The Atomic Nucleus &
Radioactive Decay
(Chapter 10)
Student Learning Outcomes
• Analyze radioactive decay processes
• Differentiate between nuclear
Fission & Fusion
How do we know the number of neutrons?
Atomic number = number of protons
Mass number = protons + neutrons
17 O−2
8
Practice
Classify each atom.
Is it an ion? Is it an isotope?
3 He
2
13
6C
34 Cl−1
17
26 Fe
13
6
+1
Li
3
What is the limit for the Strong Nuclear Force?
The protons and
neutrons in the core of
an atom are held
together by the strong
nuclear force.
Diameter < 10-15 m
The strong nuclear force
balances the electric
force in a stable atom.
FN
FE
Nuclear Force vs. Electric Force
Strong Nuclear Force
Attraction
Protons & Neutrons
Range < 10-15 m
Electric Force
Repulsion
Protons
Range > 10-15 m
Practice
1) Would a helium (He) atom or a bismuth (Bi)
atom tend to be more unstable?
2) What about a 32He atom?
What does radioactivity mean?
Radioactivity is the
transformation of an
unstable atom into a
different type of atom.
Over 60 radioactive
elements can be found
in nature.
There are 3 general
sources of radioactivity.
Earth
Cosmic Rays
Human Produced
How do we determine the new isotope?
Atoms radioactively decay by emitting high energy
particles.
XY+a
(alpha decay)
XY+b
(beta decay)
XY+g
(gamma decay)
Alpha radiation: particle (He+) is a positively
charged particle that leaves the nucleus.
Can cause damage on the surface of matter
Beta radiation: (e−) is a negatively charged
particle that leaves the nucleus.
8 Li
3
8 Be
4
Beta
Particle
Can penetrate through several millimeters of matter
Gamma Rays: (g) are high energy photons.
High Energy
Atom
No change
to nucleus
High Energy
Gamma Ray
released
Can pass through all types of matter
The Rules
Unstable
Condition
Name
Symbol
+2
a
Neutron/Proton
Beta
e–
–1
Ratio
Excited
Gamma photon 0
Nucleus
b
83+ Protons
Alpha
4
2
He+
g
The
Rules
D(Mass #) = – 4
D(Atomic #) = – 2
D(Mass #) = – 0
D(Atomic #) = + 1
No Change
Practice
Is the atom an isotope?
1.
226
−
Ra

X
+
e
88
2.
226
88Ra
3.
222
86Rn  X +
4.
14
6C
5.
36
18Ar
What makes it unstable?
What is the new atom?
 X + a
 X + b
 X + g
4 He
2
What does the half-life of an element indicate?
Half-life is the time it takes for one half of an
unstable substance to decay into a different substance.
Isotope
Half Life
15 C
6
14 C
6
2.449 seconds
5,730 years
Practice
1. An isotope of radium (Ra) has a half-life of 1620 years.
If 1000 grams were placed in a barrel, how much of the
material in the barrel would be radium after 6480 years?
2. 22589Ac (Actinium) has a half-life of 10.0 days. How
many days would it take to decrease the original amount
placed in a barrel to 1/8th of the original amount?
What is carbon dating?
Carbon dating is the process of using the known
half-life of carbon to determine age of a sample.
 Living plants and animals take in CO2 (Carbon-14)
 Carbon-14 is a radioactive isotope
6C 
14
14
7N
+ b
Practice
The half-life of carbon-14 is 5730 years.
1. If a piece of wood has 1/2 as much carbon-14 as
compared to a living tree, how old is the wood?
2. If a piece of wood has 1/32 as much carbon-14 as
compared to a living tree, how old is the wood?
A Question to Ponder
3. It is assumed the relative abundance of
carbon-14 in our atmosphere has remained
constant for the last 50,000 years. How do you
think this assumption could affect carbon
dating?
What is Nuclear Fission?
Nuclear fission is
splitting apart a nucleus.
 Electric force wins
Nuclear fission is used in
atomic bombs & nuclear
reactors.
 Nucleus flies apart
 Atomic bombs produce a
chain reaction.
 Huge amounts of energy
released
 Nuclear reactors control the
rate of fission.
A little Uranium yields a lot of energy
8.4 x 1010 Joules per atom
A Question
Nuclear energy is a clean fuel. It does not pollute
the atmosphere. What reasons may people have
for not wanting to utilize nuclear energy?
What is Nuclear Fusion?
Nuclear fusion is the process of forcing atoms
together into a new type of atom.
Requires very high temperatures and pressures
Nuclei must overcome Coulomb Barrier
Some mass is converted into energy
Nuclear Fusion occurs naturally in stars.
Image Credit: NASA
1 Second In The Sun
• 1038 Reactions
• 600 Billion kg H  He
• 4 Billion kg Mass  Energy
• 4 x 1026 Watts
http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
How is radiation measured?
Radiation is energy.
Lethal doses begin at about
400 – 500 rems
 The rem: radiation
absorbed and the possible
biological damage
Food
Electronics
Tobacco
Human exposure is about
0.2 rems/year
39 mrems
11 mrems
1,300 – 9,000 mrems