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CHAPTER 4 (1) AND 9
ATOMIC STRUCTURE AND RADIOACIVITY
PHYSICAL SCIENCE
I. Atomic Structure
A. Objectives
1. Explain Dalton’s atomic theory, and describes why it was more
successful than Democritus’s theory
2. State the charge, mass, and location, of each part of an atom according
to the modern model of the atom
3. Compare and contrast Bohr’s model with the modem model of the
atom
B. What are atoms?
1. Our understanding of atoms required many centuries
a. The idea of an atom – which means “unable to be divided” –
dates back to the Greek philosopher Democritus, who lived in the
fourth century BCE.
2. John Dalton developed the atomic theory in 1808.
a. Like Democritus, Dalton proposed that atoms could not be
divided. Dalton’s was the first atomic theory with a scientific
basis.
3. An atom is the smallest part of an element that still has the element’s
properties
4. Atoms are the building blocks of molecules
C. What’s in an atom?
1. Atoms are made of protons, neutrons, and electrons
a. At the center of each atom is a small, dense nucleus with a
positive electric charge
b. The nucleus is made of protons (a subatomic particle that has a
positive charge) and neutrons (a subatomic particle that has no
charge)
c. Moving around outside the nucleus is a cloud of electrons:
subatomic particles with negative charges
2. Unreacted atoms have no overall charge
a. Although atoms are made of charged particles, they do not have
an overall charge because they have an equal number of protons
and electrons whose charges exactly cancel
b. A helium atom is made of two protons, two neutrons, and two
electrons
D. Models of the Atom
1. Bohr’s model compares electrons to planets
a. In 1913, the Danish scientist Niels Bohr suggested that
electrons in an atom move in set paths around the nucleus much
like the planets orbit the sun in our solar system
b. In Bohr’s model, electrons can only be in certain energy levels.
2. Electrons act more like waves
a. By 1925, Bohr’s model of the atom no longer explained
electron behavior.
b. A new model was proposed, in which electrons behave more
like waves on a vibrating string than like particles
3. An electron’s exact location cannot be determined
a. It is impossible to determine both the exact location of an
electron in an atom and the electron’s speed and direction
b. The best scientists can do is calculate the chance of finding an
electron in a certain place within an atom
4. Electrons exist in energy levels
a. The number of filled energy levels an atom has depends on the
number of electrons
5. Electrons are found in orbitals within energy levels
a. An orbital is a region in an atom where there is a high
probability of finding electrons
b. An s orbital is shaped like a sphere
c. A p orbital is dumbbell shaped and can be oriented three
different ways in space
6. Every atom has between one and eight valence electrons
a. Valence electrons are found in the outermost shell of an atom
and determine the atom’s chemical properties
b. Valence electrons are the electrons in an atom that participate in
chemical bonding
II. Chapter 9 – Section 1: What is Radioactivity?
A. Objectives
1. Identify four types of nuclear radiation and their properties
2. Balance equations for nuclear decay
3. Calculate the half-life of a radioactive isotope
B. Nuclear Radiation
1. The process by which an unstable nucleus emits one or more particles
or energy in the form of electromagnetic radiation is called radioactivity
2. The particles that are released from the nucleus during radioactive
decay is called nuclear radiation
3. There are different types of nuclear radiation
4. Types of Nuclear Radiation
5. Alpha particles consist of protons and neutrons
a. an alpha particle is a positively charged atom that is released in
the disintegration of radioactive elements and that consists of two
protons and two neutrons
6. Beta particles are electrons produced from neutron decay
a. A beta particle is a charged electron emitted during
types of radioactive decay, such as beta decay
7. Gamma rays are very high energy
a. A gamma ray is a high-energy photon emitted by a nucleus
during fission and radioactive decay
8. Neutron radioactivity may occur in unstable nucleus
a. Neutron emission consists of matter that is emitted from an
unstable nucleus
b. Neutrons are able to travel farther through matter than either
alpha or beta particles
9. In nuclear decay, the sums of the mass numbers and the atomic
numbers of the decay products equal the mass number and atomic number
of the decaying nucleus
10. A nucleus gives up two protons and two neutrons during alpha decay
a. The process of the alpha decay of radium – 226 is written
follows.
11. A nucleus gains a proton and loses a neutron during beta decay
a. A beta decay process occurs when carbon-14 decays to nitrogen
– 14 by emitting a beta particle
C. Math Skills
1. Nuclear decay Actinium – 217 decays by releasing an alpha particle.
Write the equation for this decay process, and determine what element is
formed.
2. Write down the equation with the original element on the left side and
the products on the right side. Use the letter X to denote the unknown
product. Note that the mass and atomic numbers of the unknown isotope
are represented by the letters A and Z.
3. Write math equations for the atomic and mass numbers.
4. Rearrange the equations.
5. Solve for the unknown values, and rewrite the equation with all nuclei
represented.
D. Radioactive Decay Rates
1. The half-life is the time required for half of a sample of a radioactive
substance to disintegrate by radioactive decay or by natural processes.
2. Half-life is a measure of how quickly a substance decays.
a. Using half-lives, scientists can predict how old an object is.
3. Carbon – 14 is used to date materials.
E. Math Skills – Half – Life
1. Radium – 226 has a half – life of 1599 years. How long would it take
seven-eights of a radium-226 sample to decay?
2. List the given and unknown values
3. Calculate the fraction of radioactive sample remaining.
4. Calculate the number of half- lives
5. Calculate the total time required for the radioactive decay.
III. Section 2 – Nuclear Fission and Fusion
A. Objectives
1. Describe how the strong nuclear force affects the composition of a
nucleus.
2. Distinguish between fission and fusion, and provide examples of each.
3. Recognize the equivalence of mass and energy, and why small losses in
mass release large amounts of energy
4. Explain what a chain reaction is, how one is initiated, and how it can be
controlled
B. Nuclear Forces
1. Nuclei are held together by a special force
a. Protons and neutrons are tightly packed in the tiny nucleus of an
atom
b. The strong nuclear force causes protons and neutrons in the
nucleus to attract each other. This attraction is much stronger than
the electric repulsion between protons.
2. Neutrons contribute to nuclear stability
3. Too many neutrons or protons can cause a nucleus to become unstable
and decay
C. Nuclear Fission
1. Fission is the process by which a nucleus splits into two or more
fragments and releases neutrons and energy
2. One type of fission of uranium-235 can be represented by the following
equation.
3. Energy is released during a nuclear fission
a. The equivalence of mass and energy observed in nature is
explained by the special theory of relativity
b. This equivalence is expressed by the following equation
4. Neutrons released by fission can start a chain reaction
5. A nuclear chain reaction is a continuous series of nuclear fission
reactions
6. Chain reactions can be controlled
a. If there is less than a critical mass of a fissionable isotope, a
chain reaction will not occur
7. The critical mass is the minimum mass of a fissionable isotope that
provides the number of neutrons needed to sustain a chain reaction
D. Nuclear Fusion
1. Nuclear fusion is the process in which light nuclei combine at
extremely high temperature, forming heavier nuclei and releasing energy
2. Nuclear fusion occurs in the sun
a. Four hydrogen atoms fuse together in a multi-step process to
produce a helium atom and enormous energy in the form of
gamma rays
IV. Section 3 – Nuclear Radiation Today
A. Objectives
1. Describe the sources of nuclear radiation, including where it exists as
background radiation
2. List and explain three beneficial uses and three possible risks of nuclear
radiation
3. Compare and contrast the advantages and disadvantages of nuclear
energy as a power source
B. Where is Radiation?
1. Nuclear radiation is all around you. Background radiation is the
nuclear radiation that arises naturally from cosmic rays and from
radioactive isotopes in the soil and air.
2. Radiation is measured in units of rems.
3. A rem is the quantity of ionizing radiation that does as much damage to
human tissue as 1 roentgen of high-voltage X rays does.
4. Exposure varies from one location to another
5. Some activities add to the amount of nuclear radiation exposure.
C. Beneficial uses of nuclear radiation
1. Smoke detectors help to save lives
a. In a smoke alarm, a small alpha-emitting isotope detects smoke
particles in the air.
2. Nuclear radiation is used to detect diseases
a. A radioactive tracer is a radioactive material that is added to a
substance so that its distribution can be detected later.
b. Radioactive tracers are widely used in medicine.
3. Nuclear radiation therapy is used to treat cancer
a. Radiotherapy is a treatment that uses controlled doses of nuclear
radiation for treating diseases such as cancer
4. Agriculture uses radioactive tracers and radio-isotopes
a. On research farms, radioactive tracers help scientists to
understand biochemical processes in plants
D. Possible risks of nuclear radiation
1. Nuclear radiation can ionize atoms
a. ionization is a change in the number of electrons in an atom or
molecule, causing the particle to be positively or negatively
charged
2. Radiation sickness results from high levels of nuclear radiation
a. People working in radioactive areas wear a dosimeter, a device
that measures the amount of nuclear radiation exposure
b. Studies have shown a relationship between exposure to high
levels of nuclear radiation and cancer.
3. The risk depends upon the amount of radiation exposure
4. High concentrations of radon gas can be hazardous
a. Radon gas is colorless and odorless, and is produces by the
decay of uranium – 238 present naturally in soil and rock
b. Tests for radon gas are widely available
E. Nuclear Power
1. Nuclear fission has both advantages and disadvantages
a. Nuclear fission is an alternative to fossil fuels as a source of
energy
b. radioactive products of fission must be handled carefully and
nuclear waste must be safely stored
2. Nuclear fusion reactors are being tested
a. Nuclear fusion reactions are difficult to produce in the
laboratory
b. Nuclear fusion also has advantages and disadvantages