1. Part I: Chapter 25 Radioactive decay & Half Life
Dr. Williams
JCHS

2. The Nucleus
A nucleus that consists of protons and neutrons is called a nucleon.
A nucleus is characterized by two numbers, represented by letters A & Z:
A: the atomic mass number (the total number of nucleons)
Z: the atomic number (the number of protons)

3. Atoms are a few angstroms, but most of the atom is empty space.
Nucleus Size
How big is a nucleus?
Atoms are a few angstroms, but most of the atom is empty space.
angstrom ang·strom or ång·strom (āng'strəm) n. Abbr. A, Å, angst A unit of length equal to one hundred millionth (10- 8 ) of a centimeter, used especially to specify radiation wavelengths.
Since the nucleus is smaller than atom, it is a few femtometers. (one quandrillionth of a meter).

4. Nuclear Forces What holds the nucleus together?
Protons are close together; thus the gravitational force attracting them to each other is much smaller than the electrical force repelling them… so what is keeping them together?
The strong force, which only works over small distances.
A strong force is an attractive force fro protons and neutrons separated by a few femtometers. It does not work over larger distances.

5. When Nuclei break up, this is known as radioactivity.
Nucleus Stability
There is a tug-of war between the attractive force of the strong nuclear force and the repulsive electrostatic forces between protons.
As Z gets larger, stable nuclei will have more neutrons than protons. A point will be reached where stability is compromised.
Bismuth, with 83 protons and 126 neutrons is the largest stable nucleus.
Nuclei with more than 83 protons are all unstable and will eventually break up into small pieces.
When Nuclei break up, this is known as radioactivity.

6. Nucleus Splitting Einstein’s equation related energy and mass. E= mc^2
With his equation you can convert mass to energy.
In a nucleus there is binding energy, the energy needed to split the nucleus into individual protons and neutrons.
To calculate binding energy add the mass of individual protons and neutrons and subtract the mass of the nucleus, then plug into Einstein’s equation.
Delta massc^2.

7. Remember isotopes…
Atoms of the same element that have different numbers of neutrons
Most isotopes are stable – those that are radioactive have an unstable ratio of protons and neutrons in their nucleus
Emitting a particle from their nucleus creates a more stable ratio
It also changes the nucleus into a different element!

8. Two kinds of transmutation reactions:
Reactions involving the decay of the nucleus and changing into a new element are called transmutation reactions
Natural transmutation
Elements that naturally emit energy without the absorption of energy from an outside source
Artificial transmutation
Causing an otherwise stable nucleus to become radioactive
Nuclear fission and nuclear fusion
Two kinds of transmutation reactions:

9. Transmutation can occur by radioactive decay
Transmutation can also occur when particles bombard the nucleus of an atom.

10. Note - gamma radiation is not a particle it is only energy
There are several types of particles that an unstable nuclei can emit to attain a more stable atomic configuration
Table O – Symbols used in Nuclear Chemistry – shows us particles that can be emitted
Note - gamma radiation is not a particle it is only energy

11. + charge
0 charge

12. Characteristics of Radiation Types pg. 801
Each radioisotopes only goes through one type of decay mode.
The three decay modes are:
Alpha decay
The release of an alpha particle from the nucleus
Beta decay
The release of a beta particle from the nucleus
Positron decay
The release of a positron particle from the nucleus.
A positron is a particle with the mass of an electron but a positive charge. During positron emission, a proton changes to a neutron.

17. Natural Transmutation
Just as a balanced chemical reaction must always be written for a “normal” chemical reaction a balanced nuclear reaction can be written as well.
These are examples of balanced nuclear equations
One element becomes another
The particles involved are balanced
The sum of the mass numbers and the sum of the atomic numbers on each side are equal

18. Natural Transmutation
Steps for writing and balancing a nuclear reaction
Sample: Write and balance the nuclear reaction for radium-226.
Step 1: Write the notation for the radioactive isotope as the reactant
Step 2: Look on Table N for the decay mode of the radioisotope
radium – 226 = α
Step 3: Look on Table O for the symbol of the decay mode. This is one of the products, write the symbol of the particle as a product
Step 4: The second product is determined by first balancing the mass number and atomic number and then looking on the periodic table to determine what element has that atomic number
226
222 4 Rn Ra He + 2 86 88

19. Natural Transmutation
Write the balanced nuclear equation for the decay of iodine - 131
131
131 I β Xe + 53 -1 54

20. Natural Transmutation
Neon – 19 decays by positron emission. Write the balanced nuclear equation. 19 Ne β 19 F + 10 +1 9

21. Practice with Alpha Particles
Alpha decay can most simply be described like this:
1) The nucleus of an atom splits into two parts.
2) One of these parts (the alpha particle) goes zooming off into space.
3) The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons).
There are other points, but the three above are enough for this class. Here is a typical alpha decay equation:

22. Alpha Practice

23. Answers Set 1

24. Alpha Practice

25. Alpha Practice Answers 2

26. Alpha Practice

27. Alpha Practice Set 3

28. Beta Practice
Beta decay is somewhat more complex than alpha decay is. These points present a simplified view of what beta decay actually is:
1) A neutron inside the nucleus of an atom breaks down, changing into a proton.
2) It emits an electron and an anti-neutrino (more on this later) which go zooming off into space.
3) The atomic number goes UP by one and mass number remains unchanged.
Here is an example of a beta decay equation:

29. Beta Practice Set 1

30. Beta Practice Set 1 Answers

31. Beta Practice 2

32. Beta Set 2 Answers

33. Beta Practice Set 3

34. Beta Practice 3 Answers