NAME____________________________________ PER____________ DATE DUE____________

ACTIVE LEARNING I N C HEMISTRY E DUCATION "ALICE"

CHAPTER 5 ATOMS AND MOLECULES Atomic Theory Naming Compounds Writing Formulas 5-1

©1997, A.J. Girondi

NOTICE OF RIGHTS All rights reserved. No part of this document may be reproduced or transmitted in any form by any means, electronic, mechanical, photocopying, or otherwise, without the prior written permission of the author. Copies of this document may be made free of charge for use in public or nonprofit private educational institutions provided that permission is obtained from the author . Please indicate the name and address of the institution where use is anticipated. © 1997 A.J. Girondi, Ph.D. 505 Latshmere Drive Harrisburg, PA 17109 [email protected] Website: www.geocities.com/Athens/Oracle/2041

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SECTION 5.1

The Classification Of Matter

In order to work effectively with chemical concepts, it is important for you to learn the "language of chemistry" and how certain words and symbols are used by chemists. In this chapter you will learn about the meaning of terms such as element, compound, mixture, atom, molecule, ion, and polyatomic ion (or radical). You will also learn to identify chemical elements by chemical symbols. Finally, you will learn how to write chemical formulas and how to name chemical compounds. An idea that can be attributed to the ancient Greeks is the concept of the atom. They developed the idea that all of the material substance in the world was composed of fundamental building blocks that could not be divided into smaller parts. The property of being indivisible into smaller parts led them to coin the word atom, which means "indivisible." Today, we realize that atoms can be broken up into smaller pieces, but the name atom is still used to describe the fundamental unit of matter for chemists. Atoms have the special ability to combine together to form larger groups of atoms called molecules. Two types of molecules are possible - elements and compounds. When a molecule is composed of only one kind of atom, it is referred to as an element. For example, the element gold is composed of millions of gold atoms connected together. A sample of the metal copper is made of copper atoms linked together. Atoms of some elements tend to combine in pairs. These paired elements are called diatomic molecules. Diatomic means "two atoms" in a molecule. Elements that exist as diatomic molecules include hydrogen and oxygen. In the pure form they are written as H2 and O2. You will learn about other diatomic gases in chapter 6. The idea that substances in nature are composed of certain basic or fundamental elements that cannot be further reduced to simpler substances is an ancient one dating back to early Greece. The Greek elements were considered to be earth, air, fire, and water. Today, we realize that nature is much more complex than that. Of the first 92 elements, 90 are naturally-occurring ones of which all other matter is composed. Technitium (#43) amd promethium (#61) are the exceptions. We have also been able to produce at least an additional 19 elements in the laboratory. These are called human-made elements. They are more complicated than the basic ninety-two and are very unstable, which means they decompose readily into simpler elements. A listing of all of the elements is undoubtedly present somewhere in your chemistry classroom. This list is called the periodic table of elements. Each element on the table is represented by a symbol. In instances where molecules are composed of more than one kind of atom, this cluster of atoms is referred to as a compound. Examples of compounds include water (H2O), carbon dioxide (CO2), and table salt (NaCl). Although these substances consist of more than one kind of atom, they are considered to be pure substances. Elements are also considered to be pure substances. A mixture is yet another way to describe combinations of elements and compounds. A mixture is composed of materials that have been placed together but which are not chemically combined. A mixture may be composed of elements or compounds. The important thing to remember is that the substances making up a mixture can always be separated by physical methods which means without using a chemical reaction. Sand and salt can be made into a mixture by simply stirring these two substances together. The sand and salt mixture can then be separated by adding water and filtering. The salt will dissolve in the water and pass through the filter paper, while sand would not be able to pass through the filter paper. Dissolving is a physical change and filtering is a physical method of separation. The next activity will help to illustrate the difference between a mixture and a compound. Homogeneous materials are those which have the same composition throughout. Heterogeneous materials do not have the same composition throughout. Solutions such as salt water are homogeneous mixtures since they have the same composition throughout and can be separated by physical methods (such as boiling the water away). The mineral called marble is an example of a heterogeneous mixture since its composition can vary, even within one small sample as well as from one location to another on earth.

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Matter

Mixtures

Homogeneous

Pure Substances

Heterogeneous

Elements

Compounds

Solutions Figure 5.1 The Classification of Matter

ACTIVITY 5.2

Properties Of Iron In A Compound And In A Mixture

Obtain the materials labeled 5.2 from the materials shelf. Container A contains iron filings and powdered sulfur mixed together. Container B contains a chemical compound containing iron, sulfur, and oxygen (FeSO4). In the compound, iron is chemically bonded to the other elements. 1. Hold a magnet up to each container and move it around. One of the properties of elemental iron is that it is attracted to a magnet. 2. Is the iron in container A attracted to the magnet?__________ Is the iron in container B attracted to the magnet?__________ Based on your observations, do elements retain their original properties when they

form compounds with other elements?

conclusion?

{1}__________

How does this activity support your

{2}________________________________________________________________

______________________________________________________________________________

SECTION 5.3

Symbols Of The Elements

Each chemical element has a name assigned to it for the purpose of identifying it. The names for the elements have been developed as each element has been discovered, and there are many interesting and colorful names. In order to avoid having to write a long name in describing an element, it has become customary in chemistry to use a chemical symbol in place of the name. Most of the symbols are derived from the names of the elements themselves. Quite often, the symbol consists of two letters that are the first two letters in the name of the element. As an example, the element calcium is denoted by the symbol Ca. Another example is argon, which is denoted by the symbol Ar. However, there are exceptions to this practice. The element arsenic is denoted by the symbol As so that it is not confused with argon. Another exception in chemical symbols is the fact that some elements are denoted by only one letter. Thus, hydrogen is represented by H, oxygen by O, fluorine by F, and so on. In addition, the symbols for some of the elements are not related to their more modern names but have come from older names for those elements that are no longer used. 5-4

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An example is sodium which has the symbol Na. This symbol is derived from the older name "natrium" that was originally given to sodium. Other examples of this are potassium, K; mercury, Hg; iron, Fe; tungsten, W; etc. Table 5.1 contains the names and symbols for 38 of the more common elements. You are expected to MEMORIZE the names of these elements and their symbols. (Spelling counts!) Many students misspell the name of the element which has the symbol F. Write its name - spelled properly - in this space: _____________________ spelled flourine!

Remember, the symbol of this element is F - not Fl! And, it is not

You will notice that the first letter in the symbol of any element is a capital letter, while the second letter (if there is one) is in lowercase. You need to use care in writing the symbols. As an example, the element cobalt has the symbol Co. If you were to write CO as the symbol for cobalt, you would actually be writing the formula for a compound, carbon monoxide, which is very different from cobalt. Table 5.1

Selected Elements And Their Symbols

Aluminum Antimony Arsenic Barium Bismuth Bromine Calcium Carbon Cesium Chlorine Chromium Cobalt Copper Fluorine Gold Hydrogen Iodine Iron Lead

SECTION 5.4

Al Sb As Ba Bi Br Ca C Cs Cl Cr Co Cu F Au H I Fe Pb

Lithium Magnesium Manganese Mercury Nickel Nitrogen Oxygen Phosphorus Platinum Potassium Silicon Silver Sodium Strontium Sulfur Tin Titanium Tungsten Zinc

Li Mg Mn Hg Ni N O P Pt K Si Ag Na Sr S Sn Ti W Zn

Atomic Theory

Although the Greeks invented the idea of the atom about 2,500 years ago, their concept was not based on experimental evidence gathered in a laboratory. Our modern atomic theory is the result of the work of several European scientists dating back to the 1600's. The most notable among these scientists is John Dalton, an Englishman. Isaac Newton and Robert Boyle, two other Englishmen, had suggested the possibility of atoms through their work, but Dalton put the idea of the existence of atoms on a firm experimental basis. Dalton was able to show that when elements combined to form compounds, the masses of each element that went into making a compound always were present in definite ratios to each other. Thus, when he combined hydrogen with oxygen to form water, he always found that the ratio of the mass of 5-5

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hydrogen to that of oxygen was 1:8 (1 gram of H to 8 grams of O). This convinced him that the only way for 1 gram of hydrogen

+

8 grams of oxygen

9 grams of water

this to make sense was if each element consisted of basic units, or atoms, that had definite masses. According to his reasoning, if basic units of matter did not exist, then1 gram of hydrogen should be able to combine with any number of grams of oxygen. John Dalton assumed that water consisted of one hydrogen atom and one oxygen atom. Furthermore, he reasoned that if the hydrogen atom had a mass of one mass unit, then the oxygen atom must have a mass of eight mass units.

H + O

HO

Really ? ? ?

(He was incorrect, of course, in his assumption, because we now know that two atoms of hydrogen combine with one atom of oxygen to form a single molecule of water.) Water is H2O, not HO. Now if the mass ratio is 1 to 8 ( H to O) in water, and if the atom ratio is 2 to 1 in water , what this means is that the oxygen atom must have a mass which is 16 times greater than that of the hydrogen atom – not 8 times as great (as Dalton had assumed). Amedeo Avogadro (whom you will learn about later) recognized the problem in Dalton's assumptions through his work with the gas laws and was able to show that certain elements, including hydrogen and oxygen, actually existed in molecular (diatomic) form (H2 and O 2). When water is formed, two molecules of hydrogen (which is four atoms of hydrogen) react with one molecule of oxygen (which is two atoms of oxygen) to form two molecules of water:

2 H2 + O2

-----> 2 H2O

Nevertheless, Dalton's work represented the pioneering effort to experimentally establish the existence of atoms. Dalton was also aware that two elements can produce two completely different compounds. For example, carbon can combine with oxygen to form either CO (carbon monoxide) or CO2 (carbon dioxide). He found that in CO, 16 parts by weight of oxygen always combined with 12 parts by weight of carbon. In CO2, he found that 32 parts by weight of oxygen always combined with 12 parts by weight of carbon. So, if he started with two equal weights of carbon and reacted them both with different quantities of oxygen he always ended up with either a 12:16 ratio or a 12:32 ratio:

CO

If you take enough of each compound such that the mass of one element in them is the same

CO2

12 grams carbon

12 grams carbon

16 grams oxygen

32 grams oxygen then the mass of the other element present in the two compounds will be in a ratio of small whole numbers (1:2 in this case)

Notice in the example above that for oxygen, 16 to 32 is a 1 to 2 ratio. This seemed to indicate that elements could not combine in just any quantities. 12 grams of carbon have to combine with 16 grams of oxygen or with 32 grams of oxygen - nothing in between! For example, he never found 12 parts by weight of carbon combining with 24 parts by weight of oxygen. Wonder why? Maybe, he reasoned, it's because elements don't come in just any quantity. Maybe they come in definite discrete amounts or particles. This would explain the simple whole number ratio for oxygen (1:2 in this case). He called this regularity in the way two elements can combine his law of multiple proportions. This law together with the law of definite composition served as strong evidence suggesting the existence of atoms. 5-6 ©1997, A.J. Girondi

This can all sounds somewhat confusing, so here is an analogy. Suppose you go into an ice cream store and order a dish of chocolate and vanilla. When it comes you measure the mass of the ice cream to make sure you are not being cheated (you just happen to have a balance with you). You find that the dish contains 100 grams of vanilla and 100 grams of chocolate. Since you like chocolate so much, you send the dish back to the kitchen for more chocolate. This time when you get it back it contains 100 grams of vanilla and 200 grams of chocolate. Hmmm. That's interesting. Exactly a 2:1 ratio between the original amount of chocolate and the new amount. You repeat the order once more, and when you get the dish back it contains 100 grams of vanilla (starting to melt) and 300 grams of chocolate. Hey. Now the chocolate has varied in a 1:2:3 ratio - a ratio of small whole numbers. Whole numbers, mind you! Why? This leads you to believe that maybe the ice cream comes in discrete amounts - not just any amount. After all, the chocolate did not increase by fractional amounts. You ask the waiter if this might be true, and sure enough, the ice cream does come in discrete amounts! They are called scoops. Ah! The scoop theory of ice cream. Brilliant! See the analogy to Dalton's reasoning? Ice cream comes in scoops, while elements come in the form of atoms! Law of Multiple Proportions: Sometimes the same two elements can combine in different proportions to form different compounds. (Example: CO and CO2) When they do this, if you hold the mass of one element in the compounds constant, the mass of the other element present will vary in a ratio of small whole numbers. Dalton's atomic theory, first conceived in 1803, can be summarized by the following four statements: 1. An element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical to all other atoms of that element, but differ from atoms of other elements. 3. Atoms are indivisible and cannot be created or destroyed or changed into atoms of another element. 4. Chemical changes take place when atoms of elements combine with each other in new ways. Dalton's original theory was not entirely correct. For example, he thought that all atoms of a given element had exactly the same mass. Today, we know that atoms of an element do not all have the same mass. We have discovered the existence of isotopes, which are atoms of an element which have different masses. As you have discovered, elements are represented by their chemical symbols. These symbols are also used to describe chemical compounds that are formed when the elements react with each other. The use of these symbols to describe chemical compounds results in a chemical formula for the compound. This formula contains the appropriate symbols and, in addition, also has small numbers written as subscripts (below the element) that indicate how many of each kind of atom are present in a molecule of the compound. The chemical formula for water is H2O. This formula means that in the water molecule, there are two hydrogen atoms and one oxygen atom. (When only one atom is involved, it is customary not to write a subscript "1" below the element.) In other words, when there is no number, it is understood that there is only one atom of that type present. Some compounds contain only two elements and these are called binary compounds. Other compounds contain more than two elements. As an example, the elements copper (Cu), sulfur (S), and oxygen (O) combine to form a compound with the chemical formula CuSO 4. In this compound for every one atom of copper, there is one atom of sulfur and four atoms of oxygen. Problem

1. Using the same idea, how many atoms of calcium are there in calcium chromate, CaCrO4?

Calcium?__________ Chromium?__________Oxygen?__________. Problem

2. The formula for sodium acetate is NaC2H3O2. How many atoms of each element are present

in one sodium acetate molecule?

Sodium?_________ Carbon?__________ Hydrogen?__________

Oxygen?__________. 5-7

©1997, A.J. Girondi

SECTION 5.5

Oxidation Numbers Of Elements and Polyatomic Ions

We are now at the point where you are ready to learn how to write the formulas for chemical compounds. In order to accomplish this task, we will be using what are known as oxidation numbers. You will not understand where these oxidation numbers come from until you study atomic structure in a later chapter, but you will be able to use them, nonetheless. A list of some common oxidation numbers for selected elements can be found in Table 5.2 below. There are other oxidation numbers for these elements besides those listed here, but this list will suit your purposes for now.

Table 5.2

Common Oxidation Numbers of Selected Elements

(Note: Sometimes these elements can assume oxidation numbers other than those listed.)

Aluminum Antimony Arsenic Barium Bismuth Boron Bromine Calcium Carbon Cesium Chlorine Chromium Cobalt Copper Fluorine Gold Hydrogen Iodine Iron Lead

Al Sb As Ba Bi B Br Ca C Cs Cl Cr Co Cu F Au H I Fe Pb

+3 +3,+5 +3,+5 +2 +3 +3 -1,+5 +2 +2,+4 +1 -1,+5,+7 +2,+3,+6 +2,+3 +1,+2 -1 +1,+3 +1 -1,+5 +2,+3 +2, +4

Lithium Magnesium Manganese Mercury Nickel Nitrogen Oxygen Phosphorus Platinum Potassium Silicon Silver Sodium Strontium Sulfur Tin Titanium Tungsten Zinc

Li Mg Mn Hg Ni N O P Pt K Si Ag Na Sr S Sn Ti W Zn

+1 +2 +2,+4,+7 +1,+2 +2 -3,+3,+5 -2 +3,+5 +2,+4 +1 +4 +1 +1 +2 -2,+4,+6 +2,+4 +3,+4 +6 +2

(A copy of Table 5.2 (oxidation numbers) which you will be permitted to use during tests and quizzes can be found in the Reference Notebook which you were given as a part of ALICE.) You do NOT need to memorize Table 5.2. Common oxidation numbers for other elements can be found on some periodic tables and in other reference sources. Remember, you do not need to memorize oxidation numbers. Table 5.2 contains the most commonly used oxidation numbers of the elements listed. It is possible that sometimes an element will exhibit an oxidation number which is not listed in the table. There is yet another kind of fundamental unit present in some substances called an ion. An ion is a particle that carries an electric charge. In certain specific events that happen in chemistry, atoms or molecules can end up with a positive (+) or negative (-) charge. These atoms or molecules then become ions. Another name for ions which contain more than one atom is a polyatomic ion (they used to be called radicals). Examples of ions include H1+, OH 1-, NH41+, and SO42-. Ions like SO42- can also be written as SO 4-2. It means the same thing. The ion has a charge of minus two. There are oxidation numbers for polyatomic ions (see table 5.3). You ARE required to commit this list (Table 5.3) to MEMORY including the names, the formulas, and the charges!

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A table of polyatomic ions (like Table 5.3) can also be found in your ALICE Reference Notebook. You will be given a quiz to insure that you have learned the names, formulas, and charges of the polyatomic ions. After that quiz, you will be allowed to refer to the list of polyatomic ions in your reference notebook during future tests and quizzes. The reason for having you memorize them is to help you to recognize them as polyatomic ions when you see them. When you write polyatomic ions, you should write the charge with the formula. For example, sulfate should be written as SO42-, while permanganate should be written as MnO41-, etc. This list is not complete. There are many other polyatomic ions besides those listed in Table 5.3.

Table 5.3 Common Oxidation Numbers of Selected Polyatomic Ions Name

Formula

Charge

Ammonium Acetate Chlorate Perchlorate Cyanide Hydrogen carbonate

NH41+ C2H3O21ClO31ClO41CN1HCO31-

+1 -1 -1 -1 -1 -1

HSO 41OH1NO31NO21MnO 41SCN1CO32CrO4 2Cr2O72SO 42SO 32PO 43-

-1 -1 -1 -1 -1 -1 -2 -2 -2 -2 -2 -3

(or bicarbonate)

Hydrogen sulfate Hydroxide Nitrate Nitrite Permanganate Thiocyanate Carbonate Chromate Dichromate Sulfate Sulfite Phosphate

SECTION 5.6

Writing Formulas for Chemical Compounds

The process of writing a chemical formula using oxidation numbers is really rather simple. The one rule that you must remember is that "the sum of the oxidation numbers of the atoms in the formula of a compound must be zero." For example, hydrogen's oxidation number is +1 and oxygen's is -2. Therefore, in order for the oxidation numbers to add up to zero, we need two hydrogens. Two hydrogens = +2 and one oxygen = -2, so the formula for water is H2O. The subscript "2" to the right of the H indicates the presence of two hydrogen atoms. When a symbol is present without a subscript to its right, we assume that a subscript of "1" is there. We don't actually write the subscript if it is a one. Notice, too, that the element with the positive oxidation number is usually written first. Let's try more. 5-9

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Example 1: What is the formula for a compound of calcium and chlorine? Ca = +2 and Cl = -1. Therefore, in order for the oxidation numbers to add up to zero, these two elements must combine in a one to two ratio: CaCl2 Example 2: What is the formula for a compound of aluminum and oxygen? Al = +3 and O = -2. Therefore, in order for the oxidation numbers to add up to zero, these two elements must combine in a two to three ratio: Al2O3 One method used to write formulas involves the use of a lowest common multiple (LCM). In example 1 above, the lowest common multiple (disregard the signs) of the two oxidation numbers (+2 and -1) is 2. Now divide each oxidation number into the lowest common multiple (LCM) to determine the subscript for that element in the formula. For Ca: 2/2 = 1; and for chlorine: 2/1 = 2. Therefore, the formula is CaCl2. In the case of example 2 above, the LCM of the oxidation numbers involved (+3 and -2) is 6. For aluminum: 6/3 = 2; and for oxygen: 6/2 = 3. So, the formula for the compound is Al2O3. Notice how the sum adds up to zero! [2 Al = +3 X 2 = +6; 3 O = -2 X 3 = -6]. Then, (+6) + (-6) = 0. Practice now by doing problem 3.

Problem 3. Using oxidation numbers from Tables 5.2 and 5.3, write correct formulas for compounds of the following substances. Keep in mind that if one element has only a positive oxidation number, you must use a negative oxidation number for the other element. Some elements have more than one oxidation number. So, when you see a symbol followed by a Roman numeral in parentheses in the problems below, the Roman number equals the oxidation number which you should use for that element. Iron has two oxidation numbers: +2 and +3. Fe(II) refers to Fe2+. Example: Manganese can be +2, +4, or +7. Mn(IV) = Mn4+. See? The oxidation number is the same as the Roman numeral. Note, however, that the Roman numeral is not used in the formula. For example, when Mn(IV) and Cl combine, the compound's correct formula is MnCl 4. It is incorrect to include the Roman numeral in the name. Therefore, Mn(IV)Cl4 is WRONG!

a. Ba and Cl

_______________

i. Cu(I) and S

_______________

b. Hg(I) and Br

_______________

j. As(V) and O

_______________

c. Ca and O

_______________

k. C(IV) and O

_______________

d. Hg(II) and Cl

_______________

l. Sn(IV) and Br

_______________

e. Al and S

_______________

m. P(III) and I

_______________

f. Ag and S

_______________

n. As(V) and Cl

_______________

g. N(III) and O

_______________

o. Sn(II) and F

_______________

h. Na and O

_______________

p. Sr and Br

_______________

This same method is used when the formula you are trying to write contains a polyatomic ion. Just keep in mind that polyatomic ions stay together as a group and act as though they were a single atom with a single oxidation number. Whenever more than one polyatomic ion appears in a formula, it must be enclosed by parentheses with a subscript outside. 5-10

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Example 3: Write the formula for a compound of potassium and permanganate. K = +1 and MnO41- = -1. LCM = 1, so, the formula for potassium permanganate is KMnO4. Example 4: Write the formula for a compound of calcium and chlorate. Ca = +2 and ClO31- = -1. LCM = 2 So, the formula for calcium chlorate is Ca(ClO3)2 Example 5: Write the formula for a compound of ammonium and sulfate. NH41+ = +1 and SO42- = -2. LCM = 2, so, the formula for ammonium sulfate is (NH4)2SO 4 .

Problem 4. Write correct formulas for compounds of the following: a. Mg and SO42-

_______________

g. Al and SO32-

_______________

b. Sn(II) and CrO42-

_______________

h. Zn and CO32-

_______________

c. Na and HCO31-

_______________

i. Cu(II) and OH1-

_______________

d. Fe(II) and OH1-

_______________

j. Fe(III) and SO42-

_______________

e. Pb(II) and PO43-

_______________

k. Hg(I) and NO31-

_______________

f. NH41+ and Cl1-

_______________

l. NH41+ and Cr2O72-

_______________

Check your answers for problem 4, and then try problem 5 below.

Problem 5. Write correct formulas for compounds of the following: a. Mg and F

_______________

f. Bi (III) and S

______________

b. Ba and ClO31-

_______________

g. K and Cl

______________

c. N(V) and O

_______________

h. H and S

______________

d. Ca and PO43-

_______________

i. Cr(III) and C2H3O21-

______________

e. Al and OH1-

_______________

j. S (IV) and O

______________

Earlier in this chapter you determined the number of atoms found in the formulas of compounds. Problem 6 will help you to improve this skill and to better understand the meaning of chemical formulas. In each of the following problems indicate the total number of atoms in each formula. For example, the total number of atoms in the formula H2O is three. In a formula which includes parentheses, such as Ca(NO3)2, the subscript to the right of the parentheses multiplies everything inside the parentheses. The total number of atoms in Ca(NO3)2 is nine (1 Ca + 2 N + 6 O = 9). The formula Na2SO 4 has seven atoms.

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Problem 6. Determine the total number of atoms contained in each of the following formulas. a. NaCl

__________

f. Pb(C2H3O2)2

__________

b. AlPO4

__________

g. Sr(NO3)2

__________

c. Fe(ClO3)3

__________

h. Ba3(PO4)2

__________

d. Ag2SO 3

__________

i. Al(HCO3)3

__________

e. Na2Cr2O7

__________

j. (NH4)2CO3

__________

The formulas in Problem 7 below belong to a group of compounds known as "hydrates." They are compounds that have water molecules included in them. The water is tacked on to the end of the formula following a raised dot. The raised dot does NOT mean multiplication, as it might in algebra; rather, it means "plus." So, a formula such as BaCl2 •2H 2O includes 2 water molecules. The total number of atoms in this formula is nine (1 Ba, 2 Cl, 4 H, and 2 O). For each of the following hydrates, indicate the total number of atoms in each formula. Problem 7. For each of the following hydrates, indicate the total number of atoms in each formula. a. CuSO4 •5H 2O __________

SECTION 5.7

b. Na2CO3 •10H2O __________

c. CoCl2 •6H 2O ___________

Rules For Naming Compounds

There are three methods for naming chemical compounds. They are the: 1. prefix method

2. Latin name method

3. Roman numeral method

Although there are some exceptions, generally speaking, these methods are used as follows. The prefix method is most often used to name compounds which contain only nonmetals. Nonmetals are found to the right of the "staircase" on the periodic table. The Latin name method is used for compounds containing certain metals including iron (Fe), copper (Cu), tin (Sn), or mercury (Hg). The Roman numeral method is used to name all compounds which contain metals. Metals are found to the left of the "staircase" on the periodic table (excluding hydrogen).

A. The Prefix Method You should MEMORIZE the following prefixes: 1 = mon or mono 2 = di 3 = tri

4 = tetr or tetra 5 = pent or penta 6 = hex or hexa

7 = hept or hepta 8 = oct or octa 5-12

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In this method prefixes are used to indicate the number of atoms of each element present in the formula of a compound. For example, As2O5 is called diarsenic pentoxide. Note that pent is used instead of penta in order to avoid the awkward sound of the double vowel. If only one atom of the first element in the formula is present the use of "mono" is usually avoided; if there is only one atom of the second element, then the prefix "mono" is always used. For example, SO2 is called sulfur dioxide (rather than monosulfur dioxide); however, the molecule CO is called carbon monoxide. Note that if the compound is binary (contains only two elements), the name of the second element is changed so that it always ends with the suffix "ide". (Examples: sulfur dioxide and carbon monoxide.) Finish spelling the name on this binary compound: CO2 is carbon diox_______.

{3}

Problem 8. Name the following nonmetallic compounds using the prefix method. a. SO3

_________________________

e. N2O

________________________

b. As2O3

_________________________

f. SF6

________________________

c. PBr5

_________________________

g. CCl4

________________________

d. SeF2

_________________________

h. NO

________________________

Caution: Some students confuse the use of Roman numerals in names with the use of prefixes in names. Here is the difference. Roman numerals indicate the oxidation number of an element. Prefixes indicate the number of atoms of an element represented in the formula. For example, iron (III) oxide is Fe2O3. The Roman numeral (III) in the name tells us that the oxidation number of iron in this compound is +3. It does NOT mean that there are three iron atoms represented in the formula. As you can see, there are only two iron atoms represented. The compound N2O4 is called dinitrogen tetroxide. The "di" before the nitrogen means that there are two nitrogen atoms represented in the formula. The prefix does NOT indicate the oxidation number of the nitrogen.

B. The Latin Name Method You need to MEMORIZE the Latin names for the elements listed in Table 5.4. Note that the Latin name of the lower oxidation state of each element ends in "ous", while the name of the higher oxidation state ends in "ic". (You should refer to Table 5.2 for the possible oxidation states of elements.) For example, the compound CuCl2 contains copper in the +2 oxidation state [copper(II)]. So, the Latin name of this compound is cupric chloride. On the other hand, CuCl contains copper in the +1 oxidation state. Its name is cuprous chloride. Therefore, in order to use this method of naming, you must first determine the oxidation state of the metal and then choose the proper Latin name. Note that if the compound contains only two elements, the name of the second element is changed so that it always ends with the suffix "ide".

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Table 5.4

Latin Names of Four Selected Elements

Element

Symbol

Latin Name

copper (I) copper (II)

Cu1+ Cu2+

cuprous cupric

iron (II) iron (III)

Fe 2+ Fe 3+

ferrous ferric

mercury (I) mercury (II)

Hg1+ Hg2+

mercurous mercuric

tin (II) tin (IV)

Sn 2+ Sn 4+

stannous stannic

(These Latin names can also be found in your Reference Notebook.)

In problem 9 you will be asked to assign Latin names to compounds. You must first determine the oxidation number of the metal in each compound. To do this, you should first determine the oxidation number of the other element present. Note the examples below. Example 1: Let's find the Latin name for SnS2. According to Table 5.2, tin (Sn) has oxidation numbers of +2 and +4. But which of these is being used in the compound SnS 2? Well, note from Table 5.2 that sulfur (S) can have oxidation numbers of -2, +4, or +6. Since the tin has only positive oxidation numbers, we must use the negative oxidation number of the sulfur which is -2. Now, if sulfur is -2 here, and since there are two atoms of sulfur in the formula, the total oxidation number of the sulfur in SnS2 is -4. Since the total of the oxidation numbers in the formula must equal zero, the oxidation number of the single tin atom in the formula must be +4. When tin has an oxidation number of +4 its Latin name (see Table 5.4) is stannic. Thus, SnS2 is called stannic sulfide. Example 2: What is the Latin name for Cu2O? Since oxygen is -2, the total oxidation number for the copper (Cu) must be +2. Hey, but wait. There are two atoms of copper represented in the formula. Therefore, each individual copper atom must have an oxidation number of +1. Therefore, checking Table 5.4, Cu 2O is called cuprous oxide. Problem 9. Name the following using the Latin name method. Calculate the oxidation number of the metal being used, then name the compound. Ox. No. of Metal

Name

a. SnS

____________

_________________________________________

b. HgCl

____________

_________________________________________

c. FeO

____________

_________________________________________

d. CuS

____________

_________________________________________

e. HgF2

____________

_________________________________________

f. SnCl2

____________

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g. Fe2S 3

____________

_________________________________________

h. CuI

____________

_________________________________________

C. The Roman Numeral Method When naming metallic compounds using this method, first determine the possible oxidation states of the metal. If the metal has more than one positive oxidation state, then you must use a Roman numeral in the name. If the metal has only one positive oxidation state, then you should not use a Roman numeral in the name. For example, let's try to name FeCl3 according to this method. Checking a reference sheet containing oxidation states we find that iron can have states of either +2 or +3. Therefore, we must use a Roman numeral in the name. The oxidation state of iron in FeCl3 is +3. The name of this compound is iron (III) chloride. Note that when a Roman numeral is used, it is the same number as the oxidation state of the metal. That is, if the oxidation state of iron in a compound is +3, then the Roman numeral used is (III). Let's try another one. Name FeO. In FeO, the oxidation state of iron is +2. Therefore, the name of FeO is iron (II) oxide. Note that the Roman numeral is always enclosed in parentheses. Let's name the compound: BaCl2. The metal barium has only one positive oxidation state which is +2. Therefore, no Roman numeral is needed and the name is simply barium chloride. Note that if the compound contains only two elements, the name of the second element is changed so that it always ends with the suffix "ide". Problem 10. Name the following using the Roman numeral method. Use Roman numerals in the name only if needed. Check your list of oxidation numbers to see if a metal has more than one positive oxidation state. If it does, use a Roman numeral in the name.

a. MnO 2

___________________________________

b. KBr

___________________________________

c.

CrCl3

___________________________________

d.

HgS

___________________________________

e. FeBr 2

___________________________________

f.

CaF2

___________________________________

g. CrO3

___________________________________

h. CuO

___________________________________

i.

Al2O3

___________________________________

j.

Co2O3

___________________________________

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D. Compounds Containing Polyatomic Ions Polyatomic ions are charged particles which consist of more than one atom. Formulas which begin with one of these ions are named by first naming the polyatomic ion, then naming the element which follows it and changing the ending by using the suffix "ide.". Examples include NH4Cl (ammonium chloride) and (NH 4)2S (ammonium sulfide). Formulas which end with one of these polyatomic ions are named by naming the first element and then naming the polyatomic ion. Example: CaSO4, calcium sulfate. A Roman numeral is added to the name only if a metal is involved that has more than one positive oxidation number. For example, since copper can be +1 or +2, CuSO 4 is called copper (II) sulfate. However, since aluminum can only be +3, Al(NO3)3 is simply called aluminum nitrate. Some formulas consist of two polyatomic ions. They are named simply by naming the first polyatomic ion followed by the name of the second one. NH4NO3 is ammonium nitrate; (NH4)2SO 3 is ammonium sulfite. Do not use the prefix method when naming compounds containing polyatomic ions.

Problem 11. Name the following compounds which contain polyatomic ions.

a. Ca(C2H3O2)2

_____________________________________

b. Ba(NO2)2

_____________________________________

c. Fe(OH)2 (2 names)

_____________________________________ _____________________________________

d. (NH4)2O

_____________________________________

e. Ag2SO 4

_____________________________________

f. KMnO4

_____________________________________

g. CuCO3 (2 names)

_____________________________________ _____________________________________

h. NaHSO4

_____________________________________

i. NH4C2H3O2

_____________________________________

j. (NH4)3PO 4

_____________________________________

Note: Do not mix the methods for naming compounds. For example, if we name FeCl2 ferrous (II) chloride, that is wrong because we mixed the Latin name method with the Roman numeral method. Wait! There is one exception to never mixing the methods. In hydrates (described below) you will notice that the Roman numeral and prefix methods are both used in a name.

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The compounds known as hydrates, which you studied earlier in this chapter, are named as follows: CuSO 4 •5H 2O is called copper (II) sulfate pentahydrate. The prefix "penta" means "five", so pentahydrate means "five waters," which is added to the name of the compound. BaCl2 •2H 2O is called barium chloride dihydrate. The prefix "di" means "two," so dihydrate means "two waters." (You should MEMORIZE the prefixes for 1 through 8 which were listed on a previous page.)

Problem 12. Perform the following tasks dealing with hydrates. a. Name this hydrate: CoCl2 •6H 2O

_____________________________________

b. Finish writing this formula for sodium sulfate heptahydrate: Na2SO 4 • ______H2O

SECTION 5.8

Writing Correct Formulas From Names

Problem 13. Write correct formulas for the compounds named below. Use the table of oxidation numbers in this chapter or in your reference notebook. Remember that the sum of the oxidation numbers of all the atoms and/or polyatomic ions in a formula must add up to zero. a. calcium nitrate

__________________

n. ammonium fluoride _________________

b. strontium chloride

__________________

o. iron (III) sulfide

_________________

c. phosphorus triiodide __________________

p. sodium carbonate

_________________

d. silver phosphate

q. carbon tetrachloride _________________

__________________

e. dinitrogen pentoxide __________________

r. cobalt (II) chloride

_________________

f. stannous chlorate

s. ferrous phosphate

_________________

g. chromium (III) oxide __________________

t. lead (IV) oxide

_________________

h. ammonium chromate __________________

u. antimony (V) sulfide

i. cesium sulfite

__________________

v. sulfur hexabromide _________________

j. strontium fluoride

__________________

w. silver oxide

k. hydrogen iodide

__________________

x. manganese (IV) chloride

__________________

_________________

_________________ _____________

l. iron (III) chloride hexahydrate __________________________ m. calcium sulfate dihydrate

__________________________

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SECTION 5.9

More Practice With Naming Compounds

Problem 14. Name the compounds below. Before deciding upon a name, first check to see if the compound contains a metal. Use the prefix method for compounds that do not contain a metal; use the Roman numeral method for compounds that do contain a metal. (Metals are found to the left of the "staircase" on the periodic table.) Remember you should not mix the three methods of naming. Formula

Name

a. CaCl2

_______________________________________________

b. Al2S 3

_______________________________________________

c. Ba(OH)2

_______________________________________________

d. CaCO3

_______________________________________________

e. MgSO3

_______________________________________________

f. Pb3(PO4)2

_______________________________________________

g. As2O5

_______________________________________________

h. PBr3

_______________________________________________

i. KOH

_______________________________________________

j. AsCl5

_______________________________________________

k. Ag2S

_______________________________________________

l. SrCr2O7

_______________________________________________

m. CsHSO4

_______________________________________________

n. Co(NO2)2

_______________________________________________

o. SrSO4

_______________________________________________

p. NaHCO3

_______________________________________________

q. MgO

_______________________________________________

r. CrF3

_______________________________________________

s. (NH4)2Cr2O7 _______________________________________________ t. Ni(ClO3)2

_______________________________________________

u. CBr4

_______________________________________________

v. Na3PO 4

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w. N2O3

_______________________________________________

x. Pb(C2H3O2)2_______________________________________________ y. P2O5

_______________________________________________

z. KCN

_______________________________________________

aa. LiNO3

_______________________________________________

bb. Na2CrO4

_______________________________________________

Problem 15. Give two names for each of the following. Put the Latin name in the left column and the Roman numeral name in the right column. Latin Name

Roman Numeral Name

a. HgBr

_______________________________

______________________________

b. Fe(OH)2

_______________________________

______________________________

c. HgCl2

_______________________________

______________________________

d. Fe2(SO4)3

_______________________________

______________________________

e. Cu2S

_______________________________

______________________________

f. SnCrO4

_______________________________

______________________________

g. Cu(OH)2

_______________________________

______________________________

h. HgNO3

_______________________________

______________________________

i. SnO2

_______________________________

______________________________

SECTION 5.10

A Summary Of Things To Memorize

1. Symbols of selected elements (Table 5.1) 2. Names, symbols and charges of polyatomic ions (Table 5.3) 3. Selected Latin names of elements (Table 5.4) 4. Selected Greek prefixes for the numbers 1 through 8. It will also be assumed that you can recognize and write Roman numerals for at least the first eight numbers: I, II, III, IV, V, VI, VII, VIII 5-19

©1997, A.J. Girondi

SECTION 5.11

Learning Outcomes

You are now at the end of chapter 5. Check the learning outcomes below. When you feel that you have mastered all of them, arrange to take any test or quizzes on chapter 5, and then go to Chapter 6.

_____1. Distinguish between elements, compounds, heterogeneous mixtures, homogeneous mixtures, and pure substances. _____2. Distinguish between atoms, molecules, and ions. _____3. Write from memory the names (spelled correctly) and symbols of selected common elements. _____4. Be able to identify the four important ideas which composed Dalton's Atomic Theory. _____5. Write from memory the Latin names (spelled correctly) for the lower and higher oxidation states of copper, iron, mercury, and tin. _____6. Write from memory the names (spelled correctly), formulas, and charges of the common polyatomic ions. _____7. Name compounds using the Roman numeral method. _____8. Name compounds using the Latin name method. _____9. Name compounds using the prefix method. _____10. Name compounds which are hydrates. _____11. Calculate the number of atoms in the formulas of compounds (including hydrates). _____12. Use oxidation numbers to write correct formulas for compounds given their names or the elements they contain. _____13. Write from memory the common Greek prefixes. _____14. Write from memory the names and symbols of common elements.

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SECTION 5.12

Answers to Questions and Problems

Questions: {1} No; {2} Magnet is attracted to pure iron, but not attracted to iron when it is in a compound; {3} ide Problems: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11.

12. 13.

14.

15.

1,1,4 1,2,3,2 a. BaCl2; b. HgBr; c. CaO; d. HgCl2; e. Al2S 3; f. Ag2S; g. N2O3; h. Na2O; i. Cu2S; j. As2O5; k. CO2; l. SnBr4; m. PI 3 ; n. AsCl5; o. SnF2; p. SrBr2 a. MgSO4; b. SnCrO 4; c. NaHCO3 d. Fe(OH)2; e. Pb3(PO4)2; f. NH4Cl; g. Al2(SO3)3; h. ZnCO3; i. Cu(OH)2; j. Fe2(SO4)3; k. HgNO3; l. (NH4)2Cr2O7 a. MgF2; b. Ba(ClO3)2; c. N2O5; d. Ca3(PO4)2; e. Al(OH)3; f. Bi2S 3 g. KCl; h. H2S; i. Cr(C2H3O2)3 j. SO2 a. 2; b. 6; c. 13; d. 6; e. 11; f. 15; g. 9; h. 13; i. 16; j. 14 a. 21; b. 36; c. 21 a. sulfur trioxide; b. diarsenic trioxide; c. phosphorus pentabromide; d. selenium difluoride; e. dinitrogen monoxide; f. sulfur hexafluoride; g. carbon tetrachloride; h. nitrogen monoxide a. +2, stannous sulfide; b. +1, mercurous chloride; c. +2, ferrous oxide; d. +2, cupric sulfide e. +2, mercuric fluoride; f. +2, stannous chloride; g. +3, ferric sulfide; h. +1, cuprous iodide a. manganese (IV) oxide; b. potassium bromide; c. chromium (III) chloride d. mercury (II) sulfide; e. iron (II) bromide; f. calcium fluoride; g. chromium (VI) oxide; h. copper (II) oxide; i. aluminum oxide; j. cobalt (III) oxide a. calcium acetate; b. barium nitrite; c. iron (II) hydroxide, ferrous hydroxide; d. ammonium oxide; e. silver sulfate; f. potassium permanganate; g. copper (II) carbonate, cupric carbonate; h. sodium hydrogen sulfate; i. ammonium acetate; j. ammonium phosphate a. cobalt (II) chloride hexahydrate; b. Na2SO 4 • 7 H2O a. Ca(NO3)2; b. SrCl2; c. PI 3; d. Ag3PO 4; e. N2O5; f. Sn(ClO3)2; g. Cr2O3; h. (NH4)2CrO4; i. Cs2SO 3; j. SrF2; k. HI; l. FeCl3 • 6 H2O; m. CaSO4 • 2 H2O; n. NH4F; o. Fe2S 3; p. Na2CO3; q. CCl4; r. CoCl2; s. Fe3(PO4)2; t. PbO2; u. Sb2S 5; v. SBr6; w. Ag2O; x. MnCl4 a. calcium chloride; b. aluminum sulfide; c. barium hydroxide; d. calcium carbonate; e. magnesium sulfite; f. lead (II) phosphate; g. diarsenic pentoxide; h. phosphorus tribromide; i. potassium hydroxide; j. arsenic pentachloride; k. silver sulfide; l. strontium dichromate; m. cesium hydrogen sulfate; n. cobalt (II) nitrite; o.strontium sulfate; p. sodium hydrogen carbonate; q. magnesium oxide; r. chromium (III) fluoride; s. ammonium dichromate; t. nickel chlorate; u. carbon tetrabromide; v. sodium phosphate; w. dinitrogen trioxide; x. lead (II) acetate; y. diphosphorus pentoxide; z. potassium cyanide; aa. lithium nitrate; bb. sodium chromate a. mercurous bromide, mercury (I) bromide b. ferrous hydroxide, iron (II) hydroxide c. mercuric chloride, mercury (II) chloride d. ferric sulfate, iron (III) sulfate e. cuprous sulfide, copper (I) sulfide f. stannous chromate, tin (II) chromate g. cupric hydroxide, copper (II) hydroxide h. mercurous nitrate, mercury (I) nitrate i. stannic oxide, tin (IV) oxide

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SECTION 5.13

Student Notes

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