Chemistry: Matter and Change

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Chapter Menu
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
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
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Section 4-1
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.
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Section 4-1
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.

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Section 4-1
Greek Philosophers (cont.)

• Democritus (460370 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 (484322 B.C.) disagreed with
  Democritus because he did not believe empty space
  could exist.
• Aristotles views went unchallenged for 2,000
  years until science developed methods to test the
  validity of his ideas.

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Section 4-1
Greek Philosophers (cont.)
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Section 4-1
Greek Philosophers (cont.)

• John Dalton revived the idea of the atom in the
  early 1800s based on numerous chemical reactions.

• Daltons atomic theory easily explained
  conservation of mass in a reaction as the result
  of the combination, separation, or rearrangement
  of atoms.

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Section 4-1
Greek Philosophers (cont.)
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Section 4-1
Section 4.1 Assessment
Who was the first person to propose the idea that
matter was not infinitely divisible?
A. Aristotle B. Plato C. Dalton D. Democritus

• A
• B
• C
• D

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Section 4-1
Section 4.1 Assessment
Daltons theory also conveniently explained what?
A. the electron B. the nucleus C. law of
conservation of mass D. law of Democritus

• A
• B
• C
• D

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• Section quiz

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Study Guide 1
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 Daltons atomic theory is based on numerous
  scientific experiments.

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End of Section 4-1
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Section 4-2
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
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Section 4-2
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.
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Section 4-2
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.

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Section 4-2
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.

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Section 4-2
The Electron (cont.)

• This figure shows a typical cathode ray tube.
  This is like a TV tube.

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Section 4-2
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 particlethe
  electron

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Section 4-2
The Electron (cont.)

• In the early 1910s, Robert Millikan used the
  oil-drop apparatus shown below to determine the
  charge of an electron.

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Section 4-2
The Electron (cont.)

• Charges change in discrete amounts1.602 ? 1019
  coulombs, the charge of one electron (now
  equated to a single unit, 1).

• With the electrons charge and charge-to-mass
  ratio known, Millikan calculated the mass of a
  single electron.

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Section 4-2
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.

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Section 4-2
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.

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Section 4-2
The Nucleus (cont.)

• Although most of the alpha particles went through
  the gold foil, a few of them bounced back, some
  at large angles.

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• animation

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Section 4-2
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.

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Section 4-2
The Nucleus (cont.)

• The repulsive force between the positively
  charged nucleus and positive alpha particles
  caused the deflections.

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Section 4-2
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 atoms mass.

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Section 4-2
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.

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• animation

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Section 4-2
The Nucleus (cont.)

• Scientists have determined that protons and
  neutrons are composed of subatomic particles
  called quarks.

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Section 4-2
The Nucleus (cont.)

• Chemical behavior can be explained by considering
  only an atom's electrons.

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• Atom
• __________ ___________
• ____ _____ _______
• ____ _____ charge _______

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Section 4-2
Section 4.2 Assessment
Atoms are mostly ____. A. positive B. negative
C. solid spheres D. empty space

• A
• B
• C
• D

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Section 4-2
Section 4.2 Assessment
What are the two fundamental subatomic particles
found in the nucleus? A. proton and electron
B. proton and neutron C. neutron and electron
D. neutron and positron

• A
• B
• C
• D

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• Section quiz

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Study Guide 2
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.

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End of Section 4-2
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Section 4-3
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.

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Section 4-3
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 isotopes mass number
atomic mass unit (amu) atomic mass
The number of protons and the mass number define
the type of atom.
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Section 4-3
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
  elements atomic number.

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Section 4-3
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.

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Section 4-3
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.

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Section 4-3
Isotopes and Mass Number (cont.)
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Section 4-3
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.

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Section 4-3
Mass of Atoms (cont.)

• The atomic mass of an element is the weighted
  average mass of the isotopes of that element.

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Section 4-3
Section 4.3 Assessment
An unknown element has 19 protons, 19 electrons,
and 3 isotopes with 20, 21 and 22 neutrons. What
is the elements atomic number? A. 38 B. 40
C. 19 D. unable to determine

• A
• B
• C
• D

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Section 4-3
Section 4.3 Assessment
Elements with the same number of protons and
differing numbers of neutrons are known as what?
A. isotopes B. radioactive C. abundant
D. ions

• A
• B
• C
• D

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• Section quiz

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Study Guide 3
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.

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End of Section 4-3
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Section 4-4
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
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Section 4-4
Section 4.4 Unstable Nuclei and Radioactive
Decay (cont.)
radioactivity radiation nuclear
reaction radioactive decay alpha radiation
alpha particle nuclear equation beta
radiation beta particle gamma rays
Unstable atoms emit radiation to gain stability.
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Section 4-4
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.

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Section 4-4
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.

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Section 4-4
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.

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Section 4-4
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.

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Section 4-4
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.

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Section 4-4
Radioactive Decay (cont.)
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• animation

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Section 4-4
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.

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Section 4-4
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 naturemost have already decayed to
  stable forms.

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Section 4-4
Section 4.4 Assessment
A reaction that changes one element into another
is called what? A. chemical reaction B. beta
radiation C. nuclear reaction D. physical
reaction

• A
• B
• C
• D

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Section 4-4
Section 4.4 Assessment
Why are radioactive elements rare in nature?
A. They do no occur on Earth. B. Most have
already decayed to a stable form. C. They take
a long time to form. D. They are too hard to
detect.

• A
• B
• C
• D

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• Section quiz

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Study Guide 4
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 atoms nucleus
  determines its stability.

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End of Section 4-4
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Assessment Standardized Test Practice Image
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78
Study Guide 1
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 Daltons atomic theory is based on numerous
  scientific experiments.

79
Study Guide 2
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.

80
Study Guide 3
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.

81
Study Guide 4
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 atoms nucleus
  determines its stability.

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Chapter Assessment 1
Whose work led to the modern atomic theory?
A. Dalton B. Rutherford C. Einstein
D. Aristotle

• A
• B
• C
• D

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Chapter Assessment 2
Which particle is not found in the nucleus of an
atom? A. neutron B. proton C. gamma ray
D. electron

• A
• B
• C
• D

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Chapter Assessment 3
Two isotopes of an unknown element have the same
number of A. protons B. neutrons
C. electrons D. both A and C

• A
• B
• C
• D

85
Chapter Assessment 4
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 B. 7Li C. Both isotopes occur equally.
D. unable to determine

• A
• B
• C
• D

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Chapter Assessment 5
What happens when an element emits radioactive
particles? A. It gains energy. B. It gains
neutrons. C. It loses stability. D. It loses
energy.

• A
• B
• C
• D

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STP 1
What is the smallest particle of an element that
still retains the properties of that element?
A. proton B. atom C. electron D. neutron

• A
• B
• C
• D

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STP 2
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
electrons D. 70 neutrons, 70 protons, 54
electrons

• A
• B
• C
• D

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STP 3
The primary factor in determining an atom's
stability is its ratio of neutrons to ____.
A. protons B. electrons C. alpha
particles D. isotopes

• A
• B
• C
• D

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STP 4
What is the densest region of an atom?
A. electron cloud B. nucleus C. isotopes
D. atomic mass

• A
• B
• C
• D

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STP 5
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.
C. The cathode has a negative charge. D. The
cathode has no charge.

• A
• B
• C
• D

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CIM
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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