1. Radioactivity Changes in the nucleus

2. Forces within the atom Strong Nuclear Force:
a short-range force that opposes repulsion of protons & holds p+ & n◦ together in the nucleus
2. Weak Nuclear Force:
responsible for radioactive decay
3. Electromagnetic Force:
attraction b/w opposite charges of (+) nucleus and (-) electrons; keeps e- in orbit.
4. Gravity: the weakest, little or no effect

3. History 1896 – Henri Becquerel
Discovers radioactivity in Uranium minerals
1898 – Marie Curie
Coined the term “radioactivity”
Worked with pitchblende which contained U, Po and Ra; isolated Radium
Only person ever to win two Nobel prizes
1911 – Ernest Rutherford
Used alpha particles emitted from Ra in Gold Foil Experiment

4. Peaceful uses for Radioactivity
Radio dating
Sterilization
Smoke detectors
Medical diagnostics & treatment
Electric power
Synthesis of new elements

5. Chemical Reactions vs Nuclear Reactions
CHEMICAL CHANGES
Atoms interact only through their outer electrons
New compounds form but not new elements
Obey conservation laws (mass & energy)
Some E changes take place as chemical bonds break & reform

6. Chemical Reactions vs Nuclear Reactions
NUCLEAR CHANGES
Involve only the nucleus of the atom.
Isotopes are transformed into different isotopes!
Can only take place under high-energy conditions of extreme temperature & pressure: inside stars, particle accelerators & nuclear reactors.
Convert a small amount of mass to a large amount of energy according to E = mc2.
We see this energy as sunlight!

7. Nuclear Binding Energy & Mass Defect
When a nucleus if formed from nucleons, some mass changes into energy according to Einstein’s equation: E=mc2.
The E released = the nuclear binding energy: the difference between the mass of an atom and the sum of the masses of the p+, n0, & e-.
This difference is called the mass defect.

8. Nuclear Stability Question: What makes a nucleus unstable?
Answer: Not enough or too many neutrons!
Rule of Thumb:
Isotopes that are much heavier or lighter than the most common isotope are likely to be radioactive.
Probability:
We know they will decay, we know how long it will take but we do not know when!

9. Predicting Nuclear Stability
ATOMIC NUMBER
RATIO OF NEUTRONS TO PROTONS
Smaller atoms
Almost equal (1 : 1)
Larger atoms
Need more n◦ to be stable (1.5 : 1)
Greater than 83
All: more stable if…..
All unstable =radioactive
even numbers of nucleons

10. Four Types of nuclear changes:
Radioactive Decay
Fission
Fusion
Artificial Transmutation

11. Radioactive Decay
The spontaneous decay of the nucleus of an atom by emission of particles and/or radiation.
Includes:
Alpha decay
Beta decay
Electron capture
Positron emission

13. Fission
Nuclear rxns in which a heavy nucleus splits into smaller nuclei when bombarded with neutrons.
More n◦ are produced → split other heavy nuclei = a chain reaction
An uncontrolled chain rxn = a nuclear bomb
A controlled chain rxn = a nuclear reactor.
Control rods are used to absorb the n◦.
Moderators are used to slow the n◦.

14. Fission

15. Fusion
Nuclear rxns in which two or more small nuclei combine to form one larger nucleus, releasing large amts of Energy.
Think sunlight!
Some of the mass ( < 1%) of the combining particles is converted to electromagnetic radiation according to E=mc2.
The resulting nuclei are more stable.

16. Fusion

18. WARM UP
Find at least FIVE new terms in your notes that you need to learn….
Yellow PTs are for classroom use – you can leave them on the desk…. But please do not write on them! Thx.

19. Artificial Transmutation: Nuclear Bombardment
Has taught us about the structure of atoms.
Three things can happen:
1. Nucleus shatters, see what comes out.
2. Nucleus becomes metastable & gives off E, used in medical diagnostics.
3. Transmutation occurs, a new element forms.

20. Transmutation
A change in the identity of the nucleus as a result of a change in the number of its protons.

21. Penetrating ability of radioactive particles

22. Penetrating ability of radioactive particles

23. Isotope Notation
Helium – 4 OR He-4
hyphen notation for this nuclide

24. Atomic Number: p+ number, identifies element
Mass Number: total of nucleons, identifies the isotope (or nuclide)
Nucleons: Protons and neutrons.

25. Warm Up: For Plutonium-239….
The 239 is a _______ .
The number of p+, n°, & e- is _______ .
The nuclear symbol is _______ .
What do you notice about the number of neutrons in larger atoms?
What is the name of the force that holds nucleons together in the nucleus?
What is diff b/w fission and fusion?
Which happens in stars? Which in nuclear reactor?

26. Writing Nuclear Equations
Represents a nuclear reaction
The mass numbers must be equal on both sides of the arrow.
The atomic numbers must be equal on both sides of the arrow.

27. Writing Nuclear Equations
Some examples of nuclear decay equations are:

28. Radioisotopes
All elements have one or more isotopes that are unstable.
These isotopes may be natural or artificial; artificial radioisotopes are produced in nuclear bombardment rxns.

29. Cosmic Rays Streams of ionizing radiation of extraterrestrial origin.
Consist mostly of protons and alpha particles (90%) but include small amounts of other atomic nuclei like C, N, and O.
Enter the atmosphere and collide with atomic nuclei producing secondary radiation.

30. Carbon-14
Forms in the upper atmosphere when N-14 is bombarded with cosmic rays.
C-14 → CO2 → living things (through photosynthesis!)
Scientists measure the remaining amt. of C-14 in organic matter.
Used to date specimens 500 to 500,000 years old.

31. Other isotopes useful for Radiodating
Name
t1/2
Use
Tritium
12.3 years
wines
U-238
4.5 x 109 years
rocks

32. Half-Life
The time it takes for one-half the nuclei present in a sample to decay.
Because half the sample becomes stable with each half-life that passes, the decay is exponential.
Remember, we can’t predict when a nucleus will decay but we can predict how long it will take.

34. Predicting Nuclear Stability
Even and odd numbers of p+ & n◦
Stable
Even numbers of p+ & n◦
Slightly less stable
Either p+ or n◦ are uneven
Unstable
Both p+ & n◦ are uneven