1. Unit 14: Nuclear Chemistry

2. Chemical vs Nuclear
Atoms tend to attain stable electron configurations by losing, gaining, or sharing electrons
The nuclei of unstable isotopes gain stability by undergoing changes which emit large amounts of energy
Small amount of mass is converted into a large amount of energy (E = mc2)

3. Transmutation
The conversion of an atom of one element to an atom of another element
Natural transmutation: an unstable nucleus decays on its own
These atoms are called nuclides or radioactive isotopes.
Artificial transmutation: when particles bombard the nucleus of an atom and force it to be unstable

4. Natural Transmutation: What makes a nucleus unstable?
Two ways to be unstable:
An unequal ratio of neutrons:protons
Very large nuclei are unstable- elements with atomic # > 83 are unstable because they are too big

5. All radioisotopes on Table N undergo natural transmutation

6. Radioactivity
As radioisotopes break apart to turn into more stable elements, they give off different types of energy called radiation.
4 main types of radiation

7. Looking at Decay Modes Graphically

8. Balancing Nuclear Equations
235U + 1n  141Ba + X n

10. 1. Alpha radiation Alpha particles: mass of 4, charge of +2
Fairly large compared to other types of radiation, so they can be blocked easily.
Paper, clothing etc., can prevent damage from alpha radiation

11. 2. Beta Radiation Beta particles: mass of zero, charge of -1
Fairly small compared to other types of radiation
Can only be blocked by a more dense material, like wood or foil.

12. 3. Positron Emission
Mass of zero, charge of +1
The “anti-electron”

13. 4. Gamma Radiation A gamma ray is just energy (no mass, no charge)
Gamma rays cannot be blocked completely by anything
However, concrete and lead (Pb) are very dense and can block some gamma rays.

15. Half-Life
Amount of time it takes for half of an original amount of a naturally radioactive isotope to decay
Values do NOT depend on temperature, pressure, or the original amount
Original sample
(100g)
½ life
(50 g)
½ life
(25 g)
½ life
(12.5 g)
½ life

16. Half–life problems: The Arrow Method
How long does it take a g sample of K-42 to decay to 25.0 g?
What is the half life of a 25.0 g sample of a radioisotope that decays to 6.25 g in 34.6 seconds?
If 5.0 g of Co-60 is left after 15.8 years, how many grams were in the original sample?
If the half-life of a radioisotope is 3 hours, what fraction of an original amount remains unchanged after 15 hours?

17. Another type of transmutation…
Artificial transmutation: when we force an atom to be unstable by adding something to the nucleus.
2 reactants and at least 2 products
Two types: fission and fusion
Both types of nuclear reactions release huge amounts of energy by converting a small amount of mass into a large amount of energy

18. Nuclear Fission 235U + 1n  141Ba + 92Kr + 31n + ENERGY 92 0 56 36 0
A very large (heavy) nuclei breaks apart into smaller pieces
Typical reaction with Uranium
Products are very radioactive
Chain reaction – needs to be controlled well
235U + 1n  141Ba + 92Kr n + ENERGY

19. Fission

20. Nuclear Fusion 2H + 3H  4He + 1n + ENERGY 1 1 2 0
Very small nuclei combine to form a larger nucleus
Difficult because nuclei are (+) charged, and (+) charges repel
Requires high temperatures and pressures
2H + 3H  4He + 1n + ENERGY