Polyatomic Ions: 2/11/2015

Transcription

1 Polyatomic Ions: If you look at the ingredients on a package of regular plant fertilizer, you will likely see the name ammonium nitrate, NH 4 NO 3. The formula shows symbols of three nonmetals: nitrogen, hydrogen, and oxygen, yet the compound is known to have ionic properties. As a chemical formula for an ionic compound, NH 4 NO 3 does not appear to follow the pattern established in all the previous examples. Why not? Well, it does not appear to contain a metal cation and there are three types of atoms instead of two. Ammonium nitrate is just one of many ionic compounds that contain one or more polyatomic ions. 1

2 Polyatomic ion (complex ion) A group of atoms that are covalently bonded which then gain or lose electrons to become stable. The ammonium ion (NH 4+ ) consists of one nitrogen atom and four hydrogen atoms which as a group have lost one electron. A nitrate ion (NO 3- ) is one nitrogen atom and three oxygen atoms that together have gained an electron. Since complex ions have a charge they can not exist by themselves. they have to be bonded to some other ion. Many of the ingredient labels on products you find around your house have names with these endings: sodium phosphate (cleaners) calcium carbonate (antacid tablets) sodium hypochlorite (household bleach) sodium glutamate (meat tenderizer) 2

3 Do you notice that many of the polyatomic ion names have endings like -ate and -ite? When you look at the name of an ionic compound, the ion name endings are good clues to the presence of polyatomic ions. Writing Chemical Formulas for Compounds with Polyatomic Ions: The formula writing procedure follows the same approach used earlier. You have to focus on balancing the ion charges. Remember to: 1. Write the cation symbol first and the anion symbol last. 2. Balance the charges by providing the appropriate numerical subscript for each ion. 3. The main difference in writing formulas for compounds that contain polyatomic ions is that occasionally, you need to place brackets around a polyatomic ion symbol if its numerical subscript is greater than one. 3

4 Sodium chlorite 1. Sodium is in Group 1 and forms 1+ ions. sodium is Na + 2. The -ite suffix in chlorite suggests that it may be a polyatomic ion. Find the name chlorite in the table of polyatomic ions and list its formula and charge. chlorite is ClO 2-3. Since the 1+ charge on the sodium ion is balanced by the 1- charge on chlorite, one of each ion is needed in the formula. 4. The chemical formula of sodium chlorite is NaClO 2 Iron(III) Sulfate 1. The Roman numeral III indicates that the iron ion has a 3+ charge. iron(iii) is Fe Sulfate has the -ate ending so it is probably a polyatomic ion. A quick look at the table of polyatomic ions confirms this. sulfate is SO Balance the charges. (Fe 3+ ) x 2 = 6+ (SO 4 2- ) x 3 = 6- (6+) + (6-) = net charge 0 4. The sulfate symbol gets the subscript 3: (SO 4 ) 3 5. Note the use of brackets to separate the 4 from the 3. Without them the polyatomic ion formula would show 43 oxygen atoms: SO 43! 6. The chemical formula for iron(iii) sulfate is Fe 2 (SO4) 3. 4

5 Ammonium Permanganate 1. Ammonium is a polyatomic ion. A IUPAC convention is that cation names should end in -ium. ammonium is a 1+ ion: NH The -ate suffix in permanganate suggests that it may be a polyatomic ion. Locate the name permanganate in the data table and list its formula and charge: permanganate is MnO 4-3. Since the 1+ charge on the ammonium ion is balanced by the 1- charge on permanganate, one of each ion is needed in the formula. 4. The chemical formula of ammonium permanganate is NH 4 MnO 4 5. Note that parentheses are not required because just one of each ion is needed to produce a neutral unit. Write formulas for the following: 1. Sodium hydroxide 2. Potassium bicarbonate 3. Potassium carbonate 4. Magnesium hydroxide 5. Ammonium sulfate 1. NaOH 2. KHCO 3 3. K 2 CO 3 4. Mg(OH) 2 5. (NH 4 ) 2 SO 4 5

6 Writing Names for Compounds Containing Polyatomic Ions Writing names for compounds containing polyatomic ions is as straight forward as the process was for naming binary ionic compounds. 1. Begin by identifying the anion. 2. Then identify the cation. 3. Finally write the compound name as: cation first and anion second. (NH 4 ) 3 PO 4 1. Locate the PO 4 symbol in the polyatomic ion table. PO 4 3- is phosphate 2. Locate the NH4+ symbol in the polyatomic ion table. NH 4+ is ammonium 3. Write the names in this order: cation first, anion second: ammonium phosphate Important Note: The reason you locate the anion symbol and name first is to assist in identifying the charge on the cation should it be multivalent. 6

7 Cr(NO 3 ) 3 1. Locate the NO 3- symbol in the polyatomic ion table. NO 3- is nitrate 2. Locate Cr in the multivalent ion table. Note that it is multivalent (more than one possible charge). Since there are three nitrate ions, each with a 1- charge, the total negative charge in the formula is 3-. This means that the chromium ion must have a 3+ charge. Cr 3+ is chromium (III) 3. Combine the names in this order: cation first, anion second: chromium (III) nitrate Give the name for each of the following: 1. Zn(OH) 2 2. Pb(NO 3 ) 2 3. Mg(CH 3 COO) 2 4. Na 3 BO 3 5. K 2 Cr 2 O 7 1. Zinc Hydroxide 2. Lead (II) Nitrate 3. Magnesium Acetate 4. Sodium Borate 5. Potassium Dichromate 7