Chapter 3: The Structure of Matter

Lesson 1: What Are Molecules?

•You may describe sugar by its properties (color, taste, texture), but how would you describe a single grain of sugar?

•You might say it is very small. But how small is the smallest piece of sugar?

Size of Molecules•Each grain of sugar is made of even

smaller particles that are too tiny for you to see. These tiny particles are called molecules (the smallest particles of a substance that still has the properties of that substance.) Each molecule of sugar has exactly the same properties.

•How small can molecules be? They are so small that billions of them could be places side by side on a line one centimeter long!

Describing Molecules

•Imagine dividing up one drop of water into smaller and smaller drops. The smallest drop you could make that still has the properties of water would be one molecule of water. In general, all water molecules are alike.

•Ex: A molecule of water from the ocean, a raindrop and the water you drink.

• In the figure of the molecule of water, you can see it has three parts – one large part and two smaller parts. If you divide up the water molecule, it would no longer have the properties of water. Each individual part of the water molecule is called an atom (the building block of matter.) • So each water molecule has three atoms. Each kind of atom has its own properties. All matter is made of atoms.

States of Matter•You can describe matter by telling about its

properties (mass, density) but the form (solid, liquid, gas) that matter has is another of its properties.

How many solids, liquids and gases can find in the picture below:

• The trees and homes are solids. In solids, the molecules in a solid attract, or pull toward each other. In a solid, the molecules vibrate, which means that they move back and forth quickly, but stay close together.

•  • The water in the picture is a liquid. The pull between the

molecules is weaker in liquids than it is in solids. They slide past each other. A liquid has a certain volume but changes it shape because its molecules can easily move around. You can take a bottle of water and pour it into different containers, but it will still be the same amount.

•  • The helium balloon is filled with a gas. The molecules are

much farther apart than they are in a liquid or a solid. The pull between the molecules is very weak and will take the shape of its container, filling it completely. In our picture, it is the shape of the balloon. The volume of a gas can change.

Plasma•Matter can also exist in a fourth state of

matter called plasma (a very hot gas made of particles that have an electric charge.) The particles of plasma shake violently at very high temperatures. Plasma is very rare on Earth. All stars, including the sun, are balls of plasma.

Lesson 2: What Are Elements? •Most matter around us is made up of

many different kinds of atoms. However, some matter has only one kind of atom. When matter is made up of only one kind of matter, it is called an element.

•All atoms of the same element are alike. Ex: All atoms of oxygen are the same and are different from the atoms of all other elements.

•Foil is made of atoms of the element aluminum.

•Gold, silver and copper are also elements. Many of these are used to make jewelry.

Natural Elements

•Scientists know of about 109 different kinds of elements. Ninety-two (92) of these elements are called natural elements (ones found in nature).

•Ex: Oxygen (we get from the air we breathe); Calcium (keeps your bones and teeth strong)

•Not all elements are natural elements. Scientists are able to produce a few elements in laboratories. Some only last for a short time (sometimes only a fraction of a second) before changing into other elements.

Listed below are some natural elements and what they are used for:

Name Used for or found in

Copper Coins, frying pans, electrical wire

Silver Jewelry, photography

Carbon “lead” pencils, charcoal, diamonds

Helium Balloons, airships

Nitrogen Air that we breathe, fertilizers

Chlorine Bleach, table salt

Aluminum Airplanes, cookware, soft-drink cans

Neon “neon” signs

Gold Jewelry, seawater, dentistry

Mercury Thermometers, drugs, pesticides

Iron Steel, eating utensils

Elements in Water•In Lesson 1, we learned that a molecule of

water is made of three parts like those in the figure. These parts are elements. The large part of the molecule is an atom of the element oxygen. The two small parts are atoms of the element hydrogen. The atoms of the element oxygen are different from the atoms of the element hydrogen.

Lesson 3: What Are Compounds?

•When two or more atoms of different elements join together, the substance that forms is called a compound.

•A compound has properties that are different from the properties of the elements that form the compound.

•Think again about a molecule of water. In the drawing below you can see that an atom of oxygen combines with two atoms of hydrogen to form a molecule of the compound water. Water is different from the elements that form it. Water is a liquid; oxygen and hydrogen are gases.

•Another compound that probably is familiar to you is table salt. The chemical name for salt is sodium chloride. It is formed when the element sodium is combined with the element chlorine. Again, sodium chloride is very different from each of the elements it contains. Sodium is a solid and chlorine is a poisonous gas! However, when they are combined, chlorine not longer has its poisonous property.

•Compounds can have completely different properties from the elements that form it.

•Most kinds of matter on Earth are compounds. There are more than 10 million known compounds.

Name Elements in this compound Use

Table salt Sodium, chlorine Cooking

Water Hydrogen, oxygen Drinking

Sugar Carbon, hydrogen, oxygen Cooking

Baking soda Sodium, hydrogen, carbon, oxygen Baking

Epsom salts Magnesium, sulfur, oxygen Medicine

Below are some common compounds and the elements that make up each compound.

•You may wonder if you can tell by looking at a substance whether it is an element or a compound. An unknown substance must be tested in a laboratory to determine whether it is an element or a compound.

Lesson 4: How Do Scientists Know What Atoms are Like?

•Since atoms are too small to be seen with the eyes alone, people have wondered for a long time what atoms look like. Scientists have been studying atoms since the 1800’s. How do they know what an atom looks like if they can’t see them?

Using Models •Sometimes scientists can tell what things look

like by studying how they act. Ex: Can you see the wind? We can see the effects of the wind, like leaves blowing around, but we can’t actually see the wind. So, we use evidence.

•Scientists use this same evidence to study things they can’t see. Ex: By studying how atoms act, scientists decide what an atom must look like.

•They use models (an idea, a picture or an object that is built to explain how something else looks or works) to help people understand the way the objects acts.

Models of Atoms

•Scientists use models of atoms to show how atoms look and act without having to actually see them. Models have been developed over 2000 years ago. As they gather new information about atoms, scientists change their models.

•In the early 1900’s, scientists developed a model of an atom like this one. However, as scientists know more about atoms, this kind of model is still useful for describing atoms.

Hydrogen Helium

•This central part of an atom is called a nucleus. The nucleus of an atom contains small particles called protons.

•Protons are labeled with the letter p. Another symbol for a proton is a plus (+) sign.

• Electrons use the letter e or use the minus (-) sign.

•Electrons are particles in an atom that move around the outside of the protons and are smaller than the protons.

•The protons and electrons of an atom stay together because they attract each other.

• In 1932, scientists had evidence that the nucleus of an atom had another kind of particle. This particle is call a neutron. It is about the same size as a proton. Because of this new evidence, scientists changed the model of the atom.

• You can see that the electrons seem to be on a certain path around the nucleus of the atom. Scientists thought that electrons moved in different layers around protons, sometimes jumping from one layer to another.

•Today scientists use another model of atoms. It is called the electron cloud model. The dark center represents the nucleus. However, you can’t see different layers of electrons like you saw in the first models.

•This model was developed as a result of scientists gaining evidence that electrons behave in a more complicated way than they previously thought. So, they are not sure how electrons move around the nucleus. As they learn more, the model will most likely change.

•You have looked at models showing the number of protons and electrons in the atoms of a few different elements. On the following table, you will see some other elements listed with the numbers of protons and electrons in each. What do you notice about the number of protons and the number of electrons in each of the elements?

Element Number of protons Number of electrons

Hydrogen 1 1

Helium 2 2

Lithium 3 3

Beryllium 4 4

Boron 5 5

Carbon 6 6

Nitrogen 7 7

Oxygen 8 8

Fluorine 9 9

Neon 10 10

The number of protons in an atom is equal to the number of electrons in the atom.

Lesson 5: How Can You Identify Elements?•Because more than 100 elements are known, scientists need a way to identify them. One way to identify elements is by knowing their atomic numbers.

Atomic Number• The table below lists the same ten elements listed in the table

from our previous lesson. You can see that an additional column, labeled Atomic number has been added to the table.

• The atomic number of an element tells you how many protons are in each atom of the element.

Element Atomic Number Number of protons

Hydrogen 1 1

Helium 2 2

Lithium 3 3

Beryllium 4 4

Boron 5 5

Carbon 6 6

Nitrogen 7 7

Oxygen 8 8

Fluorine 9 9

Neon 10 10

Number of electrons

1

2

3

4

5

6

7

8

9

10

Notice that each element has a different number of protons and therefore a different atomic number. Ex: hydrogen has 1 proton; its atomic number is also 1.

• Look at the atom of boron: 

• How many protons does boron have? What does the table tell you the atomic number of boron is?

• For all the elements, the atomic number of the element is equal to the number of protons it has.

• Atomic Number• Number = of protons•  •  

The Mass of an Element• You learned in Chapter 1 that mass is the amount of

matter in an object.• Protons and neutrons have a greater mass than

electrons have. In fact, the mass of a proton or a neutron is about 1800 times the mass of an electron. You still cannot measure their mass on a balance scale because they are so small. Instead, scientists tell about the mass of an element by using its mass number.

• The mass number of an element is equal to the sum of the numbers of protons and neutrons in an atom of the element.

• Mass = Number of protons +• Number Number of neutrons

•The drawing below shows an atom of beryllium. You can see it has 4 protons and 5 neutrons. The atomic number of beryllium is 4 (the same as the number of protons). To find the mass number, you add the number of protons, 4, and the number of neutrons, 5. The mass number of beryllium is 9 (4 + 5 = 9).

•You have learned about protons, neutrons and electrons. You also learned about atomic numbers and mass number of elements. The table below gives a summary of information for the first ten elements.

Element Atomic Number MassNumber

Number of protons Number of electrons

Number of neutrons

Hydrogen 1 1 1 1 0

Helium 2 4 2 2 2

Lithium 3 7 3 3 4

Beryllium 4 9 4 4 5

Boron 5 11 5 5 6

Carbon 6 12 6 6 6

Nitrogen 7 14 7 7 7

Oxygen 8 16 8 8 8

Fluorine 9 19 9 9 10

Neon 10 20 10 10 10

• You can use information you know about an element to determine other information. For example, look at the atom of sodium.

•

• Find the number of protons and neutrons. How many protons are in the nucleus?

• You know that the number of electrons in an element is equal to its number of protons. How many electrons does sodium have?

• You know that the atomic number of an element is equal to the number of protons it has. What is the atomic number of sodium?