Transcript 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
54
Xe +
0
–1

or
131
53
I 
131
54
Xe +
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
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(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
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