Transcript Atomic Theory Explains Radioactivity

Chapter 7 – Science 10
 Isotopes
 Different atoms of an element that have same #protons but
different #neutrons
 Radioactive decay
 Alpha, beta, gamma + decay products
 Half life
 The time required for ½ the nuclei in a sample to decay into
another element
 A constant for radioactive isotopes
 Fission and fusion
 Fission = break apart nuclei --- Fusion = join together nuclei
 Nuclear equations
 A nuclear reaction where atoms change
 Releases particles and/or energy
 High energy rays and particles emitted by radioactive
sources
 Examples of everyday Natural Background Radiation
 Radio waves
 Microwaves
 Infrared rays
 Visible light
 UV rays
 RADIOACTIVITY is the action of releasing high
energy particles and rays from a substance as a result
of changing the nuclei of the substance.
 Willhelm Roentgen
 Discovered and named the X-ray
 Henri Becquerel
 Discovered uranium salts emitted rays that darkened
photographic paper (up to this point only light rays
could do this, and now we know light is radiation too)
 Marie Curie and Pierre Curie
 Coined the term radioactivity
 Identified new elements (Polonium and Radium)
 Led to the field of nuclear chemistry and nuclear physics
 Marie Curie won 2 Nobel prizes (only 3 others have done
this)
 All isotopes of a particular element have the same
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number of protons but different number of neutrons
All isotopes have the same atomic number
Since the number of neutrons is differs then the mass
number will differ
Mass number = atomic number + number of neutrons
Number of Neutrons = mass number – atonic number
 Nature is a mix of isotopes
 But no matter where you take your sample, the
percentage of each isotope is constant!
 Periodic tables will sometimes take this into
consideration and provide the average mass of the
element based on different isotopes.
There are many types of uranium:
235
238
A
A
Z
Z
Number of protons
Number of protons
Number of neutrons
Number of neutrons
U
92
U
92
There are many types of uranium:
235
238
U
92
U
92
A
235
A
238
Z
92
Z
92
Number of protons
92
Number of protons
92
Number of neutrons
143
Number of neutrons
146
Isotopes of any particular element contain the same
number of protons, but different numbers of neutrons.
 Atomic mass refers to average atomic mass of the
naturally occurring isotopes of any element. Since it is
an average of various isotopes, it is generally a decimal
number. Mass number, on the other hand, is the count
of protons and neutrons for any single isotope and is
always an integer.
 ALL atoms are isotopes. Naturally occurring chemical
elements are usually mixtures of isotopes, and so their
atomic masses are weighted averages of the masses of
the isotopes in the mixture
 Atoms of one element can change into atoms of
another element!
 Decay:
 Particle decay is the spontaneous process of one
elementary particle transforming into other elementary
particles.
 Radiation:
 High energy rays and particles emitted from radioactive
sources
Radiation is Energy
• The energy is given off by unstable
(radioactive) atoms and some machines.
 Radioactive atoms emit radiation because their nuclei
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are unstable.
Unstable atoms gain stability by losing energy.
The energy released is the radiation!
Atoms will undergo radioactive decay until they form
stable non-radioactive atoms.
The non-radioactive atom is now a new element!
Isotopes that are radioactive are called radioisotopes
 The “Half-Life” describes how quickly Radioactive Material
decays away with time. *more about this in chapter 7.2
It is the time required for half of the unstable atoms to
decay.
 Some Examples:
 Some natural isotopes (like uranium and thorium) have
half-lives that are billions of years,
 Most medical isotopes (like Technicium-99m) last only
a few days
7/21/2015
UCRL-PRES-149818. Understanding Radiation and it’s Effects.
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 We are constantly exposed to
low levels of radiation from
outer space, earth, and the
healing arts.
 Low levels of naturally
occurring radioactive
material are in our
environment, the food we
eat, and in many consumer
products.
 Some consumer products
also contain small amounts
of man-made radioactive
material.
Smoke
Detector
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The sievert
(symbol: Sv) is
A measure of
the health
effect of low
levels of
radiation on
the human
body.
1 Sv/year has a
4% chance of
doing major
damage.
The Gold Foil Experiment led to the discovery of the three types of
radiation: Alpha, Beta, and Gamma
 Alpha α
 The most massive of all three types
 It is the same as a helium atom
• Notice the equation is balanced!
• Notice a new element has been
formed!
• Note: you can replace He in the
equation with α
• Alpha particles have a +charge
Alpha Decay
An alpha particle is identical to that of a helium nucleus.
It contains two protons and two neutrons.
Alpha Decay
A
X
Z
A-4
4
Y
He
+
Z-2
2
unstable atom
alpha particle
more stable atom
Alpha Decay
222
226
Ra
88
Rn
86
4
He
2
Alpha Decay
A
A-4
4
226
222
4
X
Z
Ra
88
Y
+
Z-2
Rn
+
86
He
2
He
2
Alpha Decay
222
Rn
86
222
Rn
86
A
4
Y
He
+
Z
2
218
Po
+
84
4
He
2
Alpha Decay
A
230
4
234
230
4
X
Z
U
92
Th
He
+
90
2
Th
He
+
90
2
Alpha Decay
230
Th
90
230
Th
90
A
4
226
4
Y
He
+
Z
2
Ra
He
+
88
2
Alpha Decay
A
214
4
218
214
4
X
Z
Po
84
Pb
He
+
82
2
Pb
He
+
82
2
 Beta β
 A Beta particle IS an electron
 You can use 2 symbols to represent the particle
 A neutron will change into a proton and electron
 *haven’t you ever wondered why neutrons were
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neutral?
During the Beta decay the Proton stays
During the Beta decay the Electron goes
The electron shoots out with a lot of energy
Do the math….
The atomic number will increase by 1 (an extra
proton)
The atomic mass number remains the same because
the missing electron had very little mass
Beta Decay
A beta particle is a fast moving electron which is
emitted from the nucleus of an atom undergoing
radioactive decay.
Beta decay occurs when a neutron changes into a
proton and an electron.
Beta Decay
As a result of beta decay, the nucleus has one less
neutron, but one extra proton.
The atomic number, Z, increases by 1 and the mass
number, A, stays the same.
Beta Decay
218
218
Po
84
At
85
b
-1
0
Beta Decay
A
X
Z
218
Po
84
A
b
-1
218
b
-1
Y
+
Z+1
Rn
+
85
0
0
Beta Decay
234
A
b
-1
234
234
b
-1
Th
90
Th
90
Y
+
Z
Pa
+
91
0
0
Beta Decay
A
210
b
-1
210
210
b
-1
X
Z
Tl
81
Pb
+
82
Pb
+
82
0
0
Beta Decay
210
A
b
-1
210
210
b
-1
Bi
83
Bi
83
Y
+
Z
Po
+
84
0
0
Beta Decay
A
214
b
-1
214
214
b
-1
X
Z
Pb
82
Bi
+
83
Bi
+
83
0
0
 Gamma γ
 Consists of rays of high energy, short wavelength radiation
 Has no mass
 Has no charge
 The mass does not change
 Atomic number does not change
Gamma rays are the
highest energy form of
electromagnetic
radiation.
 The parent radioactive isotope needs to have excess
energy for this to occur.
 Excess energy is represented with the symbol *
 1. the sum of the mass numbers does not change
 2. the sum of the charges in the nucleus does not
change