Download Radioactivity

7.1 Atomic Theory and Radioactive
Decay
• Natural background radiation exists all around us.
 This radiation consists of high energy particles or waves being emitted from a
variety of materials.
• Radioactivity is the release of high-energy particles or waves.
 Being exposed to radioactive materials can be beneficial or harmful.
 X rays, radiation therapy, and electricity generation are beneficial.
 High-energy particles and waves damage DNA in our cells.
 When atoms lose high-energy particles and waves, ions or even new atoms can
be formed.
 High-energy waves and particles are called radiation when they leave the atom.
The
electromagnetic
epectrum
See pages 286 - 287
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Searching for Invisible Rays
• Radiation is everywhere, but can be difficult to detect.
 Roentgen named X rays with an “X” 100 years ago because they were
previously unknown.
 Becquerel realized uranium emitted seemingly invisible energy as well.
 Marie Curie and her husband Pierre named this energy radioactivity.
 Early discoveries of radiation relied on photographic equipment.
 Later, more sophisticated devices such as the Geiger-Müller counter were
developed to more precisely measure radioactivity.
Radium salts, after being
placed on a photographic
plate, leave behind the dark
traces of radiation.
See pages 288 - 289
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Isotopes and Mass Number
• Isotopes are different atoms of the same element, with the difference
between the two atoms being the number of neutrons in the nucleus.
 Isotopes have the same number of protons and therefore the same atomic
number as each other.
 By having different numbers of neutrons, isotopes have different mass
numbers.
 Isotopes of an element have the same symbol and same atomic number
 Mass number refers to the protons plus neutrons in an isotope
 Atomic mass = proportional average of the mass numbers for all isotopes of
an element.
• 19.9% of boron atoms have 5 neutrons, 80.1% have 6 neutrons
• 19.9% have a mass number of 10, and 80.1% have a mass number of 11
• (.199 * 10) + (.801*11) = 10.8 = atomic mass of boron
See page 289 - 290
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Representing Isotopes
• Isotopes are written using standard atomic notation.
 Chemical symbol + atomic number + mass number.
40
41
 Potassium has three isotopes, 39
19 K, 19 K, 19 K
 Potassium is found in nature in a certain ratio of isotopes.
 93.2% is potassium-39, 1.0% is potassium-40, and 6.7% is potassium-41
 Atomic mass = (0.932 x 39) + (0.001 x 40) + (0.067 x 41) = 39.1
See page 290
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Radioactive Decay
• Unlike all previously discovered chemical reactions, radioactivity sometimes
results in the formation of completely new atoms.
 Radioactivity results from having an unstable nucleus.
 When these nuclei lose energy and break apart, decay occurs.
 Radioactive decay releases energy from the nucleus as radiation.
 Radioactive atoms release energy until they become stable, often as different
atoms.
 An element may have only
certain isotopes that are
radioactive.
• These are called
radioisotopes.
Radioisotope uranium-238 decays in several
stages until it finally becomes lead-206.
See page 293
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Marie and Pierre Curie and
their discoveries
• Radiation is everywhere, but can be difficult to detect.
 Becquerel discovered that uranium emitted seemingly
invisible energy.
 Early discoveries of radiation relied on photographic
equipment.
 Marie Curie and her husband Pierre named this energy
radioactivity.
Radium salts, after
being placed on a
photographic plate,
leave behind the dark
traces of radiation.
Radioactive watch and a Geiger counter: http://www.youtube.com/watch?v=Q1WEkB5fKcg&feature=related
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The Curie’s continued
• Marie discovered that uranium ore
contained other radioactive elements
• Marie and Pierre have been credited
with the discoveries of
 Polonium and Radium
• They spent 4 years, processing tons
of ore before they has isolated
enough of each element to
characterize its chemical properties
• In 1903 the Curie’s were awarded
the Nobel prize in physics for these
discoveries
• Three years later tragedy struck
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Marie continued her work
• Marie took Pierre’s teaching position at the Sorbonne
• This was a first for women in history
• In honor of Pierre the Radiology Congress chose the
curie as the basic unit of radioactivity
• Marie became the first person (and still only 1 of 4) to
receive a second Nobel Prize.
• Marie continued to tirelessly investigate and promote
the use of radium as a treatment for cancer for the rest
of her life.
• Marie Curie died July 4, 1934, overtaken by pernicious
anemia no doubt caused by years of overwork and
radiation exposure
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Radiation is playing an increased importance in
diagnosing and treating certain diseases....
Medical Uses Include:
- X-rays
- Tumor Treatment
- MRI scans
- PET scans
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Three Types of Radiation
• Rutherford identified three types of radiation using an electric field.
 Positive alpha particles were attracted to the negative plate.
 Negative beta particles were attracted to the positive plate.
 Neutral gamma rays did not move towards any plate.
See page 294
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Types of Radiation
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Alpha Radiation
• Alpha radiation is a stream of alpha
particles.
 They are ___________ charged, and are the most massive of the
radiation types.
 Alpha particles are essentially the same as __________ atoms.
 Alpha particles are represented by the symbols

.
 Because it has two protons, it has a charge of 2+.
 The release of alpha particles is called __________ decay.
 Alpha particles are slow and penetrate materials much less than
the other forms of radiation. A sheet of paper will stop an
alpha
See page 294 - 295
particle.
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Alpha radiation continued
Radium-226 releases an alpha particle and becomes
Radon-222. Radon has two less protons than radium.
226
88
Ra 
222
88
Rn + 42 
or
226
88
Ra 
222
88
4
2
Rn + He
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Beta Radiation
• A beta particle is an _________ and is
negatively charged.
 Beta particles are represented by the symbols
.
 Electrons are very tiny, so beta particles are assigned a mass of 0.
 Since there is only an electron, a beta particle has a charge of 1–.
 Beta decay occurs when a ________ changes into a proton + an
electron.
 The proton stays in the ________, and the electron is __________.
 It takes a thin sheet of aluminum foil to stop a beta particle.
See page 296
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Beta Radiation continued
Iodine-131 releases a beta particle and becomes xenon-131.
A neutron has turned into a proton and the released electron.
131
53
I 
131
53
I 
131
54
Xe +
or
131
54
Xe +
0
–1

0
–1
e
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Gamma Radiation
• Gamma radiation is a ray of high-energy, short-wavelength radiation.
 Gamma radiation has _____________ and no mass, and is represented
by the symbol
 Gamma radiation is the __________-energy form of electromagnetic radiation.
 It takes thick blocks of lead or concrete to stop gamma rays.
 Gamma decay results from energy being released from a high-energy nucleus.
 Often, other kinds of radioactive decay will also release gamma radiation.
 Uranium-238 decays into an alpha particle and also releases gamma rays.
238
92
U 
234
90
Th + 42 He + 2
See page 297
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Nuclear Equations for Radioactive Decay
•
•
Nuclear equations are written like chemical equations,
but represent changes in the nucleus of atoms.
Remember these two rules when working with nuclear
equations:
1. The sum of the mass numbers does not change.
2. The sum of the charges in the nucleus does not change.
See pages 298 - 299
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(c) McGraw Hill Ryerson 2007
What is the Atomic
Theory?
• Atomic Theory is a theory of
the nature of matter, which
states that matter is
composed of discrete units
called atoms
• Atoms can be divided into
sub-atomic particles known
as Protons, Neutrons and
Electrons.
What is
Radioactivity?
•Radioactivity is the
release of highenergy particles and
rays of energy from
a substance as a
result of changes in
the nuclei of its
atoms
(c) McGraw Hill Ryerson 2007
(c) McGraw Hill Ryerson 2007
Background Radiation
UV Radiation
Solar Radiation
Exists all around us from many different sources
It is a stream of high-energy, fast-moving particles or
waves that are found in our environment
Can interact with an atom and turn it into an ion
Ion: An atomic particle that is electrically charged, either
negatively or positively.
• Eg: H+, Cl-, Cu2+ etc.
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Review: What’s in an Atom?
•
Protons
 Atomic Mass: 1 AMU
 Charge: +1
 Location: Nucleus
•
Neutrons
 Atomic Mass: 1 AMU
 Charge: Neutral
 Location: Nucleus
•
Electrons
 Atomic Mass: 5.489 × 10−4
(= 0.0005489) AMU
 Charge : -1
 Location: Outside nucleus in orbital
Note: AMU stands for Atomic Mass Units
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Isotopes
• Are different atoms of the same element, with a different
number of neutrons in the nucleus.
– Changing the # of neutrons changes the mass number
• *Remember: mass number = # protons
+ # neutrons*
– Isotopes still have the same:
• Number of protons / atomic number
• Element symbol
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How are Isotopes Represented?
• Chemists represent
isotopes using standard
atomic notation (also
called the nuclear
symbol)
=the number of protons
= the number of neutrons
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Example: The Carbon Isotopes
mass number
exact weight
percent abundance
12
12.000000
98.90
13
13.003355
1.10
Note: Carbon -14
is unstable and
radioactive and
does not exist in
nature so it is not
included in
calculating the
atomic mass of
Carbon
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Atomic Mass (the decimal #’s)
mass number
exact weight
percent abundance
12
12.000000
98.90
13
13.003355
1.10
• Atomic mass = The average of the mass numbers for all isotopes of an
element.
• From the table above we can see that:
 98.90 % (=0.9890) of carbon atoms have 6 neutrons and 6 protons equaling a
mass number of 12
 1.10 % (=0.0110) of carbon atoms have 7 neutrons and 6 protons equaling a mass
number of 13
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Types of Radioactive Decay
1.
•
•
Alpha Radiation:
Alpha particles are positively charged atomic particles and have the same
combination of particles as the nucleus of a helium (He) atom
These particles are slow moving
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How to express Alpha decay:
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2. Beta Radiation
• In Beta decay, a neutron changes into a proton and an electron
• It becomes an atom of the next higher element on the periodic table
• The mass number of the resulting isotope does not change
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How to express Beta decay:
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3. Gamma
Radiation
• Rays of high energy, short wavelength radiation
• It is not the release of particles like we saw in alpha and beta decay.
• A high energy-gamma ray is given off when an isotope falls from a
high energy state to a lower energy state.
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How to express Gamma decay:
• Results from a redistribution of energy in the nucleus
• The * indicates that the nickel nucleus has extra energy that is
released as a gamma ray
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Refer to pg 298 of the text for a summary of the three types of radioactive decay!
http://www.youtube.com/watch?v=kaS11fW7nNc
http://www.youtube.com/watch?v=SmwlzwGMMwc&feature=related
(c) McGraw Hill Ryerson 2007