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Chemistry Chapter 4
Structure of the Atom
Main Ideas
The ancient Greeks tried to explain matter,
but the scientific study of the atom began
with John Dalton in the early 1800's.
An atom is made of a nucleus containing
protons and neutrons; electrons move
around the nucleus.
The number of protons and the mass
number define the type of atom.
Unstable atoms emit radiation to gain
stability.
Early Ideas about Matter
Objectives:
• Compare and contrast the atomic models
of Democritus, Aristotle, and Dalton.
• Understand how Dalton's theory explains
the conservation of mass.
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.
Democritus
• 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
• 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
John Dalton
(English school teacher -1766-1844)
Responsible for the beginning of the
development of modern atomic theory.
Dalton revised Democritus’s theories by
performing and studying many chemical
reactions.
Through careful observations and
measurements he was able to determine
mass ratios of the elements involved in
those reactions.
John Dalton
• 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.
Dalton’s Atomic Theory
Even though some of the theory was incorrect, It
provided the basis we have today for atomic theory.
As is common with science, his theory has since
been revised and additional information learned.
Dalton’s Atomic Theory
Which parts of Dalton’s Atomic
Theory was incorrect?
Atoms are indivisible.
Atoms of a given element are
identical in size, mass and chemical
properties.
Question ?
Who was the first person to propose the
idea that matter was not infinitely
divisible?
A. Aristotle
B. Plato
C. Dalton
D. Democritus
Question?
Dalton’s theory also conveniently
explained what?
A. the electron
B. the nucleus
C. law of conservation of mass
D. law of Democritus
Defining the Atom
Objectives
• 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.
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.
• Gerd Binnig and Heinrich Rohrer won
the Nobel Prize in 1986 for the
invention of the STM
STM
Scanning tunneling
microscopy (STM) is the
highest resolution imaging
and nanofabrication
technique available. It relies
on quantum tunneling of
electrons from a sharp
metal tip to a conducting
surface. The 71 Angstrom
diameter "quantum corral"
shown above was created
and imaged with an ultrahigh vacuum cryogenic
STM. Each sharp peak in
the circle is an iron atom
resting on atomically flat
copper
STM
Cathode Ray Tube
• 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.
Cathode Ray Tube
Cathode Ray Tube
Cathode Ray Tube
Thomson received the Nobel Prize in 1906 for
identifying the first subatomic particle—the
electron. Research with the Cathode Ray Tube
determined the following:
1. Cathode rays were a stream of charged particles
2. The particles carried a negative charge.
3. Changing the metal that made up the electrodes
did not change the ray, therefore all types of
matter had these particles.
Milikan’s Oil Drop
Experiment
• In the early 1910s, Robert Millikan used
the oil-drop apparatus shown below to
determine the charge of an electron.
The Electron
• 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
Early Atomic Models
• 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
• Although most of the alpha particles went
through the gold foil, a few of them
bounced back, some at large angles.
The Nucleus
• The repulsive force between the positively
charged nucleus and positive alpha
particles caused the deflections.
The Nucleus
The Nucleus
• 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.
Niels Bohr Model
The Nucleus
• 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
• All atoms are made of three
fundamental subatomic
particles: the electron, the
proton, and the neutron.
• Atoms are spherically
shaped.
The Nucleus
• Chemical behavior can be explained by
considering only an atom's electrons.
• Atoms are mostly empty space, and
electrons travel around the nucleus held
by an attraction to the positively charged
nucleus.
The Nucleus
• Scientists have determined that protons
and neutrons are composed of subatomic
particles called quarks.
Question?
Atoms are mostly ____.
A. positive
B. negative
C. solid spheres
D. empty space
Question?
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
How Atoms Differ
Objectives:
• 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.
Atomic Number
• The number of protons in the nucleus of an
atom identifies the element and is known as
the element’s atomic number.
• Moseley (1887-1915) discovered each
element contains a unique positive charge
in their nucleus.
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
• 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
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
• The atomic mass of an element is the
weighted average mass of the isotopes of
that element.
Question?
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
B. 40
C. 19
D. unable to determine
Question ?
Elements with the same number of
protons and differing numbers of
neutrons are known as what?
A. isotopes
B. radioactive
C. abundant
D. ions
Unstable Nuclei and
Radioactive Decay
Objective:
• Explain the relationship between unstable
nuclei and radioactive decay.
• Characterize alpha, beta, and gamma
radiation in terms of mass and charge.
Radioactivity
• 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.
• Nuclear reactions can change one
element into another element.
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.
• Nuclear stability is determined by the ratio of
neutrons to protons.
Radioactive Decay
• Atoms that contain too many or too 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.
Alpha Particle
• 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.
• Found because they are deflected towards a
negavtively charged plate.
Alpha Particle
• 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.
Beta Particle
• Beta radiation is radiation that has a
negative charge and emits beta particles.
• Each beta particle is an electron with a 1–
charge.
• Found because they are deflected toward the
positive charged plate
Beta Particle
Gamma Rays
• Gamma rays are high-energy radiation with no
mass and are neutral and cannot, by
themselves, result in the formation of a new
atom.
• Gamma rays account for most of the energy lost
during radioactive decay.
Question ?
A reaction that changes one element into
another is called what?
A. chemical reaction
B. beta radiation
C. nuclear reaction
D. physical reaction
Question ?
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.
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.
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.
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.
Study Guide
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.
Question?
Whose work led to the modern atomic
theory?
A. Dalton
B. Rutherford
C. Einstein
D. Aristotle
Question?
Which particle is not found in the nucleus
of an atom?
A. neutron
B. proton
C. dust
D. electron
Question?
Two isotopes of an unknown element
have the same number of:
A. protons
B. neutrons
C. electrons
D. both A and C
Question?
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
Question?
What happens when an element emits
radioactive particles?
A. It gains energy.
B. It gains neutrons.
C. It loses stability.
D. It loses energy.
Question?
What is the smallest particle of an element
that still retains the properties of that
element?
A. proton
B. atom
C. electron
D. neutron
Question?
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
Question?
The primary factor in determining an
atom's stability is its ratio of neutrons
to ____.
A. protons
B. electrons
C. alpha particles
D. isotopes
Question?
What is the densest region of an atom?
A. electron cloud
B. nucleus
C. isotopes
D. atomic mass
Question?
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.
The End
IB 4
IB 5
IB 6
IB 7
IB 8
IB 9
IB 10
IB 11
IB 12
IB 13
IB 14
IB 15
IB 16
IB 17
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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