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Radioactivity
Dalton’s Atomic Theory (1805)
• All elements are composed of atoms, which are
indestructible and indivisible.
• All atoms of the same element are exactly alike – they have
the same mass.
• Atoms of different elements are different – they have
different masses.
• Compounds are formed by the joining of atoms of two or
more elements. In any compound, the atoms of the
elements are joined in a definite whole number ratio (1:1,
1:2, 3:2, etc)
The “Smoking Gun:”
Becquerel’s Photographic Plates (1896)
Radioisotopes
• The discovery of radioactivity by Becquerel and
the Curies showed that one of Dalton’s ideas, that
matter is indestructible and indivisible, is not
always true.
• Certain isotopes, because of their size and/or
ratio of protons and neutrons, are not stable.
• Radioisotopes have unstable, high energy nuclei
• Radioisotopes lose energy by emitting radiation
and matter
Radioactive Decay
• Radioisotopes decay from one element to
another until they are transformed into stable,
non-radioactive isotopes.
• For example, 238U decays 11 times, shedding
mass and energy each time, eventually becoming
206Pb, a stable isotope.
• Radioactive decay is spontaneous – it does not
require an input of energy.
• Another term for radioactive decay is
transmutation.
U-238 Decay Series
Three Types of Radiation
• alpha radiation
• beta radiation
• gamma radiation
α
β- , β+
ϒ
Alpha Radation
• In alpha radiation, helium nuclei are ejected
from the nucleus.
• The process studied by Curie involved
uranium-238, among other radioisotopes:
238
92
U 
 Th He
radioactive_ decay
234
90
4
2
In alpha decay, the atomic number decreases by 2, the
mass number decreases by 4.
238
92
U 
 Th He
radioactive_ decay
234
90
4
2
• The helium nucleus is known as an alpha particle. This
process is known as  emission. In this case, an atom
of uranium changes into an atom of thorium.
• The general process of atoms changing into different
elements is known as transmutation.
• These alpha particles are less dangerous than other
forms of radiation.
• Their large mass insures that they do not penetrate
very far into materials.
• This is a good thing because they steal two electrons
from other atoms they encounter.
• These alpha particles are less dangerous than
other forms of radiation.
• Their large mass insures that they do not
penetrate very far into materials.
• This is a good thing because they steal two
electrons from other atoms they encounter.
Charged Particles are deflected in
electric/magnetic fields
• Because of their positive charge, alpha particles
are easily detected in electric and magnetic fields.
Penetrating Power of Radiation
Alpha Decay Equations
Nuclide
215At
217Rn
218Fr
228U
236Pu
241Am
217Po
+1
1p
1
o n
Decay Equation
Beta Decay
• Protons and neutrons have energy states, just
like electrons
• Like electrons, nucleons tend to be more
stable in pairs
• The ratio of protons and neutrons is also
important
• If a nucleus has too may neutrons, a neutron
can eject an electron, becoming a proton.
Beta Radiation
• Beta radiation is composed of high energy
electrons, known as beta particles. A neutron
breaks down to form a proton and an beta
particle ():
1
0
n H  e  
1
1
o
1
Beta Particles and Neutrinos
• The v particle is a neutrino, a very tiny particle
of negligible mass. Note that this equation can
also be written
1
0
• Table O:
n H    
1
1
o
1
1
0
n H    
1
1
o
1
• Neutrinos are virtually undetectable and are
often left out of nuclear equations.
• The proton remains in the nucleus and is not
ejected. Thus in beta decay, the atomic
number increases by one! The most famous
example is carbon-14.
14
6
C N  e
14
7
0
1
14
6
C N  e
14
7
0
1
• Carbon-14 is radioactive. By decaying to
nitrogen-14, it attains stability.
• Beta particles are less massive, and therefore
more penetrating. They represent a greater
health risk than alpha particles.
Beta Minus Decay Equations
Nuclide
16C
19O
25Na
10Be
66Cu
71Zn
213Bi
+1
1p
o
1n
Decay Equation
Positron Emission
• There is another interesting form of beta
emission.
• A proton will transform itself into a neutron,
releasing a positron, a positively charged
electron-like particle.
• Positrons are a form of antimatter.
1
1
p n   
1
0
0
1
Positron Emission
• Artificially created radioactive nuclei of the lighter
elements often undergo positron emission:
30
15
P Si 
30
14
0
1
• Many artificial radionuclides are have too many
protons, so a proton changes into a neutron.
• In positron emission, the atomic number goes
down by 1, and the mass number remains
unchanged.
Positron Emission Equations
Nuclide
10C
14O
22Na
7Be
58Cu
63Zn
203Bi
+1
1p
o
1n
Decay Equation
Gamma Radiation
• Gamma rays are high frequency radiation –
they have no mass or charge.
• Gamma emission does not change the atomic
number or mass number of the atom, and
often accompanies  or  emission.
Gamma Radiation Often Accompanies
Other Forms of Decay
234
92
234
90
U Th He  
230
90
4
2
Th Pa   
234
91
0
1
• Gamma rays are extremely
dangerous and will pass completely
through the body, damaging cells
as they go.
• Several cm of lead is necessary to
contain gamma rays.
Which type of emission has the
highest penetrating power?
•
•
•
•
alpha
beta
positron
gamma
Natural Transmutation
• Alpha decay: Mass number ↓4, atomic
number ↓2
• Beta minus decay: mass number unchanged,
atomic number↑1
• Beta plus decay: mass number unchanged,
atomic number ↓1
The Zone of Stability
For a given element, there
is an optimal ratio of
protons to neutrons
~1:1 ratios common with
lighter elements
Heavier elements: 1: 1.5
Deflection by Magnetic/Electric Fields
The diagram represents radiation
passing through an electric field.
Which type of emanation is represented by the
arrow labeled 2?
•
•
•
•
alpha particle
beta particle
positron
gamma radiation
• Which type of radiation would be attracted to
the positive electrode in an electric field?
Which type of radiation has neither
mass nor charge?
•
•
•
•
gamma
neutron
alpha
beta
Which radioactive emanations have a
charge of 2+?
•
•
•
•
alpha particles
beta particles
gamma rays
neutrons
Famous Radioisotopes
• The type of decay is
indicated by the decay
mode.
• Homework: Write decay
equations for each
nuclide on Table N
Which
radioisotope is a
beta emitter?
•
•
•
•
90Sr
220Fr
37K
238U
Which isotope will spontaneously decay
and emit particles with a charge of +2?
•
•
•
•
53Fe
137Cs
198Au
220Fr
Positrons are spontaneously emitted
from the nuclei of
•
•
•
•
potassium-37
radium-226
nitrogen-16
thorium-232
In the equation:
• Which particle is represented by X?
• The fossilized remains of a plant were found at
a construction site. The fossilized remains
contain the amount of carbon-14 that is
present in a living plant.
• Which answer choice correctly completes the
nuclear equation for the decay of C-14?