The Structure of the Atom
Section 4.1 Early Ideas About
Matter
Section 4.2 Defining the Atom
Section 4.3 How Atoms Differ
Section 4.4 Unstable Nuclei and
Radioactive Decay
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Section 4.1 Early Ideas About Matter
• Compare and contrast the atomic models of
Democritus, Aristotle, and Dalton.
• Understand how Dalton's theory explains the
conservation of mass.
theory: an explanation supported by many
experiments; is still subject to new experimental
data, can be modified, and is considered
successful if it can be used to make predictions
that are true
Section 4.1 Early Ideas About Matter (cont.)
Dalton's atomic theory
The ancient Greeks tried to explain
matter, but the scientific study of the
atom began with John Dalton in the
early 1800's.
Greek Philosophers (cont.)
• Many ancient scholars believed matter was
composed of such things as earth, water,
air, and fire.
• Many believed
matter could be
endlessly divided
into smaller and
smaller pieces.
Greek Philosophers (cont.)
• Democritus (460–370 B.C.) was the first
person to propose the idea that matter was
not infinitely divisible, but made up of
individual particles called atomos.
• Aristotle (484–322 B.C.) disagreed with
Democritus because he did not believe empty
space could exist.
• Aristotle’s views went unchallenged for 2,000
years until science developed methods to test
the validity of his ideas.
Greek Philosophers (cont.)
Greek Philosophers (cont.)
• John Dalton revived the idea of the atom in
the early 1800s based on numerous
chemical reactions.
• Dalton’s atomic theory easily explained
conservation of mass in a reaction as the
result of the combination, separation, or
rearrangement of atoms.
Greek Philosophers (cont.)
Section 4.1 Assessment
Who was the first person to propose the
idea that matter was not infinitely
divisible?
A. Aristotle
A
0%
D
D. Democritus
C
C. Dalton
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Plato
Section 4.1 Assessment
Dalton’s theory also conveniently
explained what?
A. the electron
B. the nucleus
D
A
0%
C
D. law of Democritus
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. law of conservation of mass
Section 4.2 Defining the Atom
• Define atom.
• Distinguish between the subatomic particles in
terms of relative charge and mass.
• Describe the structure of the atom, including the
locations of the subatomic particles.
model: a visual, verbal, and/or mathematical
explanation of data collected from many
experiments
Section 4.2 Defining the Atom (cont.)
atom
cathode ray
electron
nucleus
proton
neutron
An atom is made of a nucleus containing
protons and neutrons; electrons move
around the nucleus.
The Atom
• The smallest particle of an element that
retains the properties of the element is
called an atom.
• An instrument called the scanning tunneling
microscope (STM) allows individual atoms to
be seen.
The Electron
• When an electric charge is applied, a ray of
radiation travels from the cathode to the
anode, called a cathode ray.
• Cathode rays are a stream of particles
carrying a negative charge.
• The particles carrying a negative charge are
known as electrons.
The Electron (cont.)
• This figure shows a typical cathode ray
tube.
The Electron (cont.)
• J.J. Thomson measured the effects of both
magnetic and electric fields on the cathode
ray to determine the charge-to-mass ratio
of a charged particle, then compared it to
known values.
• The mass of the charged particle was much
less than a hydrogen atom, then the lightest
known atom.
• Thomson received the Nobel Prize in 1906
for identifying the first subatomic particle—the
electron
The Electron (cont.)
• In the early 1910s, Robert Millikan used the
oil-drop apparatus shown below to
determine the charge of an electron.
The Electron (cont.)
• Charges change in discrete amounts—
1.602  10–19 coulombs, the charge of one
electron (now equated to a single unit, 1–).
• With the electron’s charge and charge-tomass ratio known, Millikan calculated the
mass of a single electron.
the mass of
a hydrogen
atom
The Electron (cont.)
• Matter is neutral.
• J.J. Thomson's plum pudding model of the
atom states that the atom is a uniform,
positively changed sphere containing
electrons.
The Nucleus
• In 1911, Ernest Rutherford studied how
positively charged alpha particles
interacted with solid matter.
• By aiming the particles at
a thin sheet of gold foil,
Rutherford expected the
paths of the alpha
particles to be only
slightly altered by a
collision with an electron.
The Nucleus (cont.)
• Although most of the alpha particles went
through the gold foil, a few of them
bounced back, some at large angles.
The Nucleus (cont.)
• Rutherford concluded that atoms are
mostly empty space.
• Almost all of the atom's positive charge and
almost all of its mass is contained in a dense
region in the center of the atom called the
nucleus.
• Electrons are held within the atom by their
attraction to the positively charged nucleus.
The Nucleus (cont.)
• The repulsive force between the positively
charged nucleus and positive alpha
particles caused the deflections.
The Nucleus (cont.)
• Rutherford refined the model to include
positively charged particles in the nucleus
called protons.
• James Chadwick received the Nobel Prize in
1935 for discovering the existence of
neutrons, neutral particles in the nucleus
which accounts for the remainder of an
atom’s mass.
The Nucleus (cont.)
• All atoms are made of three
fundamental subatomic
particles: the electron, the
proton, and the neutron.
• Atoms are spherically
shaped.
• Atoms are mostly empty
space, and electrons travel
around the nucleus held by
an attraction to the positively
charged nucleus.
The Nucleus (cont.)
• Scientists have determined that protons
and neutrons are composed of subatomic
particles called quarks.
The Nucleus (cont.)
• Chemical behavior can be explained by
considering only an atom's electrons.
Section 4.2 Assessment
Atoms are mostly ____.
A. positive
B. negative
C. solid spheres
D
C
A
0%
B
D. empty space
A. A
B. B
C. C
0%
0%
0%
D. D
Section 4.2 Assessment
What are the two fundamental subatomic
particles found in the nucleus?
A. proton and electron
B. proton and neutron
D
A
0%
C
D. neutron and positron
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. neutron and electron
Section 4.3 How Atoms Differ
• Explain the role of atomic number in determining the
identity of an atom.
• Define an isotope.
• Explain why atomic masses are not whole numbers.
• Calculate the number of electrons, protons, and
neutrons in an atom given its mass number and
atomic number.
Section 4.3 How Atoms Differ (cont.)
periodic table: a chart that organizes all known
elements into a grid of horizontal rows (periods)
and vertical columns (groups or families) arranged
by increasing atomic number
atomic number
atomic mass unit (amu)
isotopes
atomic mass
mass number
The number of protons and the mass
number define the type of atom.
Atomic Number
• Each element contains a unique positive
charge in their nucleus.
• The number of protons in the nucleus of an
atom identifies the element and is known as
the element’s atomic number.
Isotopes and Mass Number
• All atoms of a particular element have the
same number of protons and electrons but
the number of neutrons in the nucleus can
differ.
• Atoms with the same number of protons but
different numbers of neutrons are called
isotopes.
Isotopes and Mass Number (cont.)
• The relative abundance of each isotope is
usually constant.
• Isotopes containing more neutrons have a
greater mass.
• Isotopes have the same chemical behavior.
• The mass number is the sum of the protons
and neutrons in the nucleus.
Isotopes and Mass Number (cont.)
Mass of Atoms
• One atomic mass unit (amu) is defined as
1/12th the mass of a carbon-12 atom.
• One amu is nearly, but not exactly, equal to
one proton and one neutron.
Mass of Atoms (cont.)
• The atomic mass of an element is the
weighted average mass of the isotopes of
that element.
Section 4.3 Assessment
An unknown element has 19 protons, 19
electrons, and 3 isotopes with 20, 21 and
22 neutrons. What is the element’s atomic
number?
A. 38
A
0%
D
D. unable to determine
C
C. 19
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. 40
Section 4.3 Assessment
Elements with the same number of
protons and differing numbers of
neutrons are known as what?
A. isotopes
A
0%
D
D. ions
C
C. abundant
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. radioactive
Section 4.4 Unstable Nuclei and
Radioactive Decay
• Explain the relationship between unstable nuclei and
radioactive decay.
• Characterize alpha, beta, and gamma radiation in
terms of mass and charge.
element: a pure substance that cannot be broken
down into simpler substances by physical or chemical
means
Section 4.4 Unstable Nuclei and
Radioactive Decay (cont.)
radioactivity
alpha particle
radiation
nuclear equation
nuclear reaction
beta radiation
radioactive decay
beta particle
alpha radiation
gamma rays
Unstable atoms emit radiation to gain
stability.
Radioactivity
• Nuclear reactions can change one element
into another element.
• In the late 1890s, scientists noticed some
substances spontaneously emitted radiation,
a process they called radioactivity.
• The rays and particles emitted are called
radiation.
• A reaction that involves a change in an atom's
nucleus is called a nuclear reaction.
Radioactive Decay
• Unstable nuclei lose energy by emitting
radiation in a spontaneous process called
radioactive decay.
• Unstable radioactive elements undergo
radioactive decay thus forming stable
nonradioactive elements.
Radioactive Decay (cont.)
• Alpha radiation is made up of positively
charged particles called alpha particles.
• Each alpha particle contains two protons and
two neutrons and has a 2+ charge.
Radioactive Decay (cont.)
• The figure shown below is a nuclear
equation showing the radioactive decay of
radium-226 to radon-222.
• The mass is conserved in nuclear equations.
Radioactive Decay (cont.)
• Beta radiation is radiation that has a
negative charge and emits beta particles.
• Each beta particle is an electron with a 1–
charge.
Radioactive Decay (cont.)
Radioactive Decay (cont.)
• Gamma rays are high-energy radiation
with no mass and are neutral.
• Gamma rays account for most of the energy
lost during radioactive decay.
Radioactive Decay (cont.)
• Atoms that contain too many or two few
neutrons are unstable and lose energy
through radioactive decay to form a stable
nucleus.
• Few exist in nature—most have already
decayed to stable forms.
Section 4.4 Assessment
A reaction that changes one element into
another is called what?
A. chemical reaction
B. beta radiation
D
A
0%
C
D. physical reaction
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. nuclear reaction
Section 4.4 Assessment
Why are radioactive elements rare in
nature?
A. They do no occur on Earth.
A
0%
D
D. They are too hard to detect.
C
C. They take a long time to form.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Most have already decayed to a
stable form.
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Concepts in Motion
Section 4.1 Early Ideas About Matter
Key Concepts
• Democritus was the first person to propose the
existence of atoms.
• According to Democritus, atoms are solid,
homogeneous, and indivisible.
• Aristotle did not believe in the existence of atoms.
• John Dalton’s atomic theory is based on numerous
scientific experiments.
Section 4.2 Defining the Atom
Key Concepts
• An atom is the smallest particle of an element that
maintains the properties of that element.
• Electrons have a 1– charge, protons have a 1+ charge,
and neutrons have no charge.
• An atom consists mostly of empty space surrounding
the nucleus.
Section 4.3 How Atoms Differ
Key Concepts
• The atomic number of an atom is given by its
number of protons. The mass number of an atom is
the sum of its neutrons and protons.
atomic number = number of protons = number of electrons
mass number = atomic number + number of neutrons
• Atoms of the same element with different numbers of
neutrons are called isotopes.
• The atomic mass of an element is a weighted average of
the masses of all of its naturally occurring isotopes.
Section 4.4 Unstable Nuclei and
Radioactive Decay
Key Concepts
• Chemical reactions involve changes in the electrons
surrounding an atom. Nuclear reactions involve
changes in the nucleus of an atom.
• There are three types of radiation: alpha (charge of 2+),
beta (charge of 1–), and gamma (no charge).
• The neutron-to-proton ratio of an atom’s nucleus
determines its stability.
Whose work led to the modern atomic
theory?
A. Dalton
B. Rutherford
D
A
0%
C
D. Aristotle
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. Einstein
Which particle is not found in the nucleus
of an atom?
A. neutron
B. proton
D
A
0%
C
D. electron
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. gamma ray
Two isotopes of an unknown element
have the same number of:
A. protons
B. neutrons
D
A
0%
C
D. both A and C
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. electrons
Lithium has an atomic mass of 6.941 and
two isotopes, one with 6 neutrons and one
with 7 neutrons. Which isotope is more
abundant?
A. 6Li
A
0%
D
D. unable to determine
C
C. Both isotopes occur equally.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. 7Li
What happens when an element emits
radioactive particles?
A. It gains energy.
B. It gains neutrons.
D
A
0%
C
D. It loses energy.
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. It loses stability.
What is the smallest particle of an element
that still retains the properties of that
element?
A. proton
A
0%
D
D. neutron
C
C. electron
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. atom
How many neutrons, protons, and
electrons does 12454Xe have?
A. 124 neutrons, 54 protons, 54 electrons
B. 70 neutrons, 54 protons, 54 electrons
C. 124 neutrons, 70 protons, 54 electronsA. A
D. 70 neutrons, 70 protons, 54 electrons B. B
D
C
B
A
0%
C. C
0%
0%
0%
D. D
The primary factor in determining an
atom's stability is its ratio of neutrons
to ____.
A. protons
A
0%
D
D. isotopes
C
C. alpha particles
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. electrons
What is the densest region of an atom?
A. electron cloud
B. nucleus
C. isotopes
D
C
A
0%
B
D. atomic mass
A. A
B. B
C. C
0%
0%
0%
D. D
Why are electrons attracted to the cathode
in a cathode ray tube?
A. The cathode is more stable.
B. The cathode has a positive charge.
D
A
0%
C
D. The cathode has no charge.
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. The cathode has a negative charge.
Click on an image to enlarge.
Table 4.3
Properties of Subatomic Particles
Figure 4.12 Rutherford's Experiment
Figure 4.14 Features of an Atom
Figure 4.21 Types of Radiation
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