1. Nuclear and Atomic Physics
Intro: Atomic Structure and a little history. 
Print off the topic outline for this unit by tomorrow!

2. Early atomic structure…
JJ Thomson: “Plum pudding” model
An atom is a mixture of positive and negative charges
Image from:

3. Atomic Structure
1909: Rutherford—Worked with Geiger and Marsden on the gold foil experiment
A very thin gold foil was placed in the center of a chamber that had photo detecting material on its inner surface.
Alpha particles (essentially a helium nucleus) were shot toward the gold foil
The particles were detected on the inner surface of the chamber, and any scattering was noted

4. Image from: http://bhs. smuhsd

5. Gold foil experiment… Results: Conclusion:
Most alpha particles were detected at very small scattering angles (essentially went through the foil, but were deflected)
Some larger-angle scattering occurred, sometimes large enough that the alpha particle seemed to reflect back nearly on its original path.
Conclusion:
Atoms have a dense, positively charged center (the nucleus) and the electrons must be in the space surrounding the nucleus

6. Rutherford’s Atom… “Planetary model”:
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7. Planetary Model Massive, positively charged nucleus
Electrons orbited much like planets around the Sun
The Coulombic (electrostatic) force of attraction between the positive protons and the negative electrons kept the electrons in orbit

8. Planetary Model—problems…
According to a theory of electromagnetism, accelerating charges will emit energy in the form of electromagnetic radiation
Radiating energy would cause the electron to have a lower total energy and therefore would cause it to orbit a smaller distance from the nucleus…
Electron would spiral in and crash into the nucleus

9. Neils Bohr—atomic postulates
Studied Hydrogen atom
Determined that there are certain defined energy states in which the electron can exist
In one of these states, the electron will not radiate its energy and will remain in a stable orbit
Energy can only be lost if the electron transitions into a state of lower energy

10. Emission and Absorption Spectra
All elements will emit light in characteristic colors when heated
Scottish physicist Thomas Melville—first to study emitted light ( )
Heat source = flame
Passed emitted light through prism
Pattern produced was significantly different than white light passed through spectrum
Bright line spectrum

11. Emission and Absorption spectra give the same “fingerprint” for an element, but in different ways

12. Alpha particles directed at a thin gold foil will most likely...
pass directly through the foil with no deflection
be reflected straight back from the solid foil
pass through the foil wil a small amount of deflection
be deflected at large angles as it passes through the foil

13. Large deflections of alpha particles in Geiger and Marsden's scattering experiment suggested...
atoms consist of a small negative nucleus surrounded by protons
atoms consist of a small positive nucleus surrounded by electrons
atoms consist of a small neutral nucleus surrounded by electrons and protons
atoms consist of a large positive mass with embedded electrons

14. The Rutherford model of the atom has the positive charge...
spread uniformly throughout the atom's volume
circling the nucleus as positive electrons
concentrated in a central nucleus
none of the above

15. Isotopes
Nuclear isotopes (a.k.a nuclides) have specific nuclear notation:
Z = atomic number (= # protons)
A = mass number ( Nucleon number)
(= #protons + # neutrons)
X = chemical symbol of the element

16. Isotopes
Most elements have more than one isotope (although not always a stable one!)
Isotopes are atoms of the SAME ELEMENT with DIFFERENT numbers of NEUTRONS
Atomic number is ALWAYS the same for any isotope—only the mass number (nucleon number) changes
Evidence for neutrons using isotopes: there is no other way to logically explain the difference in mass for various atoms of a particular element.

17. Isotope Practice: Isotope Mass Number Atomic Number # of protons
# of neutrons
# of electrons
8939Y 89 39 50
Cl-1 37 17 20 18
3516S 35 16 19
Co+2 60 27 33 25
Isotope
Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 50
Cl-1 37 17 20 18
3516S 35 16 19 60 27 25
Isotope
Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 50
Cl-1 37 16 19 60 27 25
Isotope
Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y
Cl-1 37 16 19 60 27 25
Isotope
Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 50
Cl-1 37 17 20 18 16 19 60 27 25

18. Graviton (hypothetical)
Nuclear Interactions
Fundamental Forces
Type
Relative Strength
Field Particle
Gravitational 1
Graviton (hypothetical)
Weak nuclear
1032
W+/- and Z0
Electromagnetic
1036
Photon
Strong nuclear
1038
Gluons

19. Fundamental Forces… Evidence: Strong nuclear force:
protons do stay together in stable nuclei, even though the electromagnetic forces between them would suggest they would repel
Weak nuclear force:
evidence suggested during beta decay (where a neutron disintegrates into a proton and an electron…)

20. Radioactive Decay Discovered in 1896 by Antoine Henri Becquerel
Inspired by discovery of X-rays, wanted to know connection between those and fluorescent or phosphorescent materials
Experiment:
Photographic paper wrapped in black paper to keep out light…
Salt samples (such as Uranium) placed on the covered paper
Also exposed the wrapped paper to sunlight for several hours…

21. Ionizing Radiation—because rays could ionize gas molecules
Results:
Photographic plate was NOT exposed due to the sunlight
Outlines of the uranium sample clearly visible on plate:
THEN manipulated:
Temperature
Amount of light
Other physical
and chemical changes
NO EFFECT!
Ionizing Radiation—because rays could ionize gas molecules

22. Radioactive Decay
Marie Curie (and husband, Pierre)—followed Becquerel’s experiments to look for other substances with the same properties as Uranium…
Isolated Thorium (~1898)
Discovered Radium and Polonium…won Nobel Prize in Chemistry (1903)
1899—Rutherford discovered that Uranium emits 2 kinds of radiation (“alpha and beta rays”)
1900—gamma rays discovered as a 3rd type of radiation by Paul Villard

23. Types of Radiation
Ionizing power: the ability of radiation to knock electrons out of orbit when they collide with another atom.
Alpha particles (a)
Helium nucleus
Charge = +2e (the same as 2 protons)
Mass = 4u (1u = mass of a nucleon)
Type of energy: all kinetic
velocity ~ 0.05c
Penetration: stopped by a sheet of paper
Range: a few centimeters
Ionizing power—largest of the 3 types of radiation…very dangerous if ingested!

24. Very fast moving electron—emitted from the nucleus Charge = -1e
Beta Particles (b)
Very fast moving electron—emitted from the nucleus
Charge = -1e
Mass = 1/1850 u
Energy = all kinetic
(velocity up to 99% speed of light)
Penetration: will be stopped by a few mm of aluminum
Range: a few meters through air
More penetrating than Alpha particles, but less ionizing.

25. Gamma Rays (g) High energy electromagnetic radiation Charge = neutral
(very high frequency, very short wavelength)
Charge = neutral
Mass = 0
Energy: Photon Energy (proportional to the frequency of the ray)
Velocity = speed of light (c)
Penetration: can be stopped by several cm of lead or by a meter or more of concrete
Range: there is no maximum range
Lowest ionizing power of the 3 types of radiation

26. Particles can be identified based on how they interact with a magnetic field:
Alpha particles will curve slightly
Beta particles will be deflected significantly, and in the opposite direction from alpha
Gamma rays—no charge, so no deflection at all