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

4 H + 2 0 e- He + energy11

-1 24

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.

He + He Be + energy42 2

4 84

He + Be C + energy42

84

126

Other Fusion Reactions

• Hydrogen Bomb or possible Fusion nuclear reactor reaction

• 3H + 2H 4He + 1n

• New elements discovered:

• 20Ca + 95Am 115Uup

• 115Uup 113Uut + 2He

1 1 2 0

4

Balancing Nuclear Equations

• Mass numbers and Atomic numbers must add up on both sides of the reaction arrow.

• 100Fm 54Xe + ____ + 4 0n256 140 1

For Atomic numbers 100 = 54 + X X = 46

For Mass Numbers: 256 = 140 + X + 4 X = 112

46Pd112

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

decay occur as ONE atom tries to increase it’s stability by getting rid of a few neutrons, or protons & neutrons.

• The product is an alpha, beta, or gamma particle and ONE new atom. There is only ONE thing on the left hand side.

• Nuclear reactions involve more than just getting rid of a few protons or neutrons. The new atoms produced are VERY different elements than the reactant.

• Nuclear reactions must be started, so there are 2 things on the left hand side.– Nuclear fission: makes 2

or more much smaller atoms

– Nuclear fusion: makes 1 much larger atom

Nuclear Fission

What are the differences between the 2 above nuclear equations??

Alpha Decay