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Chapter 6
Introduction to Atoms
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Section 1 Development of the Atomic Theory
Section 2 The Atom
Concept Map
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Chapter 6
Section 1 Development of the
Atomic Theory
Bellringer
In your Science Journal, draw a picture that shows
what you think an atom looks like. On your drawing,
include labels for the locations of protons, neutrons,
and electrons.
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Chapter 6
Section 1 Development of the
Atomic Theory
What You Will Learn
• There have been different models of the atom over
time.
• The atomic theory has changed as scientists have
experimented and discovered new information about
the atom.
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Chapter 6
Section 1 Development of the
Atomic Theory
The Beginning of Atomic Theory
• Matter is made of particles that we call atoms.
• An atom is the smallest particle into which an
element can be divided and still have the properties
of that element.
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Chapter 6
Introduction to Atoms
Atom
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Chapter 6
Section 1 Development of the
Atomic Theory
Dalton’s Atomic Theory Based on
Experiments
• By the late 1700s, scientists had learned that
elements combine in certain proportions based on
mass to form compounds.
• Dalton’s atomic theory, published in 1803, suggested
that elements combine in certain proportions because
they are made of atoms.
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Chapter 6
Section 1 Development of the
Atomic Theory
Thomson’s Discovery of Electrons
• In 1897, Thomson’s experiments led him to conclude
that there must be negatively charged particles inside
the atom. These particles are now called electrons.
• An electron is a subatomic particle that has a
negative charge.
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Chapter 6
Introduction to Atoms
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Chapter 6
Section 1 Development of the
Atomic Theory
Rutherford’s Atomic “Shooting Gallery”
• In 1909, Rutherford designed an experiment to study
the parts of an atom.
• The results of Rutherford’s gold-foil experiment
suggested that atoms are not all the same
throughout. He concluded that atoms have different
parts.
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Chapter 6
Introduction to Atoms
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Chapter 6
Section 1 Development of the
Atomic Theory
The Nucleus and the Electrons
• Rutherford’s results led him to conclude that most
matter in an atom is found in one very small area.
• In 1911, Rutherford revised the atomic theory and
proposed that in the center of the atom is a tiny,
extremely dense, positively charged area called the
nucleus.
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Chapter 6
Section 1 Development of the
Atomic Theory
The Nucleus and the Electrons, continued
• In physical science, the nucleus is an atom’s central
region which is made of protons and neutrons.
• From Rutherford’s results, the important idea
emerged that atoms are mostly empty space with a
tiny, massive nucleus at the center and electrons
surrounding the nucleus.
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Chapter 6
Section 1 Development of the
Atomic Theory
The Nucleus and the Electrons, continued
• In 1913, Bohr’s results led him to propose that
electrons move around the nucleus in definite paths.
• According to modern atomic theory, electrons are
likely to be found moving in certain regions around
the nucleus of an atom. These regions are called
electron clouds, or orbitals.
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Chapter 6
Section 1 Development of the
Atomic Theory
The Nucleus and the Electrons, continued
• Electron clouds are regions around the nucleus of
an atom where electrons are likely to be found. Each
electron cloud has a definite energy level.
• Therefore, each electron in an atom has a definite
energy based on which electron cloud the electron is
moving in.
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Chapter 6
Introduction to Atoms
Electron Cloud
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Chapter 6
Section 1 Development of the
Atomic Theory
The Size of an Atom
• Atoms are so small that light waves are too large to
be used to observe them. Scientists use scanning
tunneling electron microscopes to provide images of
atoms.
• However, these images are not an actual picture of
the atom. They show an image of the surface of a
material at the atomic level.
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Chapter 6
Section 2 The Atom
Bellringer
An atom is the smallest particle of an element.
Atoms are made of protons, neutrons, and electrons.
Write a paragraph in your Science Journal explaining
how the two sentences above relate to one another.
Are they both true? If so, how is that possible?
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Chapter 6
Section 2 The Atom
What You Will Learn
• Protons, neutrons, and electrons make up atoms.
• All atoms of a given element have the same number
of protons in the nucleus.
• Isotopes of an element differ by the number of
neutrons in the nucleus.
• Atomic mass is an average of the masses of all of the
naturally occurring isotopes of an element.
• Four forces are at work in atoms.
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Chapter 6
Section 2 The Atom
The Parts of an Atom
• Almost all kinds of atoms are made of the same three
particles. These particles are protons, neutrons, and
electrons.
• Protons, neutrons, and electrons are called
subatomic particles because they are each much
smaller than an atom.
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Chapter 6
Section 2 The Atom
The Parts of an Atom, continued
• The number of subatomic particles that are in an
atom and the way the particles interact determine the
properties of an atom.
• A proton is a subatomic particle that has a positive
charge and that is located in the nucleus of an atom.
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Chapter 6
Section 2 The Atom
The Parts of an Atom, continued
• A neutron is a subatomic particle that has no charge
and that is located in the nucleus of an atom.
• An electron is a subatomic particle that has a
negative charge and that is found outside the nucleus
in electron clouds.
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Chapter 6
Section 2 The Atom
The Parts of an Atom, continued
• The charges of protons and electrons are opposite
but equal, so the charges cancel out. If the numbers
of electrons and protons become unequal, the atom
becomes a charged particle called an ion.
• The SI unit that is used to express the mass of a
particle in an atom is the atomic mass unit (amu).
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Chapter 6
Introduction to Atoms
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Chapter 6
Section 2 The Atom
Atoms and Elements
• There are more than 110 different elements. The
atoms of each of these elements are different from
the atoms of all other elements.
• Electrons and protons are found in all atoms.
• An atom does not have to have equal numbers of
neutrons and protons.
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Chapter 6
Section 2 The Atom
Atoms and Elements, continued
• The atomic number is the number of protons in the
nucleus of an atom.
• All atoms of the same element have the same atomic
number.
• The atomic number of each element is listed on the
periodic table.
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Chapter 6
Introduction to Atoms
Atomic Number
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Chapter 6
Section 2 The Atom
Isotopes
• An isotope is an atom that has the same number of
protons as other atoms of the same element do, but
has a different number of neutrons.
• Atoms that are isotopes of each other are always the
same element, because isotopes of the same
element always have the same number of protons.
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Chapter 6
Section 2 The Atom
Isotopes, continued
• Some isotopes of an element have special properties
because they are unstable. An unstable atom is an
atom with a nucleus that will change over time. This
type of atom is radioactive.
• Isotopes of an element share most of the same
chemical and physical properties.
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Chapter 6
Section 2 The Atom
Isotopes, continued
• Each isotope of an element can be identified by its
mass number. The mass number is the sum of the
protons and neutrons in the nucleus of an atom.
• 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.
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Chapter 6
Introduction to Atoms
Mass Number
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Chapter 6
Introduction to Atoms
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Chapter 6
Section 2 The Atom
Isotopes, continued
• Most elements contain a mixture of two or more
isotopes.
• 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 6
Section 2 The Atom
Forces in Atoms
• There are four basic forces that are at work
everywhere in nature, even within the atom.
These forces are gravitational force,
electromagnetic force, strong force, and weak
force.
• These four forces work together to give an atom
its structure and properties.
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Chapter 6
Introduction to Atoms
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Chapter 6
Introduction to Atoms
Concept Map
Use the terms below to complete the concept map
on the next slide.
nucleus
mass number
isotopes
protons
atoms
electrons
atomic number
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Chapter 6
Introduction to Atoms
Concept Map
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Chapter 6
Introduction to Atoms
Concept Map
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