Unit 7: Structureof Matter

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Vocabulary

Study the vocabulary words and definitions below.

compound ....................................... a substance formed when two or moreelements combine chemically

element ........................................... a substance that cannot be broken downinto a simpler form by ordinarychemical means

formula ........................................... the way a chemist tells how two or moreelements are combined to make acompoundExample: H2O is the formula for water

hydrogen (H) .................................. the lightest and most abundant of allelements; occurs as a gas when not inother substances

mixtures .......................................... two or more substances put together; nochemical reaction takes place and theyare easily separated

oxygen (O) ...................................... an element found as a gas when not inother substances; it has an atomicnumber of eight and is involved inburning and rusting

symbols ........................................... the letters used by scientists to representthe names of the elements

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Introduction

You have seen how scientists represent the reactions that createsubstances. You may have wondered how two substances with oxygen inthem (like water and sugar) could be so different. In this unit, we willdiscuss what properties these substances have that make them unique.

Elements

By now we know that matter has mass, volume, and density. We alsoknow that matter can be a solid, liquid, or a gas. We have also learnedsome of the physical and chemical properties of matter. We experimentedto show that chemical changes produce new substances. However, whatmakes up matter? Think about water. Water can be broken down intohydrogen and oxygen. The substances of hydrogen and oxygen cannot bebroken down by chemical means. These substances are called elements.Elements cannot be broken down by chemical action. All substances are

made of elements.

If you look at all the buildings around you,you see that they come in many differentshapes and sizes. But there are similaritiesbetween the buildings. Think of a pyramidand a castle. Both are made of stone blocks,but the blocks have been arranged in verydifferent ways. By doing this, the builders

made the structure they wanted. You can think of elements as buildingblocks. On Earth, we have discovered about 120 elements. While some ofthe elements can only be found in very special labs, these are all theelements that we know exist. Everything is made from these elements.

Some substances are made of only a single element. Aluminum (Al), gold(Au), oxygen (O), and hydrogen (H) are examples of substances with asingle element.

examples of substances with a single element

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8 P8 N

1 atomof oxygen

1 P

1 P

2 atomsof hydrogen

water molecule broken down intoits elements

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Most elements are solid under normal conditions. Few are liquid. Themercury (Hg) used in thermometers is normally liquid.

Many other elements are gases under normal conditions. Oxygen (O) andhydrogen (H) are just two of the elements that are gases at roomtemperature.

Scientists have a special way of writing the names of elements. They useletters instead of writing the whole word. The letters are called symbols.Here are some of the common ones.

Copper

Aluminum

Iron

Mercury

Oxygen

Hydrogen

Silver

Gold

Carbon

Cu

Al

Fe

Hg

O

H

Ag

Au

C

Elements Symbols

Each of the elements has its own symbol. Each element has at least oneproperty that makes it different from another element.

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Compounds

Many substances are made from more thanone element. Elements can unite with eachother. The elements form new substances thatare very difficult to separate. The newsubstances are called compounds. Acompound has chemical and physicalproperties that are uniquely its own. It maylook totally different from the elements thatformed it. As you have seen, the atoms of twoelements, hydrogen (H) and oxygen (O),combine to form water.

Sugar is a compound formed by atoms of carbon (C), hydrogen (H), andoxygen (O).

carbon

sugar(a compound)

hydrogen

oxygen

C6

O6

C6

O6

H12H

12

Sugar and water do not look like the elements that formed them. Whencompounds are formed, the elements always combine in the sameproportions. A formula tells how elements combine to form compounds.The formula for water is H2O. Compounds always have formulas.

Mixtures

It is possible to combine two elements or compounds without producingnew substances. No chemical change takes place. These substances arecalled mixtures. Mixtures can be separated. Each substance in the mixturekeeps its own properties. If you mix iron filings with sand, you couldseparate them because there has been no chemical reaction. There is nonew compound; there is only iron and sand.

8 P8 N

1 P

oxygen

1 P

hydrogenhydrogen

Water Molecule

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If we took hydrogen and combusted it with oxygen, water would beformed. Water does not have the same properties as hydrogen and oxygenbecause it is a different compound. Water is always made from twohydrogen atoms and one oxygen atom. Water cannot be made any otherway because it is not a mixture.

On the other hand, a mixture can be made in many different ways. Air is amixture. The elements in the air are not always the same. Tossed salad is amixture too; salads do not always have the same ingredients. Mixtures donot have formulas. They are not formed by chemical changes.

A tossed salad is a mixture too; salads do not alwayshave the same ingredients.

Summary

Now we know that elements are the simplest forms of substance. Gold(Au) is an element. Compounds are formed when a chemical change takesplace between two or more elements. Mixtures are formed when two ormore substances are put together. No chemical change takes place. Theparts of a mixture can easily be separated.

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Practice

Use the periodic table of the elements on pages 98-99 to identify each of theelements whose symbols appear below. Write the name and the atomic numberfor each element on the line provided.

1. C

2. Au

3. Ag

4. Hg

5. Cu

6. Fe

7. H

8. O

9. Al

element's name &atomic number

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Lab Activity: Part 1

Investigate:• You will differentiate between a compound

and a mixture and separate the substances in a mixture using physical means.

Facts:• The substances in mixtures do not combine

chemically.

Materials:• sulfur• paper• iron filings• ring stand and clamp

• Bunsen burner• magnets• test tube

Part 1

1. Pour some sulfur onto a sheet of paper.

2. Add some iron filings. Mix the sulfur and the iron filings together.

a. Did a chemical change take place? ________________________

b. Are any new substances formed? _________________________

c. Did the iron and the sulfur keep their own properties? ______

3. Move a magnet near the sulfur and the iron filings.

a. Can you separate the iron from the sulfur? _________________

b. Did the iron and the sulfur form a mixture or a compound?

________________________________________________________

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Lab Activity: Part 2

1. Mix the iron filings and the sulfur on a sheet of paper.

2. Pour the mixture into a test tube.

3. Place the tube in clamp on a ring stand.

4. Heat the tube until it begins to glow.

5. Let the test tube cool.

6. Remove the substance from the test tube.

a. Can you see the iron? ___________________________________

b. Can you see the sulfur? __________________________________

c. Could you separate the iron from the sulfur using a magnet?

______________________________________________________

d. Did you make a new substance? __________________________

e. Is this new substance a mixture or a compound? ____________

Note: This new substance is called Iron Sulfide.

f. What are the two elements that formed the substance?

______________________________________________________

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Practice

Use the list below to complete the following statements. One or more terms will beused more than once.

120 copper hydrogen oxygenaluminum element laboratories silvercarbon elements liquidchemical gold mercury

1. An is a substance that cannot be broken

down into a simpler form and from which other substances may be

made.

2. There are about different kinds of

elements.

3. All substances are made from .

4. is an example of a solid element.

5. Mercury is an element that is normally in a

form or state.

6. changes produce new substances.

7. Some elements are only found in .

8. Au is the symbol for .

9. Cu is the symbol for .

10. C is the symbol for .

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11. Al is the symbol for .

12. Ag is the symbol for .

13. O is the symbol for .

14. H is the symbol for .

15. Hg is the symbol for .

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Practice

Write True if the statement is correct. Write False if the statement is not correct.

__________ 1. Two or more elements combine chemically to form asubstance.

__________ 2. Sugar is a mixture, not an element.

__________ 3. Compounds are very easy to separate.

__________ 4. Hydrogen and oxygen combine to form water.

__________ 5. Compounds have the same properties as the elementsfrom which they are formed.

__________ 6. A compound is formed when two or more substancesare put together and no chemical change takes place.

__________ 7. All mixtures have formulas.

__________ 8. Mixtures can easily be separated.

__________ 9. Oxygen is a compound.

_________ 10. Air is a mixture.