Chapter Introduction
Lesson 1 Discovering Parts
of an Atom
Lesson 2 Protons, Neutrons,
and Electrons—
How Atoms Differ
Chapter Wrap-Up
What are atoms, and
what are they made of?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1. The earliest model of an atom
contained only protons and electrons.
2. Air fills most of an atom.
3. In the present-day model of the atom,
the nucleus of the atom is at the center
of an electron cloud.
Do you agree or disagree?
4. All atoms of the same element have
the same number of protons.
5. Atoms of one element cannot be
changed into atoms of another
element.
6. Ions form when atoms lose or gain
electrons.
Discovering Part of an Atom
• What is an atom?
• How would you describe the size of an
atom?
• How has the atomic model changed
over time?
Discovering Part of an Atom
• atom
• neutron
• electron
• electron cloud
• nucleus
• proton
Early Ideas About Matter
Democritus (460–370 BC) believed that
matter is made of small, solid objects
called atomos, from which the English
word atom is derived.
Early Ideas About Matter (cont.)
• Aristotle (384–322 BC) did not believe
that empty space exists, but instead
believed that all matter is made of fire,
water, air, and earth.
• Because Aristotle was so influential,
his ideas were accepted and
Democritus’s ideas about atoms were
not studied again for more than 2,000
years.
Dalton’s Atomic Model
John Dalton combined data from his own
scientific research with data from the
research of other scientists to propose a
new atomic theory.
The Atom
An atom is the smallest piece of an
element that still represents that element.
What is a copper atom?
The Atom (cont.)
• Atoms of different elements are different
sizes, but all are very, very small.
• You cannot see atoms with just your
eyes or even with most microscopes.
How would you describe the
size of an atom?
The Atom (cont.)
• The 1981 invention of a high-powered
microscope, called a scanning tunneling
microscope (STM), enabled scientists to
see individual atoms for the first time.
• Scientists have learned that atoms are
not the smallest particles of matter.
Following his experiments with cathode ray
tubes, scientist J.J. Thomson concluded that
cathode rays were made of small, negatively
charged particles which he called electrons.
Thomson—Discovering Electrons
An electron is a particle with one negative
charge (1–).
electron
from Greek electron, means “amber,”
the physical force so called because
it first was generated by rubbing
amber. Amber is a fossilized
substance produced by trees.
Thomson—Discovering Electrons (cont.)
• Because atoms are neutral, or not
electrically charged, Thomson proposed
that atoms also must contain a positive
charge that balances the negatively
charged electrons.
• Thomson’s proposed atom was a sphere
with a positive charge evenly spread
throughout and negatively charged
electrons within it.
Thomson’s model of the atom contained a
sphere of positive charge with negatively
charged electrons within it.
Rutherford—Discovering
the Nucleus
Scientist Ernest Rutherford set up
experiments to test Thomson’s atomic
model and to learn more about what
atoms contain.
Rutherford expected the positive alpha
particles to travel straight through the foil
without changing direction.
Some alpha particles traveled in a straight
path, as expected. But some changed
direction, and some bounced straight back.
Rutherford—Discovering
the Nucleus (cont.)
Given the results of the gold
foil experiment, how do you
think an actual atom differs
from Thomson’s model?
Rutherford—Discovering
the Nucleus (cont.)
• Rutherford concluded that most of an
atom’s mass and positive charge is
concentrated in a small area in the
center of the atom called the nucleus.
• Additional research showed that the
positive charge in the nucleus was made
of positively charged particles called
protons.
Rutherford—Discovering
the Nucleus (cont.)
• A proton is an atomic particle that has
one positive charge (1+).
• Negatively charged electrons move in
the empty space surrounding the
nucleus.
Rutherford’s model contains a small, dense,
positive nucleus. Tiny, negatively charged
electrons travel in empty space around the
nucleus.
Discovering Neutrons
• James Chadwick discovered that, in
addition to protons, the nucleus also
contained neutrons.
• A neutron is a neutral particle that exists
in the nucleus of an atom.
Bohr’s Atomic Model
• Niels Bohr proposed that electrons
move in circular orbits, or energy levels,
around the nucleus.
• Electrons closer to the nucleus have
less energy than electrons farther away
from the nucleus.
Bohr’s Atomic Model (cont.)
• More research showed that, although
electrons have specific amounts of
energy, energy levels are not arranged
in circular orbits.
• When an electron moves from a higher
energy level to a lower energy level,
energy is released—sometimes as
visible light.
In Bohr’s model of the atom, electrons
move in circular orbits around the atom.
Bohr’s Atomic Model (cont.)
How did Bohr’s model of the
atom differ from Rutherford’s?
The Modern Atomic Model
• In the modern atomic model, electrons
form an electron cloud.
• An electron cloud is an area around an
atomic nucleus where an electron is
most likely to be.
In this atom, electrons are more likely
to be found closer to the nucleus than
farther away.
The Modern Atomic Model (cont.)
How has the model of the atom
changed over time?
Quarks
• Protons and neutrons are made of
smaller particles called quarks.
• Scientists theorize that there are six
types of quarks: up, down, charm,
strange, top, and bottom.
• Protons are made of two up quarks and
one down quark.
Quarks (cont.)
• Neutrons are made of two down quarks
and one up quark.
• The current atomic model might change
with the invention of new technology
that aids the discovery of new
information.
• If you were to divide an element into
smaller and smaller pieces, the
smallest piece would be an atom.
• Atoms are so small that they can be
seen only by using very powerful
microscopes.
• Scientists now know that atoms
contain a dense, positive nucleus
surrounded by an electron cloud.
Which term describes a particle
with one negative charge?
A. atom
B. electron
C. nucleus
D. proton
Whose model of the atom
contained a sphere of positive
charge with negatively charged
electrons within it?
A. Dalton
B. Democritus
C. Rutherford
D. Thomson
Which term refers to an area
around an atomic nucleus where
an electron is most likely to be?
A. electron cloud
B. neutron
C. nucleus
D. proton
Do you agree or disagree?
1. The earliest model of an atom
contained only protons and electrons.
2. Air fills most of an atom.
3. In the present-day model of the atom,
the nucleus of the atom is at the center
of an electron cloud.
Protons, Neutrons, and
Electrons—How Atoms Differ
• What happens during nuclear decay?
• How does a neutral atom change
when its number of protons, electrons,
or neutrons changes?
Protons, Neutrons, and
Electrons—How Atoms Differ
• atomic number
• radioactive
• isotope
• nuclear decay
• mass number
• ion
• average atomic
mass
The Parts of the Atom
• The mass of
electrons is
much smaller
than the mass
of protons or
neutrons.
• Most of the
mass of an
atom is found
in the nucleus.
Different Elements—Different
Numbers of Protons
• The number of protons in an atom of an
element is the element’s atomic
number.
• The atomic number is the whole number
listed with each element on the periodic
table.
• Atoms of different elements contain
different numbers of protons.
Different elements have different atomic
numbers.
Different Elements—Different
Numbers of Protons (cont.)
• Neutral atoms of different elements also
have different numbers of electrons.
• In a neutral atom, the number of
electrons equals the number of protons;
therefore, the number of positive charges
equals the number of negative charges.
Neutrons and Isotopes
• Atoms of the same element can have
different numbers of neutrons.
• Isotopes are atoms of the same element
that have different numbers of neutrons.
• Most elements have several isotopes.
Neutrons and Isotopes (cont.)
isotope
from Greek isos, means “equal”;
and topos, means “place”
Neutrons and Isotopes (cont.)
• The mass number of an atom is the
sum of the number of protons and
neutrons in an atom.
Mass number = number of protons + number of neutrons
• An isotope is often written with the
element name followed by the mass
number.
Neutrons and Isotopes (cont.)
The average atomic
mass of an element is
the average mass of the
element’s isotopes,
weighted according to
the abundance of each
isotope.
Radioactivity
• Marie Curie called elements that
spontaneously emit radiation
radioactive.
• Henri Becquerel and Pierre and Marie
Curie discovered that the radiation
released by uranium was made of
energy and particles.
Radioactivity (cont.)
• This radiation came from the nuclei of
the uranium atoms.
• When uranium releases radiation, it
changes to a different element.
Radioactivity (cont.)
• Nuclear decay is a process that occurs
when an unstable atomic nucleus
changes into another more stable
nucleus by emitting radiation.
• Nuclear decay can produce three
different types of radiation—alpha
particles, beta particles, and gamma
rays.
An alpha particle is made of two protons and
two neutrons. When an atom releases an
alpha particle, its atomic number decreases
by two.
When beta decay occurs, a neutron changes
into a proton and a high-energy electron
called a beta particle. The atomic number
of an atom increases by one because it has
gained a proton.
Because gamma rays do not contain
particles, the release of gamma rays does
not change one element into another
element.
Radioactivity (cont.)
What happens during
radioactive decay?
Radioactivity (cont.)
• The energy released by radioactive
decay can be both harmful and beneficial
to humans.
• Radiation therapy can be beneficial to
humans by destroying harmful cells such
as cancer cells.
Ions—Gaining or Losing Electrons
• An ion is an atom that is no longer
neutral because it has gained or lost
electrons.
• An ion can be positively or negatively
charged depending on whether it has
lost or gained electrons.
• When a neutral atom loses one or more
electrons, it has more protons than
electrons and as a result, has a positive
charge.
• An atom with a positive charge is
called a positive ion.
• When a neutral atom gains one or more
electrons, it now has more electrons than
protons and as a result, has a negative
charge.
• An atom with a negative charge is
called a negative ion.
Ions—Gaining or Losing Electrons
(cont.)
How does a neutral atom change
when its number of protons or
electrons changes?
• Different elements contain different
numbers of protons.
• Two isotopes of a given element
contain different numbers of neutrons.
• When a neutral atom gains or loses
an electron, it becomes an ion.
Where is most of the mass of
an atom found?
A. electrons
B. neutrons
C. nucleus
D. protons
Which term refers to the sum
of the number of protons and
neutrons in an atom?
A. atomic number
B. average atomic mass
C. isotope
D. mass number
What term did Marie Curie use
to describe elements that
spontaneously emit radiation?
A. ion
B. isotopes
C. nuclear decay
D. radioactive
Do you agree or disagree?
4. All atoms of the same element have
the same number of protons.
5. Atoms of one element cannot be
changed into atoms of another
element.
6. Ions form when atoms lose or gain
electrons.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
An atom is the
smallest unit of an
element and is made
mostly of empty space.
It contains a tiny
nucleus surrounded
by an electron cloud.
Lesson 1: Discovering Parts of the Atom
• If you were to divide an element into smaller and
smaller pieces, the smallest piece would be an
atom.
• Atoms are so small that they can be seen only by
powerful scanning microscopes.
• The first model of the
atom was a solid sphere.
Now, scientists know that
an atom contains a dense
positive nucleus
surrounded by an electron
cloud.
Lesson 2: Protons, Neutrons, and
Electrons—How Atoms Differ
• Nuclear decay occurs when
an unstable atomic nucleus
changes into another more
stable nucleus by emitting
radiation.
• Different elements contain
different numbers of protons.
Two isotopes of the same
element contain different
numbers of neutrons. When a neutral atom gains or
loses an electron, it becomes an ion.
Which term describes a neutral
particle that exists in the nucleus
of an atom?
A. atom
B. electron
C. neutron
D. proton
Who discovered that, in addition
to protons, the nucleus also
contained neutrons?
A. Thomson
B. Rutherford
C. Chadwick
D. Bohr
Protons and neutrons are made
of smaller particles called what?
A. electron cloud
B. isotope
C. nucleus
D. quarks
Which term refers to the process
that occurs when an unstable
atomic nucleus changes into
another more stable nucleus
by emitting radiation?
A.
B.
C.
D.
radiation
radioactivity
nuclear decay
radiation therapy
Which describes an atom with a
positive charge?
A. positive ion
B. negative ion
C. isotope
D. quarks
Which term refers to the smallest
piece of an element that still
represents that element?
A. quark
B. nucleus
C. electron
D. atom
Who concluded that most of an
atom’s mass and positive charge
is concentrated in the nucleus?
A. Aristotle
B. Bohr
C. Chadwick
D. Rutherford
Which is an atomic particle that
has one positive charge?
A. proton
B. nucleus
C. neutron
D. electron
Which term refers to an atom that
is no longer neutral because it
has gained or lost electrons?
A. ion
B. isotope
C. neutron
D. proton
Which refers to the average mass
of an element’s isotopes,
weighted according to the
abundance of each isotope?
A. atomic number
B. mass number
C. relative mass
D. average atomic mass