (NH 4 ) 2 SO 4.

Naming Compounds Containing Polyatomic Ions  The monatomic ions in the compound will be named just as they are in binary compounds.  Use the chart on p.178 of your text to identify the polyatomic ion. Its name is given next to its formula.  Name the cation first, then the anion.  If the cation is a transition metal, be sure to indicate its charge with a Roman numeral.

Example: K 2 CO 3  The cation is the K +, potassium ion.  The anion, CO 3 2-, is listed in the chart on page 178. Its name is carbonate.  Therefore, this compound is potassium carbonate.

Example: Cr(OH) 2  The cation is chromium, which can have more than one possible charge.  The anion is OH -, hydroxide. Its charge is shown on the table on page 178.  Since there are 2 OH - ions, the total negative charge is -2.  The total positive charge must therefore be +2.  Since there is only one chromium ion, it must have a charge of +2.  Therefore, the name of this compound is Chromium (II) hydroxide.

Example: Fe 2 (CO 3 ) 3  The cation is iron, which can have more than one possible charge.  The anion is CO 3 2-, carbonate. Its charge is shown on the table on page 178.  Since there are 3 CO 3 2- ions, the total negative charge is -6.  The total positive charge must therefore be +6.  Since there are two ions, each one must have a charge of +3.  Therefore, the name of this compound is Iron (III) carbonate.

Hydrates  Hydrates are ionic compounds with water molecules incorporated into their crystal lattices.  These ionic compounds can be binary compounds or compounds involving polyatomic ions.  The water molecules are known as the water of hydration.  The remaining compound is known as anhydrous.

Formulas of Hydrates  The formulas for hydrates indicate the number of water molecules incorporated into the crystal lattice  For example, CaSO 4 ·2H 2 O means that there are two water molecules for every formula unit of CaSO 4 (two water molecules for every one Ca 2+ and SO 4 2- )  The formula for anhydrous Calcium sulfate is CaSO 4.

Names of Hydrates  Hydrates are named by adding the appropriate Greek prefix to the word hydrate and then adding this word to the end of the name. CaSO 4 ·2H 2 O is Calcium sulfate dihydrate. CoSO 4 ·7H 2 O is Cobalt (II) sulfate heptahydrate.