Unit 2 – The Atom
Nuclear Chemistry
Fusion and Fission
Nuclear Fission
• The splitting of a nucleus into smaller
fragments is called nuclear fission.
• Heavy atoms (mass number>60) tend to
break into smaller atoms, thereby
increasing their stability.
• Nuclear fission releases a large amount of
energy.
Nuclear Fission
Mass Defect = Energy!
• Example- the energy released from the nuclear
reaction of 1kg of uranium is equivalent to the
energy released during the combustion of 4
billion kilograms of coal.
• This large amount of energy is due to the mass
defect
• Every time fission occurs, there is a difference
between the mass of the starting atoms and the
smaller atom products. This difference in mass
is converted into energy by Einstein’s equation :
• E = mc2
Fission Chain Reactions
• One fission reaction can lead to more
fission reactions in a process called a
chain reaction.
• Example - The fission of Uranium-235
Chain Reaction of Uranium-235
• A chain reaction can only occur if the starting
material has enough mass to sustain a chain
reaction. This amount is called the critical mass.
• Nuclear Fission is what occurs in Nuclear
Reactors and Atomic Bombs.
• The Nuclear reactor is a controlled fission
reaction, the bomb is not.
Nuclear Fusion
• The combining of atomic nuclei to form a
larger atom is called fusion
• Nuclear fusion occurs in the sun where
hydrogen atoms fuse to form helium
1
1
0 e-
4 H + 2 -1

4
2
He + energy
Nuclear Fusion
Fusion
• Fusion reactions also release very large
amount of energy but require extremely
high temperatures to start.
• Nuclear fusion also occurs in new stars
and is how all of our elements were made.
4
2
4
2
He +
He +
4
2
He 
8
4
Be 
8
4
Be + energy
12
6
C + energy
Other Fusion Reactions
• Hydrogen Bomb or possible Fusion
nuclear reactor reaction
•
3H
1
+ 12H 
4He
+ 10n
2
• New elements discovered:
• 20Ca + 95Am  115Uup
•
115Uup

113Uut
4
+ 2He
Balancing Nuclear Equations
• Mass numbers and Atomic numbers must
add up on both sides of the reaction arrow.
256
140
112
46
1
Pd
• 100Fm  54Xe + ____ + 4 0n
For Atomic numbers 100 = 54 + X
X = 46
For Mass Numbers: 256 = 140 + X + 4
X = 112
Chemical
Reactions
• Involve changes in
electrons
– Acids & Bases,
combustion, displacement
• The same atoms appear
on both sides of the
reaction.
• Follows Dalton’s Law of
Conservation of Mass
• Small amount of energy
generated
– Burning fossil fuels
Nuclear
Reactions
• Involve changes in the
nucleus
– Nuclear fusion, nuclear
fission
• New atoms appear as
products of the reaction.
• Breaks Dalton’s Law of
Conservation of Mass
• Large amount of energy
generated
– 1 million times more than
chemical reactions
– Nuclear fusion on the sun
– Nuclear fission for reactors
Decay vs. Nuclear Reactions
• Alpha, beta, and gamma • Nuclear reactions involve
decay occur as ONE
more than just getting rid of
atom tries to increase
a few protons or neutrons.
it’s stability by getting rid
The new atoms produced
of a few neutrons, or
are VERY different
protons & neutrons.
elements than the reactant.
• The product is an alpha, • Nuclear reactions must be
beta, or gamma particle
started, so there are 2
and ONE new atom.
things on the left hand side.
There is only ONE thing
– Nuclear fission: makes 2
on the left hand side.
or more much smaller
atoms
– Nuclear fusion: makes 1
much larger atom
Nuclear Fission
Alpha Decay
What are the differences between the 2 above
nuclear equations??