Name
CHAPTER 4
Class
Date
Atoms
SECTION
3 Modern Atomic Theory
KEY IDEAS
As you read this section, keep these questions in mind:
• How are electrons organized in an atom?
• Can the exact location of an electron be determined?
• How do electrons move between energy levels?
What Is the Modern Model of the Atom?
The modern model of the atom is very different from
the model proposed by Rutherford. Remember that in
Rutherford’s atomic model, electrons could be found at any
distance from the nucleus. Today, scientists know that
electrons are found at only a few specific distances from the
nucleus. In addition, we know today that it is impossible to
determine exactly where an electron is at any specific time.
READING TOOLBOX
Summarize As you read this
section, underline the main
ideas in each paragraph.
When you finish reading,
write a summary of the
section using the underlined
ideas.
BOHR’S MODEL OF THE ATOM
In 1913, Danish physicist Niels Bohr showed that
electrons can be found only in certain energy levels, or
regions, around the nucleus. Electrons must gain energy
to move to a higher energy level. They must lose energy to
move to a lower level.
You can use a house, such as the one in the figure
below, to help you understand Bohr’s model. Imagine that
the nucleus of an atom is in a deep basement. Electrons
can be on any floor, but they cannot be between floors.
Electrons gain energy by riding up the elevator, and lose
energy by riding down.
READING CHECK
1. Describe What must
happen to an electron in
order for it to move to a
higher energy level?
4th energy
level
3rd energy
level
2nd energy
level
1st energy
level
In the Bohr model, electrons can
be found only in certain energy
levels around the nucleus.
EHHDBG@<EHL>K
2. Infer If an electron moves
from the third energy level to
the second energy level, has
it gained or lost energy?
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Atoms
Name
SECTION 3
Class
Date
Modern Atomic Theory continued
THE MODERN ATOMIC MODEL
READING CHECK
3. Explain Why didn’t
scientists use Bohr’s atomic
model to describe all the
elements?
READING CHECK
In Bohr’s model, electrons orbited the nucleus like
planets orbit a star. However, Bohr’s atomic model worked
only for hydrogen. Other scientists realized that other
elements were better described by combining Bohr’s ideas
about energy levels with ideas about probability.
Imagine a spinning propeller on a plane, such as the
one shown below. If someone asked you where a certain
propeller blade was at a certain time, it would be difficult
to answer. However, you could tell the person that the
blade was probably somewhere within the blurred region
at the front of the plane.
In a similar way, we cannot know the exact position,
speed, and direction of an electron at the same time.
This is because electrons exist as “clouds” around the
nucleus. Therefore, scientists can determine only how
likely it is that an electron is in a certain place. This
region is called an orbital. An orbital is a region in
which an electron is most likely to be.
4. Define What is an orbital?
The shaded region is an orbital. The
orbital is the region in which an
electron is most likely to be.
EHHDBG@<EHL>K
5. Identify Label the orbital
in the figure.
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Modern Atomic Theory continued
How Do Electrons Fill Energy Levels?
Within an atom, electrons have different amounts of
energy and exist in different energy levels. Recall that there
are many possible energy levels in an atom. Each energy
level can hold a specific number of electrons. For example,
the first energy level can hold up to two electrons. The more
electrons an atom has, the more energy levels are “filled.”
The figure below shows how many electrons different
energy levels can hold.
Electrons fill energy levels in order from lowest energy
to highest energy. For example, lithium atoms have three
electrons. Two of the electrons fill the first energy level. The
third electron is located in the second energy level.
8g^i^XVa I]^c`^c\
6. Infer How is the modern
atomic model similar to Bohr’s
model of the atom?
Electron Energy Levels
32e–
Energy level 4
18e–
Energy level 3
8e
2e–
Energy level 2
Energy level 1
–
EHHDBG@<EHL>K
Each energy level can
hold a different number
of electrons.
The electrons in the outer energy level of an atom are
called valence electrons. Valence electrons determine the
chemical properties of an atom. For example, all atoms with
one valence electron behave in a similar way.
Each energy level contains one or more subshells.
There are four different kinds of subshells. Each kind of
subshell is represented by a different letter: s, p, d, or f.
Each subshell has a different shape.
The lower the energy level, the fewer subshells are in the
energy level. For example, the first energy level contains
only an s subshell. The third energy level contains s, p, and
d subshells.
Each subshell contains one or more orbitals. Each orbital
can hold up to two electrons. An s subshell contains only
one orbital, so it can hold up to two electrons.
A p subshell contains three orbitals, as shown in the figures on the next page. The different orbitals have different
orientations, or directions, in space. Each orbital can hold
up to two electrons. Therefore, the three orbitals in the
p subshell together can hold up to six electrons.
7. Describe An atom
contains nine electrons. In
which energy levels are the
electrons located?
READING CHECK
8. Define What are valence
electrons?
READING CHECK
9. Describe How many
electrons can an s subshell
hold?
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Modern Atomic Theory continued
y
y
y
z
z
z
EHHDBG@<EHL>K
10. Compare How are the
three p orbitals different from
one another?
x
x
x
There are three kinds of p orbitals.
The shapes of the d and f orbitals are very complex. It
is very difficult to show them in a drawing. There are five
different kinds of d orbitals. Therefore, all the d orbitals
together can hold up to ten electrons. There are seven
different kinds of f orbitals, so all the f orbitals together can
hold up to 14 electrons.
KXcb8Yflk@k
Discuss It can be difficult to
understand energy levels and
orbitals. After you read about
energy levels and orbitals,
discuss any questions you
have with a small group.
EHHDBG@<EHL>K
11. Describe An atom has
electrons in the first and
second energy levels. What
is the greatest number of
electrons it could have?
ELECTRONS AND ORBITALS
Every energy level contains a certain number of
orbitals. Each orbital can hold two electrons. Therefore,
the number of orbitals determines the total number of
electrons in that energy level. For instance, the second
energy level of an atom has four orbitals: one s orbital
and three p orbitals. Therefore, the second energy level
can hold up to eight electrons. The figure below shows
the maximum number of electrons in each energy level.
Energy
level
Number of
Total number
orbitals by type of orbitals
s
p
d
1
1
2
1
3
3
1
3
5
4
1
3
5
f
7
Maximum
number of
electrons
1
2
1+3=4
8
1+3+5=9
18
1 + 3 + 5 + 7 = 16
32
How Can Electrons Move in an Atom?
An electron is never found between energy levels.
Instead, it “jumps” from one level to the next. What
makes an atom move from one energy level to another?
Electrons move between energy levels when an atom
gains or loses energy.
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Modern Atomic Theory continued
GROUND STATE AND EXCITED STATE
The lowest state of energy of an electron is called its
ground state. When an electron gains energy, it moves to
an excited state in a higher energy level. Electrons gain
energy by absorbing photons. A photon is the smallest
unit of light energy. It is a little bit like an atom of light.
When an electron falls to a lower energy level, it releases
one photon.
Photons have different energies. The energy of the
photon affects which energy level an electron can move
to. The higher the energy of the photon, the higher the
energy level the electron can jump to.
READING CHECK
12. Define What is the
ground state of an electron?
Photon is
absorbed
Key:
Electron
Energy level
When an electron absorbs a photon, the electron can jump to a
higher energy level.
EHHDBG@<EHL>K
13. Identify On the figure,
label the correct illustrations
with “ground state” or
“excited state.”
ATOMS AND LIGHT
The energy of a photon is related to the wavelength of
the light. Atoms of a given element can only gain or lose
energy in specific amounts. These amounts of energy
correspond to light of certain wavelengths, or colors.
Therefore, the colors of light that are emitted or absorbed
by an element can be used to identify the element.
For example, neon signs use energy from electricity
to excite atoms of neon gas. The atoms of neon first gain
the energy and then release it in the form of light. The
wavelength of visible light determines the color of light.
Fireworks are another example. The colors of fireworks
are produced by burning compounds of magnesium,
aluminum, and sodium salts.
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Section 3 Review
SECTION VOCABULARY
orbital a region in an atom where there is a
high probability of finding electrons
photon a unit or quantum of light; a particle of
electromagnetic radiation that has zero rest
mass and carries a quantum of energy
valence electron an electron that is found in the
outermost shell of an atom and that determines
the atom’s chemical properties
1. Apply Concepts An atom’s valence electrons are in the second energy level. Which
two orbitals could the valence electrons be in?
2. Describe Relationships How are energy levels and orbitals related?
3. Describe An atom of nitrogen contains seven electrons. Describe the number of
electrons in each energy level in an atom of nitrogen.
4. Infer How many valence electrons does an atom of nitrogen have? Explain your
answer.
5. Explain According to the modern atomic theory, what can scientists not know
about an electron?
6. Apply Concepts Aluminum has an atomic number of 13. In which energy levels are
the electrons in an aluminum atom? (Hint: Remember that atoms have the same
number of electrons as protons.)
7. Identify How many valence electrons does an atom of aluminum have? Explain
your answer.
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Atoms