Chemistry: Matter and Change

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CHEMISTRY Matter and Change
Chapter 4 The Structure of the Atom
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Table Of Contents
CHAPTER4
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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Early Ideas About Matter
SECTION4.1

• 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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Early Ideas About Matter
SECTION4.1
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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Early Ideas About Matter
SECTION4.1
Greek Philosophers

• 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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Early Ideas About Matter
SECTION4.1
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, from which the English word atom
  is derived.

• 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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Early Ideas About Matter
SECTION4.1
Greek Philosophers (cont.)
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Early Ideas About Matter
SECTION4.1
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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Early Ideas About Matter
SECTION4.1
Greek Philosophers (cont.)
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Section Check
SECTION4.1
Who was the first person to propose the idea that
matter was not infinitely divisible?
A. Aristotle B. Plato C. Dalton D. Democritus
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Section Check
SECTION4.1
Daltons theory also conveniently explained
what? A. the electron B. the nucleus C. law of
conservation of mass D. law of Democritus
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Defining the Atom
SECTION4.2

• 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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
The Electron (Cont.)

• This figure shows a typical cathode ray tube.

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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
The Electron (Cont.)

• Matter is neutral. You know that matter is
  neutral from everyday experiences. You do not
  receive an electric shock (except under certain
  conditions) when you touch an object.
• If electrons are negative, then how is matter,
  which is made up of electrons, neutral?
• J.J. Thomson proposed a model of the atom to
  answer this question.

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Defining the Atom
SECTION4.2
The Electron (Cont.)

• J.J. Thomson's plum pudding model of the atom
  states that the atom is a uniform, positively
  charged sphere containing electrons.

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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
The Nucleus (cont.)

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

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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
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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Defining the Atom
SECTION4.2
The Nucleus (cont.)

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

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Defining the Atom
SECTION4.2
The Nucleus (cont.)

• Scientists do not yet understand if or how quarks
  affect chemical behavior.
• Chemical behavior can be explained by considering
  only an atom's electrons.

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Section Check
SECTION4.2
Atoms are mostly ____. A. positive B. negative
C. solid spheres D. empty space
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Section Check
SECTION4.2
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
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How Atoms Differ
SECTION4.3

• 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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How Atoms Differ
SECTION4.3
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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How Atoms Differ
SECTION4.3
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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How Atoms Differ
SECTION4.3
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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How Atoms Differ
SECTION4.3
Isotopes and Mass Number (Cont.)

• In nature, most elements are found as mixtures of
  isotopes. Usually, the relative abundance of
  each isotope is constant.
• Ex. In a banana, 93.26 is potassium-39, 6.73 is
  potassium-41 and 0.01 is potassium-40. In
  another banana or in a different source of
  potassium, the percentage composition of the
  potassium isotopes will still be the same.

• Isotopes containing more neutrons have a greater
  mass.
• Isotopes of an atom have the same chemical
  behavior.

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How Atoms Differ
SECTION4.3
Isotopes and Mass Number (Cont.)

• The mass number is the sum of the protons and
  neutrons in the nucleus.

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How Atoms Differ
SECTION4.3
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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How Atoms Differ
SECTION4.3
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 Check
SECTION4.3
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
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Section Check
SECTION4.3
Elements with the same number of protons and
differing numbers of neutrons are known as what?
A. isotopes B. radioactive C. abundant
D. ions
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SECTION4.4
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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SECTION4.4
Unstable Nuclei and Radioactive Decay
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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SECTION4.4
Unstable Nuclei and Radioactive Decay
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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SECTION4.4
Unstable Nuclei and Radioactive Decay
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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SECTION4.4
Unstable Nuclei and Radioactive Decay
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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SECTION4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)

• The figure shown below is a nuclear equation
  showing the radioactive decay of radium-226 to
  radon-222.

4 He 2

• An alpha particle is equivalent to a helium-4
  nucleus and is represented by 4He or .
• Thus, showing mass is conserved in a nuclear
  equation.

2
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SECTION4.4
Unstable Nuclei and Radioactive Decay
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.
• During Beta decay, a neutron is converted to a
  proton and an electron. The electron is emitted
  and the proton stays in the nucleus.

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SECTION4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)
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SECTION4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)

• Gamma rays are high-energy radiation with no mass
  and are neutral.
• They usually accompany alpha and beta radiation.

• Gamma rays account for most of the energy lost
  during radioactive decay.

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SECTION4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)

• Because gamma rays are massless, the emission of
  gamma rays by themselves cannot result in the
  formation of a new atom.

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SECTION4.4
Unstable Nuclei and 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 naturemost have already decayed to
  stable forms.

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Section Check
SECTION4.4
A reaction that changes one element into another
in the nucleus of an atom is called what?
A. chemical reaction B. beta radiation
C. nuclear reaction D. physical reaction
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Section Check
SECTION4.4
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.
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CHAPTER4
The Structure of the Atom
Resources
Chemistry Online Study Guide Chapter
Assessment Standardized Test Practice
60
Early Ideas About Matter
SECTION4.1
Study Guide
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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Defining the Atom
SECTION4.2
Study Guide
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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How Atoms Differ
SECTION4.3
Study Guide
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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Unstable Nuclei and Radioactive Decay
SECTION4.4
Study Guide
Key Concepts

• Chemical reactions involve changes in the
  electrons surrounding the nucleus of 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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CHAPTER4
The Structure of the Atom
Chapter Assessment
Whose work led to the modern atomic theory?
A. Dalton B. Rutherford C. Einstein
D. Aristotle
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CHAPTER4
The Structure of the Atom
Chapter Assessment
Which particle is not found in the nucleus of an
atom? A. neutron B. proton C. gamma ray
D. electron
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CHAPTER4
The Structure of the Atom
Chapter Assessment
Two isotopes of an unknown element have the same
number of A. protons B. neutrons
C. electrons D. both A and C
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CHAPTER4
The Structure of the Atom
Chapter Assessment
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
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CHAPTER4
The Structure of the Atom
Chapter Assessment
What happens when an element emits radioactive
particles? A. It gains energy. B. It gains
neutrons. C. It loses stability. D. It loses
energy.
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CHAPTER4
The Structure of the Atom
Standardized Test Practice
What is the smallest particle of an element that
still retains the properties of that element?
A. proton B. atom C. electron D. neutron
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CHAPTER4
The Structure of the Atom
Standardized Test Practice
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
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CHAPTER4
The Structure of the Atom
Standardized Test Practice
The primary factor in determining an atom's
stability is its ratio of neutrons to ____.
A. protons B. electrons C. alpha
particles D. isotopes
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CHAPTER4
The Structure of the Atom
Standardized Test Practice
What is the densest region of an atom?
A. electron cloud B. nucleus C. isotopes
D. atomic mass
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CHAPTER4
The Structure of the Atom
Standardized Test Practice
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.
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