1. Ch. 18: The Nucleus
Review
21.1: Nuclear Stability and Radioactive Decay
21.2 Kinetics of Decay
21.3 Nuclear Transformations

2. Review Nucleus contains protons and neutrons Atomic number (Z) - # p+
Mass number (A) - # p+ + # n0
Isotopes- differ in number of neutrons only
Same Z but different A
Nuclide- specific type of atom, member of a group of isotopes
Nuclear symbol notation

3. Radioactivity and Stability
A nucleus that decomposes forming another nucleus and one or more particles
All nuclides are unstable with 84 p+ or more
Lightweight nuclides are stable with equal numbers of n0 and p+
Heavy nuclides should have ratio >1 to be stable

4. Radioactivity and Stability

5. Types of Decay- A changes
Alpha particle production
α particle: helium nucleus
Spontaneous fission
Splitting of a heavy nucleus into 2 lighter nuclides that are about the same size

6. Types of Decay- A is Constant
Beta Particle Production
β particle is an electron
Can assume the mass is zero
Net effect : changing a n0 into a p+

7. Types of Decay- A is Constant
Gamma Ray Production
γ ray is collection of high energy photons
Occurs with other types of decay
Helps a nucleus release extra energy so it can relax to a lower energy state

8. Types of Decay- A is Constant
Positron Production
Occurs for nuclides below the line of stability
Positron is a positive particle with same mass of electron
Also called antiparticle of electron
Net effect: change p+ into n0

9. Types of Decay- A is Constant
Electron Capture
One of the inner electrons in an atom is captured by nucleus
Gamma rays always produced
Decay Series
When several types of decay occur until a stable nuclide is produced

10. Writing Equations 116C produces a positron
21483Bi produces a beta particle
23793Np produces an alpha particle

11. Writing Equations Beta particle (electron) Electron capture Positron
Positron production

12. Kinetics of Decay
Rate of decay is directly proportional to number of nuclides available
All are first order
Constant half-life

13. Example
If the half-life of a decay is 67.0 hours, how much of a mg sample will remain after 335 hours?
335 / 67 = 5 half-life’s
1.000 mg  mg 
0.250 mg  mg 
0.062 mg  mg

14. Nuclear Transformations
Change of one element into another
Scientists have been able to use this to make the periodic table larger by creating new elements
Since 1940, have been able to make transuranium elements (93-112)

15. 18.4 Detection and Uses of Radioactivity 18.5 Thermodynamic Stability
18.6 Nuclear Fission and Fusion
18.7 Effects of Radiation

16. Carbon-14 Dating Used to date items made out of natural fibers
Created by Willard Libby in 1940s
Based on the decay of naturally existing carbon-14 isotope by β-particle production
It is also created

17. Carbon-14 Dating
These happen at the same rate as long as the plant is alive but when it dies, the decay happens more rapidly than the creation
Ratio of 14-C to 12-C decreases
Most accurate for pieces older than 10,000 years

18. Medical Applications Radiotracers
Radioactive nuclides that can be traced in people by monitoring their radioactivity
Thallium-201
For assessing heart damage from heart attacks
Is taken up by healthy heart tissue only

19. Medical Applications Iodine-131 For diagnosing thyroid problems
Patients drink a solution of 131-I and the uptake is monitored

20. Thermodynamic Stability
Can be determined by calculating the change in potential energy if the nucleus is made from individual particles
We can create energy changes by comparing the sum of the masses: mass defect
Mass of 168O – mass of (8 10n n)
Convert amu on periodic table to g (1amu=1.66x10-24 g)
x10-23 – [8( x10-24) + 8( x10-24)]
-2.269x10-25 g/nucleus = g/mol : lost when 1 mol of 16-O is formed

21. Thermodynamic Stability
Find energy (J) using E=mc2
E = (-1.366x10-4kg)(3.00x108m/s) = -1.23x1013J/mol
Binding energy
Energy required to decompose this nucleus into its particles
Often in MeV / nucleon
mass must be in kg!

22. Thermodynamic Stability

23. Nuclear Fission and Fusion
Splitting a heavy nucleus into 2 smaller nuclei with smaller mass numbers
Can use neutrons to create instability
Neutrons produced are used to cause more fission
Produces a huge amount of energy

24. Nuclear Fission and Fusion

25. Nuclear Fission and Fusion
Combination of 2 light nuclei to form a heavier, more stable nucleus
Stars produce their energy using this
Requires very high temperatures
Must be shot at each other to get close enough

26. Effects of Radiation
Any sort of energy transferred to cells can break bonds and cause damage
Radioactive species are sources of high energy particles so can be very harmful
Types
Somatic: cause illness, cancer, death
Genetic: produce damage in offspring

27. Factors in Effects of Radiation
The more energy, the more damage
How deep it goes into body
γ rays > β particles (1 cm) > α particles (skin)
How easily they attract electrons from biomolecules (ionization)
γ rays cause less than α particles
How long it stays inside body