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CHEMISTRY
Matter and Change
Chapter 4: The Structure of the Atom
CHAPTER
4
Table Of Contents
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
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.
SECTION
4.1
Early Ideas About Matter
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.
SECTION
4.1
Early Ideas About Matter
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, from
which the English word atom is derived.
• 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.
SECTION
4.1
Early Ideas About Matter
Greek Philosophers (cont.)
SECTION
4.1
Early Ideas About Matter
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.
SECTION
4.1
Early Ideas About Matter
Greek Philosophers (cont.)
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
atom
cathode ray
electron
nucleus
proton
neutron
An atom is made of a nucleus containing
protons and neutrons; electrons move
around the nucleus.
SECTION
4.2
Defining the Atom
The Atom
• *is called an atom.
• An instrument called the scanning tunneling
microscope (STM) allows individual atoms to
be seen.
SECTION
4.2
Defining the Atom
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 *.
• The particles carrying a negative charge are
known as *.
SECTION
4.2
Defining the Atom
The Electron (Cont.)
• This figure shows a typical cathode ray
tube.
SECTION
4.2
Defining the Atom
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
SECTION
4.2
Defining the Atom
The Electron (Cont.)
• In the early 1910s, Robert Millikan used the
oil-drop apparatus shown below to
determine the charge of an electron.
SECTION
4.2
Defining the Atom
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
SECTION
4.2
Defining the Atom
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.
SECTION
4.2
Defining the Atom
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.
SECTION
4.2
Defining the Atom
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.
SECTION
4.2
Defining the Atom
The Nucleus (cont.)
• Although most of the alpha particles went
through the gold foil, a few of them
bounced back, some at large angles.
SECTION
4.2
Defining the Atom
The Nucleus (cont.)
• Rutherford concluded that atoms are
mostly empty space.
• *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.
SECTION
4.2
Defining the Atom
The Nucleus (cont.)
• The repulsive force between the positively
charged nucleus and positive alpha
particles caused the deflections.
SECTION
4.2
Defining the Atom
The Nucleus (cont.)
• Rutherford refined the model to include
positively charged particles in the nucleus
called *.
• James Chadwick received the Nobel Prize in
1935 for discovering the existence of
neutrons, *.
SECTION
4.2
Defining the Atom
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.
SECTION
4.2
Defining the Atom
The Nucleus (cont.)
• Scientists have determined that protons
and neutrons are composed of subatomic
particles called quarks.
SECTION
4.2
Defining the Atom
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.
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
How Atoms Differ
4.3
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
atomic mass unit (amu)
isotopes
atomic mass
mass number
The number of protons and the mass
number define the type of atom.
SECTION
4.3
How Atoms Differ
Atomic Number
• Each element contains a unique positive
charge in their nucleus.
• *is known as the element’s atomic number.
SECTION
4.3
How Atoms Differ
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.
• *isotopes.
SECTION
4.3
How Atoms Differ
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 potassium40. 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.
SECTION
4.3
How Atoms Differ
Isotopes and Mass Number (Cont.)
• The mass number is *.
SECTION
4.3
How Atoms Differ
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.
SECTION
4.3
How Atoms Differ
Mass of Atoms (cont.)
• The atomic mass of an element *
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
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.
SECTION
4.4
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 *
• *are called radiation.
• *is called a nuclear reaction.
SECTION
4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay
• Unstable nuclei lose energy by emitting
radiation in a spontaneous process called *
• Unstable radioactive elements undergo
radioactive decay thus forming stable
nonradioactive elements.
SECTION
4.4
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.
SECTION
4.4
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.
• An alpha particle is equivalent to a helium-4
nucleus and is represented by 42He or
.
• Thus, showing mass is conserved in a
nuclear equation.
SECTION
4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)
• Beta radiation is *.
• 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.
SECTION
4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)
SECTION
4.4
Unstable Nuclei and Radioactive Decay
Radioactive Decay (cont.)
• Gamma rays are *.
• They usually accompany alpha and beta
radiation.
• Gamma rays account for most of the energy lost
during radioactive decay.
SECTION
4.4
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.
SECTION
4.4
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 nature—most have already
decayed to stable forms.
SECTION
Early Ideas About Matter
4.1
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 Dalton’s atomic theory is based on numerous
scientific experiments.
SECTION
Defining the Atom
4.2
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.
SECTION
How Atoms Differ
4.3
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.
SECTION
4.4
Unstable Nuclei and Radioactive Decay
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 atom’s nucleus
determines its stability.