1. Isotopes and Nuclear Reactions
Alpha, Beta, and Gamma Decay

2. Definition of Isotope
Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

3. Average Atomic Mass
The atomic mass of an element is the average of all of the isotopes
It is based on the relative abundance of each type of isotope- much like weighted grades!

4. Example #1: Carbon mass number exact weight percent abundance 12
98.90 13
1.10
Example #4: Silicon
mass number exact weight percent abundance
The answer for silicon:

5. Most of the isotopes which occur naturally are stable.
A few naturally occurring isotopes and all of the man-made isotopes are unstable.
Unstable isotopes can become stable by releasing different types of particles.
This process is called radioactive decay and the elements which undergo this process are called radioisotopes.

6. The electrons are held onto the atom by an electromagnetic attraction force.
This is because electrons are negative and protons (the nucleus) are positive.
Inside the nucleus, protons and neutrons are held together by a nuclear strong force.
Since protons are all positive, the electromagnetic force present is a force of repulsion, so the nuclear strong force is a much stronger force over a much smaller distance.

7. The strong force is felt only between particles inside the nucleus.
As the nuclei of atoms become more massive, the diameter of the nucleus increases.
Once the mass of a nucleus reaches a certain point, the strong force is not strong enough to hold the nucleus together in a stable way and the atom becomes unstable.

9. Radioactive Decay Radioactive decay results in the emission of either:
an alpha particle (a),
a beta particle (b),
or a gamma ray(g).

10. Alpha Decay
An alpha particle is identical to that of a helium nucleus.
It contains two protons and two neutrons.

11. Alpha Decay Rn
222 86 He 4 2 Ra
226 88

12. X Y + He Alpha Decay A Z A - 4 Z - 2 4 2 unstable atom (parent)
alpha particle
(radiation)
more stable atom
(daughter)

13. Alpha Decay X A Z Y
A - 4
Z - 2 + He 4 2 Ra
226 88 Rn
222 86 + He 4 2

14. Alpha Decay Rn
222 86 + Y A Z He 4 2 Rn
222 86 He 4 2 + Po
218 84

15. Alpha Decay X A Z + Th
230 90 He 4 2 He 4 2 U
234 92 + Th
230 90

16. Alpha Decay Th
230 90 + Y A Z He 4 2 He 4 2 + Ra
226 88 Th
230 90

17. Alpha Decay X A Z + Pb
214 82 He 4 2 He 4 2 + Pb
214 82 Po
218 84

18. Beta Decay
A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay.
Beta decay occurs when a neutron changes into a proton and an electron.

19. Beta Decay
As a result of beta decay, the nucleus has one less neutron, but one extra proton.
The atomic number, Z, increases by 1 and the mass number, A, stays the same.

20. Beta Decay b -1 At
218 85 Po
218 84

21. Beta Decay X A Z Y
Z + 1 + b -1 Po
218 84 At 85 + b -1

22. Beta Decay Th
234 90 Y A Z + b -1 Th
234 90 Pa 91 + b -1

23. Beta Decay X A Z Pb
210 82 + b -1 Tl
210 81 Pb 82 + b -1

24. Beta Decay Bi
210 83 Y A Z + b -1 Bi
210 83 Po 84 + b -1

25. Beta Decay X A Z Bi
214 83 + b -1 Pb
214 82 Bi 83 + b -1

26. Gamma Decay
Gamma rays are not charged particles like a and b particles.
* They are electromagnetic radiation with high frequency.
When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable.

27. Penetrating Power of Radiation