Name
Class
CHAPTER 10
Date
Introduction to Atoms
2 The Atom
SECTION
BEFORE YOU READ
Tennessee Science
Standards
After you read this section, you should be able to answer
these questions:
GLE 0807.Inq.5
GLE 0807.9.2
GLE 0807.12.4
•
•
•
•
What are the parts of an atom?
How do atoms of different elements differ?
What are isotopes?
What forces work inside atoms?
How Small Is an Atom?
An atom is very small. If you think about the size of
a penny, it contains 2  1022 atoms of copper and zinc.
This number is written as 20,000,000,000,000,000,000,000
atoms. That’s 20 thousand billion billion atoms, which is
much more than the number of people on Earth. So, each
atom must be very small.
Even things that are very thin are made up of a large
number of atoms. If you had a piece of aluminum foil,
it might seem very thin to you. However, it is still about
50,000 atoms thick.
STUDY TIP
Compare In your notebook,
make a table to compare
the features of protons,
neutrons, and electrons.
Include mass, charge, and
location in an atom.
What Are the Parts of an Atom?
Even though atoms are small, they are made of even
smaller particles. Atoms are made of three different
types of particles: protons, neutrons, and electrons.
These particles can be seen in the figure below.
READING CHECK
1. List What are the three
types of particles in an atom?
Parts of an Atom
are negatively charged particles
found in electron clouds outside the nucleus.
are positively
charged particles in the
nucleus of an atom.
are
particles in the
nucleus of an
atom that have
no charge.
TAKE A LOOK
When the model of an atom is shown in a book, it does not show the correct
scale of the particles in an atom. If protons were the size of those in an illustration, the electrons would need to be hundreds of feet away from the nucleus.
2. Identify Fill in the names
of the subatomic particles on
the blank lines in the figure.
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SECTION 2
Class
Date
The Atom continued
THE NUCLEUS
READING CHECK
3. Identify What is the
SI unit for the mass of
subatomic particles?
READING CHECK
4. Describe Where are
electrons found in an atom?
TAKE A LOOK
5. Identify Fill in the charge
and mass of a neutron.
Protons are the particles inside the nucleus of an
atom that give the nucleus its positive charge. The
mass of a proton is about 1.7  10-24 g. This is the same
as 0.0000000000000000000000017 g. This is a very small
number. Since this number is so small, scientists
created a new unit to measure the masses of the particles
in atoms. This is called the atomic mass unit (amu). A
proton has a mass of about 1 amu.
There are other particles inside the nucleus as well.
Neutrons are particles in the nucleus of an atom that do
not have an electric charge. A neutron has a little more
mass than a proton does. However, the difference in mass
is so small that the mass of a neutron can be thought of
as 1 amu.
OUTSIDE THE NUCLEUS
Electrons are the negatively charged particles in
atoms. They are found in electron clouds outside the
nucleus. Electrons are much smaller than protons and
neutrons. You would need about 1,840 electrons to equal
the mass of a proton. Electrons are so small that they are
usually not included in calculating the mass of an atom.
The masses of an electron and a proton are different,
but their electric charges are the same size. However,
the signs of the charges are opposite. Electrons have a
negative charge (1), and protons have a positive charge
(1). Neutrons, on the other hand, have no charge at all.
Because each atom has an equal number of electrons and
protons, atoms are neutral (they have no charge).
Particle
Charge
Mass (amu)
Proton
1
1
1–
1/1,840
+
Neutron
Electron
If an atom gains or loses an electron, it becomes an
ion. An ion formed by losing an electron has a positive
charge. This kind of ion has more protons than electrons.
An ion formed by gaining an electron has a negative
charge because it has more electrons than protons.
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The Atom continued
How Are Atoms of Different Elements
Different?
There are more than 110 known elements. Each element has atoms that are different from atoms of all the
other elements. All atoms of the same element have the
same number of protons. Atoms of different elements
have different numbers of protons.
READING CHECK
6. Define How do atoms of
different elements differ?
STARTING SIMPLE
The simplest atom is hydrogen. All atoms have protons and electrons, and a hydrogen atom has just one of
each. It doesn’t have a neutron. You can build a hydrogen
atom by putting a proton in the center of the atom for the
nucleus. Then, put one electron in the electron cloud. You
have just built a hydrogen atom.
ADDING NEUTRONS
An atom with two positively charged protons in its
nucleus is an atom of helium. Every helium atom has two
protons. You must also add two neutrons. So that it has a
neutral charge, you need to add two electrons outside the
nucleus. A model of this atom is shown below.
Proton
Electron
A helium nucleus must have
neutrons in it to keep the
protons from moving apart.
TAKE A LOOK
7. Describe Why must a
helium nucleus have neutrons?
Neutron
BUILDING BIGGER ATOMS
Because the protons repel each other, the atoms of
every element, except hydrogen, must have neutrons in
their nuclei. For example, most helium atoms have two
protons and two neutrons.
The number of protons is not always the same as the
number of neutrons. In fact, nuclei usually have more
neutrons than protons. For example, most fluorine atoms
have nine electrons, nine protons, and ten neutrons. Most
uranium nuclei have 92 protons and 146 neutrons.
READING CHECK
8. Explain Why must nuclei
with more than one proton
also have neutrons?
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The Atom continued
PROTONS AND ATOMIC NUMBER
READING CHECK
9. Define What does the
atomic number tell you
about an atom?
The number of protons in the nucleus is called the
atomic number of the atom. All atoms of an element
have the same atomic number. Every hydrogen atom has
only one proton in its nucleus, so hydrogen has an atomic
number of 1. Every carbon atom has six protons in its
nucleus, so carbon has an atomic number of 6.
Comparison of a Hydrogen Atom and a Helium Atom
Electron
Electron
Protons
Proton
Neutrons
Electron
Element
Hydrogen
Helium
Number of protons
ph07ci_ats000202a
1st pass
S. Toole
1/5/6
Number of
neutrons
TAKE A LOOK
10. Identify In the table, fill
in the blank boxes for each
element.
Number of
electrons
ph07ci_ats000203a
1st pass
S. Toole
1/5/6
Atomic number
What Are Isotopes?
READING CHECK
11. Describe What are
isotopes?
Most hydrogen atoms only have a proton in their
nucleus. However, about one hydrogen nucleus out of
10,000 also has a neutron. The atomic number of this atom
is 1, so the atom is still hydrogen. The nucleus has two particles, a proton and a neutron. This nucleus has a greater
mass than a hydrogen atom that has only a proton in it.
Atoms that have the same number of protons but a
different number of neutrons are called isotopes. The
two hydrogen atoms in the figure on the next page are
isotopes of each other. They are both hydrogen because
each has only one proton. Because they have a different
number of neutrons, they have different masses.
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The Atom continued
Isotopes of Hydrogen
Proton
Electron
Neutron
TENNESSEE
STANDARDS CHECK
Proton
Electron
GLE 0807.9.2 Explain that
matter has properties that are
explained by the structure and
arrangement of its atoms.
Word Help: structure
the way in which a whole
is put together
This isotope is a hydrogen
atom that has one proton
in its nucleus.
This isotope is a hydrogen atom
that has one proton and one
neutron in its nucleus.
PROPERTIES OF ISOTOPES
12. Explain Why doesn’t it
matter which nonradioactive isotope of oxygen you
breathe?
There are a small number of isotopes for each element
found in nature. Some isotopes have special properties.
For example, some isotopes are unstable, and their nuclei
change over time. Atoms with this type of nucleus are
called radioactive. A radioactive atom can break down,
giving off small particles and energy.
Isotopes of an element share the same chemical and
most of the same physical properties. For example, there
are three isotopes of oxygen. The most common isotope
of oxygen has eight neutrons in its nucleus. Other oxygen
isotopes have nine or ten neutrons. All three isotopes are
colorless, odorless gases at room temperature. Each isotope can combine with another substance as it burns.
DIFFERENCES BETWEEN ISOTOPES
Since isotopes contain different numbers of neutrons,
they have different masses. The mass number of an isotope is the sum of the protons and neutrons in an atom.
Electrons are not included in the mass number because
they are so small. The figure below shows two isotopes
of boron. The isotope on the left has a mass number of
10. The isotope on the right has a mass number of 11.
Math Focus
Two Isotopes
of Boron
Protons: 5
Neutrons: 5
Electrons: 5
Mass number 
protons  neutrons  10
Boron-10
Protons: 5
Neutrons: 6
Electrons: 5
Mass number 
protons  neutrons  11
13. Determine Boron has
an atomic number of 5. How
many neutrons are found in
boron-10, and how many in
boron-11?
Boron-11
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The Atom continued
NAMING ISOTOPES
You can identify an isotope of an element by its mass
number. For example, a hydrogen atom with one proton
and no neutrons has a mass number of 1. Its name is
hydrogen-1. Hydrogen-2 has one proton and one neutron.
A carbon isotope with a mass number of 12 is called
carbon-12. If you know that the atomic number for carbon is 6, you can calculate the number of neutrons in carbon-12. You would subtract the atomic number from the
mass number. For carbon-12, the number of neutrons is
12 minus 6, or six.
Isotope
TAKE A LOOK
14. Identify Fill in the table
with the isotope by using the
mass number and atomic
number columns.
Mass number
Atomic number
2
1
4
2
13
6
16
8
What Is the Atomic Mass of an Element?
Critical Thinking
15. Explain Can the mass
number of an atom ever be
smaller than its atomic number? Explain your answer.
Math Focus
16. Determine Calculate the
atomic mass of boron, which
occurs as 20% boron-10 and
80% boron-11. Show your
work.
For most elements, there are usually two or more
isotopes that exist. For example, copper is made of
copper-63 atoms and copper-65 atoms. This means that
the number of neutrons differs from atom to atom. The
atomic mass of an element is the weighted average of
the masses of all the natural isotopes of that element.
The percentage of each isotope is usually known. So,
a weighted average uses these percentages of each isotope present for that element. For example, a sample of
copper is about 69% copper-63 atoms and 31% copper-65
atoms. So, the atomic mass of copper is 63.6 amu.
Now, let’s try a problem. In nature, the percentage of
chlorine-35 is 76%, and chlorine-37 is 24%. What is the
atomic mass of chlorine?
Step 1: Multiply the mass number of each isotope by its
percentage in decimal form.
(35  0.76)  26.60
(37  0.24)  8.88
Step 2: Add the amounts together.
(35  0.76)  26.60
(37  0.24)  8.88
35.48 amu
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The Atom continued
What Forces Affect the Particles in Atoms?
Because charged particles attract and repel one
another, there must be other forces that hold atoms and
nuclei together. Scientists have discovered that there are
four basic forces in nature. These forces work together to
give atoms their structure and properties.
ELECTROMAGNETIC FORCE
The electromagnetic force causes objects with like
charges to repel each other. It also causes objects with
opposite charges to attract each other. Protons and
electrons are attracted to one another because of the
electromagnetic force. The electromagnetic force holds
the electrons around the nucleus.
GRAVITATIONAL FORCE
Gravitational force pulls objects toward one another.
It depends on the masses of the objects and the distance
between them. Since the particles in an atom are so small,
the gravitational force has almost no effect within atoms.
STRONG FORCE
The strong force holds the nucleus together. Inside the
nucleus, the attraction of the strong force is greater than
the repulsion of the electromagnetic force. If there were
no strong force, protons in the nucleus would repel one
another, and the nucleus would fly apart.
Say It
Discuss What would happen
if each of the basic forces did
not exist? Taking the forces
one at a time, in a small
group, discuss what would
happen if that force didn’t
exist.
Critical Thinking
17. Infer In some radioactive
nuclei, a neutron can change
into an electron and a proton. The electron leaves the
nucleus, but the proton does
not. What happens to the
identity of the atom when
this happens?
WEAK FORCE
The weak force is important in radioactive atoms. In
certain unstable atoms, a neutron can change into a proton and an electron. The weak force plays an important
role in this change.
Forces in the Atom
Description
Force
Force that affects changes of particles in
the nucleus
Attractive interaction between objects
with mass
TAKE A LOOK
Attractive force between particles in
the nucleus
18. Identify Fill in the
names of the four forces in
the table.
Attractive or repulsive force between
objects with opposite charges
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Section 2 Review
Date
GLE 0807.Inq.5, GLE 0807.9.2, GLE 0807.12.4
SECTION VOCABULARY
atomic mass the mass of an atom expressed in
atomic mass units
atomic mass unit a unit of mass that describes
the mass of an atom or molecule
atomic number the number of protons in the
nucleus of an atom; the atomic number is the
same for all atoms of an element
isotope an atom that has the same number of
protons (or the same atomic number) as other
atoms of the same element do but that has
a different number of neutrons (and thus a
different atomic mass)
mass number the sum of the numbers of
protons and neutrons in the nucleus of an atom
neutron a subatomic particle that has no charge
and that is located in the nucleus of an atom
proton a subatomic particle that has a positive
charge and that is located in the nucleus of an
atom
1. Compare Why do two isotopes of an element have the same atomic number but
different mass numbers?
2. Identify What are the three types of particles, and their charge, that can be found
in an atom?
3. Apply Why does every element except hydrogen need at least one neutron in
its nucleus?
4. Compare The atomic number of carbon is 6 and the atomic number of nitrogen
is 7. How do the number of neutrons and protons in the nuclei of carbon-14 and
nitrogen-14 differ?
5. Calculate What is the atomic mass of gallium, which occurs naturally as
60% gallium-69 and 40% gallium-71? Show your work.
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