Chapter 3: The Structure of Matter
Chapter 3: The Structure of Matter
Chapter 3: The Structure of
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!
• 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
• 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
How many solids, liquids and gases can find in the
• 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.
• 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
• 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.
• 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
• 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:
Used for or found in
Coins, frying pans, electrical wire
“lead” pencils, charcoal, diamonds
Air that we breathe, fertilizers
Bleach, table salt
Airplanes, cookware, soft-drink cans
Jewelry, seawater, dentistry
Thermometers, drugs, pesticides
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
• 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
• 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.
Below are some common compounds and
the elements that make up each compound.
Elements in this compound
Carbon, hydrogen, oxygen
Sodium, hydrogen, carbon, oxygen
Magnesium, sulfur, oxygen
• 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?
• 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
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
• 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.
• This central part of an atom is called a nucleus.
The nucleus of an atom contains small particles
• 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 (-)
• 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
• 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
Number of protons
Number of electrons
The number of protons in an atom is equal to the
number of electrons in the atom.
Lesson 5: How Can You Identify
• Because more than 100 elements are
known, scientists need a way to
identify them. One way to identify
elements is by knowing their atomic
• 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.
Number of protons
Number of electrons
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.
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.
Number of protons +
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.
Number of protons
• 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?