3 Writing Formulas and Naming Compounds

Transcription

1 3 Writing Formulas and Naming Compounds 6(D) Before You Read Shakespeare asked, What s in a name? In this section, you are going to learn what the names of chemical compounds can tell you. On the lines below, explain what your name tells about you. What You ll Learn how to determine oxidation numbers how to write formulas and names for ionic compounds how to write formulas and names for covalent compounds Read to Learn Binary Ionic Compounds Alchemists were early scientists who tried to turn lead into gold. They never succeeded, but they did develop some laboratory methods and equipment that scientists still use today. The alchemists also used symbols to write formulas. The first formulas you will learn to write are for binary ionic compounds. A binary compound is a compound made of two elements. An example of a binary compound is potassium iodide, a compound added to table salt. What are oxidation numbers? Before you can write the correct formula of a compound, you need to know which elements combine to make that compound. All elements in a certain group of the periodic table have the same number of electrons in their outer energy levels. So, elements in the same group all gain or lose the same number of electrons. Metals always lose electrons, and nonmetals always gain electrons when they form ions. The oxidation number of an atom tells you how many electrons an atom gains, loses, or shares to become stable. It is the same as the charge on the ion. The charge on the ion in an ionic compound is the same as its oxidation number. For example, a sodium ion has a charge of 1+ and an oxidation number of 1+. A chloride ion has a charge of 1 and an oxidation number of 1. Focus As you read the text under each heading, write a question that your teacher might ask on a quiz. Exchange your questions with a partner and take each other s quizzes. 329

2 1+ Hydrogen 1 H Lithium 3 Li Sodium 11 Na Potassium 19 K Rubidium 37 Rb Beryllium 4 Be Magnesium 12 Mg Calcium 20 Ca Strontium 38 Sr Boron 5 B Aluminum 13 Al Gallium 31 Ga Indium 49 In Carbon 6 C Silicon 14 Si Germanium 32 Ge Tin 50 Sn Nitrogen 7 N Phosphorus 15 P Arsenic 33 As Antimony 51 Sb Oxygen 8 O Sulfur 16 S Selenium 34 Se Tellurium 52 Te Fluorine 9 F Chlorine 17 Cl Bromine 35 Br Iodine 53 I 0 Helium 2 He Neon 10 Ne Argon 18 Ar Krypton 36 Kr Xenon 54 Xe Cesium 55 Cs Barium 56 Ba Thallium 81 Tl Lead 82 Pb Bismuth 83 Bi Polonium 84 Po Astatine 85 At Radon 86 Rn Francium 87 Fr Radium 88 Ra Take a Look 1. Identify Highlight the oxidation numbers on the periodic table. Take a Look 2. Compare Circle the Roman numerals and the oxidation numbers for each element in the table. How are oxidation numbers related to the periodic table? Look at the periodic table above. Notice the numbers above each column. These are the oxidation numbers for the elements in the column. Notice how the oxidation numbers fit with the periodic table groupings. How do transition elements form ions? Part of the periodic table is not included in the figure above. Some elements in this section can have more than one oxidation number. The table shows some of these elements and their oxidation numbers. Because these elements can have more than one oxidation number, you must include more information when you name them. When naming these compounds, the oxidation number is expressed in the name as a roman numeral. For example, when iron with an oxidation number of 3+ combines with oxygen, the compound they form is iron(iii) oxide. Name Some Special Ions Oxidation Number Name Oxidation Number Copper(I) 1+ Chromium(II) 2+ Copper(II) 2+ Chromium(III) 3+ Iron(II) 2+ Lead(II) 2+ Iron(III) 3+ Lead(IV)

3 How many positive and negative ions must a formula have? Remember that ionic compounds are neutral. The ions in an ionic compound have charges, but the compound itself does not. The formula for an ionic compound must have the right number of positive and negative ions to make the charges balance. For example, sodium chloride has one sodium ion with a charge of 1+ and one chloride ion with a charge of 1. The 1+ balances the 1. The correct formula for sodium chloride is NaCl. The formula tells you that one positively charged ion combines with one negatively charged ion and makes a neutral compound. What about the ionic compound calcium fluoride? A calcium ion has a charge of 2+. A fluoride ion has a charge of 1-. The charge 1- does not balance 2+. You need to have two fluoride ions for every calcium ion to balance the charges. The formula for the neutral compound calcium fluoride, CaF 2, shows exactly that. There is one calcium ion and two fluoride ions in the compound. How do you find the correct subscripts? Sometimes you need to use math skills to write a formula correctly. In the compound aluminum oxide, there are aluminum ions and oxygen ions. But, how many of each one? From the periodic table, you can get the information about aluminum and oxygen that you need to write the formula. By its place on the periodic table, you can tell that aluminum has an oxidation number of 3+. That means the aluminum ion has a charge of 3+. Oxygen has an oxidation number of 2, so its ion has a charge of 2. You must find the least common multiple of 3 and 2 to balance the charges. The least common multiple of 3 and 2 is 6. Multiply 3+ by 2 to equal 6+. You need two aluminum ions to have a charge of 6+. Multiply 2 by 3 to equal 6. You need three oxygen ions to have a charge of 6-. The charges balance. The correct formula for the neutral compound aluminum oxide is Al 2 O 3. How do you write the formula of an ionic compound? You learned how to use the oxidation number to find the charge on an ion. You also learned that the charges on the ions must balance to form a neutral compound. Now you can write formulas for ionic compounds. Follow rules 1, 2, and 3 on the next page. 3. Infer What does the formula CaF 2 tell you about the compound? Apply Math 4. Calculate What is the least common multiple of 5 and 2? 331

4 5. Restate What will a compound be when its positive and negative charges balance? 6. Apply Write the formula for calcium chloride. Use the periodic table to identify the positive and negative ions. Writing Formulas You have learned how to find oxidation numbers and their least common multiples. Now you can write formulas for ionic compounds. What is the formula for an ionic compound containing sodium and oxygen? Use these rules to figure it out: 1. Write the symbol of the element that has the positive oxidation number or charge. Sodium is a goup 1 element. It has an oxidation number of Write the symbol of the element with the negative oxidation number or charge. Except for hydrogen, all nonmetals have negative oxidation numbers. Oxygen has an oxidation number of The compound should be neutral. To make it neutral, the positive charges have to balance the negative charges. It takes two sodium ions to balance one oxygen ion. Thus, the formula becomes Na 2 O. Now use these rules to write the formula for lithium nitride. Lithium and nitrogen are the two atoms that make this compound. Look at the periodic table in the back of this book. Lithium is in Group 1, so it forms ions with a 1+ charge. Write the symbol for lithium, Li, first. Find the oxidation number of nitrogen. Nitrogen is in group 15. It forms ions with a charge of 3. You now can write LiN. Can you stop now? Look at the charges of the two ions. Do 1+ and 3 balance? No, you cannot stop yet. Use the number of the charge of a nitrogen ion as the subscript for Li. Use the number of the charge of a lithium ion as the subscript for N. That gives LiN. When an element has no subscript, it means only one ion is in the compound. Do 3(1+) and 3 balance? Yes, this is the correct formula. How do you name a binary ionic compound? When you know the formula, you can write the name of a binary ionic compound by following these rules: 1. Write the name of the positive ion. 2. Look to see if the positive ion is listed in the Special Ions table. If it is not in the table, go right to Step 3. If it is, the ion can have more than one oxidation number. To find the correct oxidation number, look at the formula of the compound. The charge of the compound is always zero. The negative ion can only have one possible charge. From the table, pick the charge of the positive ion that balances the negative charge. Write that positive ion s symbol along with the correct roman numeral in parentheses. 332

5 3. Write the root name of the negative ion. The root is the first part of the element s name. For example, the root name of chlorine is chlor-. The root name of oxygen is ox-. 4. Add the ending -ide to the root name. For example, oxygen becomes oxide. Do not use subscripts in the name of an ionic compound. Subscripts can be used in the formula to figure out the charge on a metal ion with more than one positive charge. 7. Determine What is the root name of oxygen? Elements in Binary Compounds Element Oxygen Phosphorus Nitrogen Sulfur -ide Name Oxide Phosphide Nitride Sulfide How do you use these rules? Use these rules to write the name of the compound CuCl. Find the name of the positive ion on the periodic table. Cu is the symbol for copper. Is copper in the Special Ions table? Yes, and the copper ion can have a 1+ or a 2+ charge. To find the charge on the copper in CuCl, look at the negative ion. Cl is the symbol for chlorine. Chlorine is in group 17 of the periodic table. That means it has an oxidation number of 1. From the formula, you can see that there is only one chloride ion in the compound. To balance a 1 charge on the chloride ion, the copper ion must have a 1+ charge. Now you can write copper(i) as the first part of the name. Write the root name of the negative ion. The root name for chlorine is chlor-. Add -ide to the root. That gives you chloride. The correct name of CuCl is copper(i) chloride. Compounds with Polyatomic Ions Not all ionic compounds are binary compounds. The formula for baking soda used in cooking is NaHCO 3. Baking soda is an ionic compound that is not binary. Compounds like baking soda are made of more than two elements. These compounds have polyatomic ions. A polyatomic ion is a charged group of atoms that are bonded together by a covalent bond. The prefix polymeans many and polyatomic means many atoms. The polyatomic ion in baking soda is the bicarbonate or hydrogen carbonate ion. The symbol of this polyatomic ion is HCO Apply Write the name of the compound FeI

6 Take a Look 9. Explain Use the table to determine what elements make up acetate. Apply Math 10. Determine How many 2+ ions does it take to balance the charge on one 4 ion? Polyatomic Ions Charge Name Formula 1+ ammonium NH acetate chlorate hydroxide nitrate 2- carbonate sulfate C 2 H 3 O 2 - CIO 3 - OH - NO 3 - CO 3 2- SO phospate PO 4 3- How do you name a polyatomic compound? To write the name of a compound with a polyatomic ion, first write the name of the positive ion. If the positive ion is polyatomic, use the table above to find its name. Next, write the name of the negative ion. Again, if it is polyatomic, look up its name in the table. What is the name of the compound K 2 SO 4? K is the symbol of the positive ion. It is not polyatomic. K is the symbol for potassium. The negative ion is SO 4 2. It is polyatomic. Use the table again. K 2 SO 4 is potassium sulfate. Now try naming the compound Sr(OH) 2. You can see that the positive ion is not polyatomic. Find the name for the symbol Sr on the periodic table. It is strontium. The negative ion is polyatomic. From the table of polyatomic ions, you can see that OH is named the hydroxide ion. The name of Sr(OH) 2 is strontium hydroxide. How do you write the formula of a polyatomic compound? To write the formula for a compound with a polyatomic ion, use the rules for writing the formula of a binary compound, but add one more thing. If you need to show more than one polyatomic ion, put parentheses around the formula for the ion before you write the subscript. Write the formula for barium chlorate. First, write the symbol of the positive ion. The symbol for barium is Ba. Barium is in group 2, so it forms a 2+ ion. 334

7 Balancing Negative Charges Now write the formula for the negative ion. The Polyatomic Ions table shows you the formula for the chlorate ion is ClO 3. Are the charges on these two ions balanced? No, 2+ does not balance 1. It takes two chlorate ions to balance the 2+ charge on the barium ion. The formula for barium chlorate is Ba(ClO 3 ) 2. Compounds with Added Water Some ionic compounds have water molecules as part of their structure. A hydrate is a compound that has water chemically attached to its ions and written into its formula. The word hydrate comes from a word that means water. What are common hydrates? When a solution of cobalt chloride evaporates, pink crystals form. The crystals have six water molecules for each unit of cobalt chloride. The formula for this compound is CoCl 2 6H 2 O. The compound s name is cobalt chloride hexahydrate. The prefix hexa- means six, so hexahydrate means six waters. You can remove water from these crystals by heating them. The blue compound that results from heating contains no water. It is called anhydrous, meaning without water. When anhydrous CoCl 2 is exposed to water, even water in the air, it absorbs water and becomes pink again. Plaster of paris becomes a hydrate when water is added. It becomes calcium sulfate dihydrate, which is also known as gypsum. The water added to the powder becomes part of the compound. Formulas for Hydrates To write a formula for a hydrate, first write the formula for the compound. Then make a dot followed by the number of water molecules. The dot stands for the ratio of compound-to-water molecules. For example, the formula for calcium sulfate dihydrate is CaSO 4 2H 2 O, a compound of calcium sulfate that contains two water molecules. Naming Binary Covalent Compounds Covalent compounds form between elements that are nonmetals. Some nonmetals can form many compounds. For example, nitrogen and oxygen can form N 2 O, NO, NO 2, and N 2 O 5. Using the rules you learned earlier, all these compounds would be called nitrogen oxide. Now you will learn how to give each of these compounds a different name. 11. Analyze How many water molecules does the hydrate LiNO 2 H 2 O have? 12. Identify What is the element N? What is the compound NO? 335

8 Take a Look 13. Use Numbers What does the prefix tetra- mean? How are prefixes used to name covalent compounds? The table below lists some Greek prefixes used to name covalent compounds made with the same elements. These prefixes tell how many atoms of each element are in a compound. For example, the compound NO 2 is nitrogen dioxide. The prefix di- tells you that there are two oxygen atoms in the compound. N 2 O is dinitrogen oxide. The compound has two nitrogen atoms. The name of the compound N 2 O 5 is dinitrogen pentoxide. This name uses two prefixes. There are two nitrogen atoms, so dinitrogen is used. There are also five oxygen atoms, so pentoxide is used. What rules apply in naming covalent compounds? Drop the last vowel of the prefix when the second element of the compound begins with a vowel. In pentoxide, the a is dropped from penta-. There is a prefix to use when a compound has only one atom of an element. The prefix is mono-. Many times mono- is not used. Instead, it is understood that if no prefix is used, there is only one atom of that element in a compound. In some cases, mono- is used for emphasis. Carbon monoxide is one example. Prefixes for Covalent Compounds Number of Atoms Prefix 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- 336

9 After You Read Mini Glossary binary compound: a compound made of two elements hydrate: a compound that has water chemically attached to its ions and written into its formula oxidation number: the number that tells how many electrons an atom gains, loses, or shares to become stable polyatomic ion: a charged group of atoms that are bonded together by a covalent bond Review 1. Study the terms and definitions in the Mini Glossary. Write a sentence that explains in your own words what a polyatomic ion is. 2. Complete the flow chart with the steps used in writing the name of a binary ionic compound. To write the name of a binary ionic compound