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Chapter 9
Introduction to Atoms
Table of Contents
Section 1 Development of the Atomic Theory
Section 2 The Atom
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Chapter 9
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 9
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 9
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 9
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 9
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 9
Section 1 Development of the
Atomic Theory
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Chapter 9
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 9
Section 1 Development of the
Atomic Theory
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Chapter 9
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 9
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.
•http://youtu.be/zYeRSgiy
pbc
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Chapter 9
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 9
Section 2 The Atom
Objectives, continued
• State how isotopes differ.
• Calculate atomic masses.
• Describe the forces within an atom.
•http://youtu.be/O5iaw5WNuB0
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Chapter 9
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 9
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 9
Section 2 The Atom
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Chapter 9
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 9
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 9
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 9
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 9
Section 2 The Atom
Isotopes, continued
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Chapter 9
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.
•http://youtu.be/O5iaw5WNuB0
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Chapter 9
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 9
Section 2 The Atom
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