Writing Formulas and Naming What You’ll Learn Compounds ...

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

Copyright ? McGraw-Hill Education. Permission is granted to reproduce for classroom use.

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.

Reading Essentials ? Chemical Bonds 329

1+

Hydrogen

1

2+

H

Lithium Beryllium

3

4

Li

Be

Sodium Magnesium

11

12

Na

Mg

Potassium Calcium

19

20

K

Ca

Rubidium Strontium

37

38

Rb

Sr

Cesium

55 Cs

Barium

56 Ba

Francium Radium

87

88

Fr

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.

0

3+ 4+ 3- 2- 1-

Boron

5 B

Carbon

6 C

Nitrogen

7 N

Oxygen

8 O

Fluorine

9 F

Aluminum Silicon Phosphorus Sulfur

13

14

15

16

Al

Si

P

S

Chlorine

17 Cl

Gallium Germanium Arsenic

31

32

33

Ga

Ge

As

Selenium Bromine

34

35

Se

Br

Indium

49 In

Tin Antimony Tellurium Iodine

50

51

52

53

Sn

Sb

Te

I

Thallium Lead Bismuth Polonium Astatine

81

82

83

84

85

Tl

Pb

Bi

Po

At

Helium 2 He

Neon 10 Ne

Argon 18 Ar

Krypton 36 Kr

Xenon 54 Xe

Radon 86 Rn

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

Copper(I) Copper(II)

Iron(II) Iron(III)

Some Special Ions

Oxidation Number

Name

1+

Chromium(II)

2+

Chromium(III)

2+

Lead(II)

3+

Lead(IV)

Oxidation Number

2+ 3+ 2+ 4+

Reading Essentials ? Chemical Bonds 330

Copyright ? McGraw-Hill Education. Permission is granted to reproduce for classroom use.

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, CaF2, 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 Al2O3.

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.

Think it Over

3. Infer What does the formula CaF2 tell you about the compound?

Apply Math

4. Calculate What is the least common multiple of 5 and 2?

Copyright ? McGraw-Hill Education. Permission is granted to reproduce for classroom use.

Reading Essentials ? Chemical Bonds 331

Think it Over

5. Restate What will a compound be when its positive and negative charges balance?

Think it Over

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 1+.

2. 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 2-.

3. 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 Na2O.

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.

Copyright ? McGraw-Hill Education. Permission is granted to reproduce for classroom use.

Reading Essentials ? Chemical Bonds 332

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.

Think it Over

7. Determine What is the root name of oxygen?

Elements in Binary Compounds

Element

-ide Name

Oxygen

Oxide

Phosphorus Phosphide

Nitrogen

Nitride

Sulfur

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.

Copyright ? McGraw-Hill Education. Permission is granted to reproduce for classroom use.

Compounds with Polyatomic Ions

Not all ionic compounds are binary compounds. The formula for baking soda used in cooking is NaHCO3. 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 HCO3-.

Think it Over

8. Apply Write the name of the compound FeI2.

Reading Essentials ? Chemical Bonds 333

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