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Chapter 11
Section 1 Development of the
Atomic Theory
Objectives
• Describe some of the experiments that led to the
current atomic theory.
• Compare the different models of the atom.
• Explain how the atomic theory has changed as
scientists have discovered new information about the
atom.
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Chapter 11
Section 1 Development of the
Atomic Theory
The Beginning of the Atomic Theory
• What Is an Element? Around 440 BCE, a Greek
philosopher named Democritus thought that you would
eventually end up with a particle that could not be cut.
He called this particle an atom.
• From Aristotle to Modern Science Aristotle,
another Greek philosopher, disagreed with
Democritus’s ideas. He believed that you would never
end up with a particle that could not be cut.
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Chapter 11
Section 1 Development of the
Atomic Theory
The Beginning of the Atomic Theory,
continued
• From Aristotle to Modern Science Democritus was
right, though: Matter is made of particles, which we call
atoms. An atom is the smallest particle into which an
element can be divided and still be the same
substance.
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Chapter 11
Section 1 Development of the
Atomic Theory
Dalton’s Atomic Theory Based on
Experiments
• Dalton’s Theory John Dalton published his atomic
theory in 1803. His theory stated that all substances
are made of atoms. Atoms are small particles that
cannot be created, divided, or destroyed. Atoms of the
same element are exactly alike, and atoms of different
elements are different. Atoms join with other atoms to
make new substances.
• Not Quite Correct The atomic theory was then
changed to describe the atom more correctly.
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Chapter 11
Section 1 Development of the
Atomic Theory
Thomson’s Discovery of Electrons
• Negatively Charged Particles Thomson
experimented with a cathode-ray tube like the one
shown on the next slide. He discovered negatively
charged particles that are now known as electrons.
• Like Plums in Pudding After learning that atoms
contain electrons, Thomson proposed a new model of
the atom. Thomson thought that electrons were mixed
throughout an atom, like plums in a pudding.
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Chapter 11
Section 1 Development of the
Atomic Theory
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Chapter 11
Section 1 Development of the
Atomic Theory
Rutherford’s Atomic “Shooting Gallery”
• Negatively Charged Particles In 1909, Ernest
Rutherford aimed a beam of small, positively charged
particles at a thin sheet of gold foil. The next slide
shows his experiment.
• Surprising Results Rutherford expected the
particles to pass right through the gold in a straight
line. To Rutherford’s great surprise, some of the
particles were deflected.
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Chapter 11
Section 1 Development of the
Atomic Theory
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Chapter 11
Section 1 Development of the
Atomic Theory
Where Are the Electrons?
• Far from the Nucleus
Rutherford proposed that in the
center of the atom is a tiny,
positively charged part called
the nucleus.
• Bohr’s Electron Levels In
1913, Niels Bohr proposed that
electrons move around the
nucleus in certain paths, or
energy levels.
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Chapter 11
Section 1 Development of the
Atomic Theory
Where Are the Electrons?, continued
• The Modern Atomic
Theory According to the
current theory, there are
regions inside the atom
where electrons are likely
to found. These regions
are called electron
clouds.
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Chapter 11
Section 2 The Atom
Objectives
• Describe the size of an atom.
• Name the parts of an atom.
• Describe the relationship between numbers of
protons and neutrons and atomic number.
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Chapter 11
Section 2 The Atom
Objectives, continued
• State how isotopes differ.
• Calculate atomic masses.
• Describe the forces within an atom.
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Chapter 11
Section 2 The Atom
How Small Is an Atom?
• Three One-Hundred-Millionths of a Centimeter!
Scientists know that aluminum is made of averagesized atoms. An aluminum atom has a diameter of
about 0.00000003 cm.
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Chapter 11
Section 2 The Atom
What Is an Atom Made Of?
• The Nucleus Protons are positively charged
particles in the nucleus. Neutrons are the particles of
the nucleus that have no electrical charge.
• Outside the Nucleus Electrons are the negatively
charged particles in atoms. Electrons are found around
the nucleus within electron clouds. All the structures of
the atom can be seen on the next slide.
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Chapter 11
Section 2 The Atom
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Chapter 11
Section 2 The Atom
How Do Atoms of Different Elements Differ?
• Starting Simply The
hydrogen atom has one
proton and one electron.
• Now for Some
Neutrons The helium
atom has two protons, two
neutrons, and two
electrons.
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Chapter 11
Section 2 The Atom
How Do Atoms of Different Elements Differ?
• Building Bigger Atoms For bigger atoms, simply
add protons, neutrons, and electrons.
• Protons and Atomic Number The number of
protons in the nucleus of an atom is the atomic number
of that atom. All atoms of an element have the same
atomic number.
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Chapter 11
Section 2 The Atom
Isotopes
• Isotopes are atoms that have the same number of
protons but have different numbers of neutrons.
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Chapter 11
Section 2 The Atom
Isotopes, continued
• Properties of Isotopes An unstable atom is an atom
with a nucleus that will change over time. This type of
isotope is radioactive.
• Telling Isotopes Apart You can identify each
isotope of an element by its mass number. The mass
number is the sum of the protons and neutrons in an
atom.
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Chapter 11
Section 2 The Atom
Isotopes, continued
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Chapter 11
Section 2 The Atom
Isotopes, continued
• Naming Isotopes To identify a specific isotope of an
element, write the name of the element followed by a
hyphen and the mass number of the isotope.
• Calculating the Mass of an Element The atomic
mass of an element is the weighted average of the
masses of all the naturally occurring isotopes of that
element.
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Chapter 11
Section 2 The Atom
Forces in Atoms
• Four Basic Forces Four basic forces are at work
everywhere, even within the atom. These forces are
gravitational force, electromagnetic force, strong force,
and weak force.
• These forces work together to give an atom its
structure and properties.
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Chapter 11
Section 2 The Atom
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Chapter 11
Introduction to Atoms
Concept Map
Use the terms below to complete the concept map
on the next slide.
a nucleus
mass number
isotopes
protons
atoms
electrons
atomic number
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Chapter 11
Introduction to Atoms
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Chapter 11
Introduction to Atoms
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End of Chapter 11 Show
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Chapter 11
Standardized Test Preparation
Reading
Read each of the passages. Then, answer the questions
that follow each passage.
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Chapter 11
Standardized Test Preparation
Passage 1 In the Bohr model of the atom, electrons
can be found only in certain energy levels. Electrons
“jump” from one level to the next level without
passing through any of the regions in between.
When an electron moves from one level to another,
it gains or loses energy, depending on the direction
of its jump. Bohr’s model explained an unusual
event. When electric charges pass through atoms of
a gaseous element, the gas produces a glowing
light, like in a neon sign.
Continued on the next slide
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Chapter 11
Standardized Test Preparation
Passage 1, continued If this light is passed through
a prism, a pattern of lines appears, each line having
a different color. The pattern depends on the
element—neon has one pattern, and helium has
another. In Bohr’s model, the lines are caused by
electron jumps from higher to lower energy levels.
Because only certain jumps are possible, electrons
release energy only in certain quantities. These
“packets” of energy produce the lines that are seen.
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Chapter 11
Standardized Test Preparation
1. In the Bohr model of the atom, what limitation
is placed on electrons?
A the number of electrons in an atom
B the electrons’ being found only in certain energy
levels
C the size of electrons
D the speed of electrons
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Chapter 11
Standardized Test Preparation
1. In the Bohr model of the atom, what limitation
is placed on electrons?
A the number of electrons in an atom
B the electrons’ being found only in certain energy
levels
C the size of electrons
D the speed of electrons
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Chapter 11
Standardized Test Preparation
2. What causes the colored lines that appear
when the light from a gas is passed through a
prism?
F packets of energy released by electron jumps
G electrons changing color
H atoms of the gas exchanging electrons
I There is not enough information to determine the
answer.
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Chapter 11
Standardized Test Preparation
2. What causes the colored lines that appear
when the light from a gas is passed through a
prism?
F packets of energy released by electron jumps
G electrons changing color
H atoms of the gas exchanging electrons
I There is not enough information to determine the
answer.
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Chapter 11
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Passage 2 No one has ever seen a living dinosaur,
but scientists have determined the appearance of
Tyrannosaurus rex by studying fossilized skeletons.
Scientists theorize that these extinct creatures had
big hind legs, small front legs, a long, whip-like tail,
and a mouth full of dagger-shaped teeth. However,
theories of how T. rex walked have been harder to
develop. For many years, most scientists thought
that T. rex plodded slowly like a big, lazy lizard.
Continued on the next slide
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Passage 2, continued However, after studying wellpreserved dinosaur tracks and noticing skeletal
similarities between certain dinosaur fossils and
living creatures like the ostrich, many scientists now
theorize that T. rex could turn on the speed. Some
scientists estimate that T. rex had bursts of speed of
32 km/h (20 mi/h)!
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1. According to this passage, where does most
of what we know about the appearance of
Tyrannosaurus rex come from?
A fossilized skeletons
B dinosaur tracks
C living organisms such as the ostrich
D living specimens of T. rex
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Chapter 11
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1. According to this passage, where does most
of what we know about the appearance of
Tyrannosaurus rex come from?
A fossilized skeletons
B dinosaur tracks
C living organisms such as the ostrich
D living specimens of T. rex
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Chapter 11
Standardized Test Preparation
2. How did scientists conclude that T. rex could
probably move very quickly?
F They measured the speed at which it could run.
G They compared fossilized T. rex tracks with T. rex
skeletons.
H They studied dinosaur tracks and noted
similarities between ostrich skeletons and T. rex
skeletons.
I They measured the speed at which ostriches could
run.
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Chapter 11
Standardized Test Preparation
2. How did scientists conclude that T. rex could
probably move very quickly?
F They measured the speed at which it could run.
G They compared fossilized T. rex tracks with T. rex
skeletons.
H They studied dinosaur tracks and noted
similarities between ostrich skeletons and T. rex
skeletons.
I They measured the speed at which ostriches could
run.
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Chapter 11
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Interpreting Graphics
Use the diagram of an atom below to answer the
questions that follow.
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Chapter 11
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1. The black circles in the center of the model
represent neutrons. What do the white circles in
the center represent?
A electrons
B protons
C nuclei
D atoms
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Chapter 11
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1. The black circles in the center of the model
represent neutrons. What do the white circles in
the center represent?
A electrons
B protons
C nuclei
D atoms
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2. What is the mass number of the atom shown
in the model?
F 3
G 7
H 9
I 11
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2. What is the mass number of the atom shown
in the model?
F 3
G 7
H 9
I 11
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3. What is the overall charge of the atom shown
in the model?
A 12
B 11
C 0
D 21
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3. What is the overall charge of the atom shown
in the model?
A 12
B 11
C 0
D 21
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Math
Read each question, and choose the best answer.
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Chapter 11
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1. Aimee, Mari, and Brooke are 163 cm, 171 cm,
and 175 cm tall. Which of the following
measurements is a reasonable average height of
these three friends?
A 170 cm
B 175 cm
C 255 cm
D 509 cm
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Chapter 11
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1. Aimee, Mari, and Brooke are 163 cm, 171 cm,
and 175 cm tall. Which of the following
measurements is a reasonable average height of
these three friends?
A 170 cm
B 175 cm
C 255 cm
D 509 cm
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Chapter 11
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2. A certain school has 40 classrooms. Most of
the classrooms have 25 to 30 students. Which of
the following is a reasonable estimate of the
number of students that go to this school?
F 40 students
G 100 students
H 1,100 students
I 2,000 students
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Chapter 11
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2. A certain school has 40 classrooms. Most of
the classrooms have 25 to 30 students. Which of
the following is a reasonable estimate of the
number of students that go to this school?
F 40 students
G 100 students
H 1,100 students
I 2,000 students
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3. Jenna is setting up a fish tank in her room.
The tank is the shape of a rectangular prism. The
height of the tank is 38 cm, the width is 23 cm,
and the length is 62 cm. The tank is filled with
water to a point that is 7 cm from the top. How
much water is in the tank?
A 44,206 cm3
B 48,070 cm3
C 54,188 cm3
D 64,170 cm3
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Chapter 11
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3. Jenna is setting up a fish tank in her room.
The tank is the shape of a rectangular prism. The
height of the tank is 38 cm, the width is 23 cm,
and the length is 62 cm. The tank is filled with
water to a point that is 7 cm from the top. How
much water is in the tank?
A 44,206 cm3
B 48,070 cm3
C 54,188 cm3
D 64,170 cm3
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4. Which of the following is equal to 85?
F 88888
G 55555555
H 58
I 88888
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Chapter 11
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4. Which of the following is equal to 85?
F 88888
G 55555555
H 58
I 88888
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Chapter 11
Section 1 Development of the
Atomic Theory
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Chapter 11
Section 1 Development of the
Atomic Theory
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Chapter 11
Section 2 The Atom
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Chapter 11
Section 2 The Atom
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Chapter 11
Section 2 The Atom
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Chapter 11
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Chapter 11
Section 2 The Atom
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