Download - Nuclear and Atomic Physics
Nuclear and Atomic Physics
Intro: Atomic Structure and a little history.
Print off the topic outline for this unit by tomorrow!
Early atomic structure…
JJ Thomson: “Plum pudding” model An atom is a mixture of positive and negative
charges
Image from: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec11.html
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
Image from: http://bhs.smuhsd.org/science-dept/marcan/apchemistry/structure_ML_MD.html
Gold foil experiment… Results:
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
Rutherford’s Atom…
“Planetary model”:
Image from: http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html
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
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
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
Emission and Absorption Spectra
All elements will emit light in characteristic colors when heated
Scottish physicist Thomas Melville—first to study emitted light (1726-1753) Heat source = flame Passed emitted light through prism Pattern produced was significantly different
than white light passed through spectrum Bright line spectrum
Emission and Absorption spectra give the same “fingerprint” for an element, but in different ways
Alpha particles directed at a thin gold foil will most likely...
A. pass directly through the foil with no deflection
B. be reflected straight back from the solid foilC. pass through the foil wil a small amount of
deflectionD. be deflected at large angles as it passes
through the foil
Large deflections of alpha particles in Geiger and Marsden's scattering
experiment suggested...A. atoms consist of a small negative nucleus
surrounded by protonsB. atoms consist of a small positive nucleus
surrounded by electronsC. atoms consist of a small neutral nucleus
surrounded by electrons and protonsD. atoms consist of a large positive mass with
embedded electrons
The Rutherford model of the atom has the positive charge...
A. spread uniformly throughout the atom's volume
B. circling the nucleus as positive electronsC. concentrated in a central nucleusD. none of the above
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
AZ X
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.
Isotope Practice:Isotope Mass
NumberAtomic Number
# of protons
# of neutrons
# of electrons
8939Y
Cl-1 37
16 19 16
60 27 25
Isotope Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 39 50 39
Cl-1 37
16 19 16
60 27 25
Isotope Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 39 50 39
Cl-1 37 17 17 20 18
16 19 16
60 27 25
Isotope Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 39 50 39
Cl-1 37 17 17 20 18
3516S 35 16 16 19 16
60 27 25
Isotope Mass Number
Atomic Number
# of protons
# of neutrons
# of electrons
8939Y 89 39 39 50 39
Cl-1 37 17 17 20 18
3516S 35 16 16 19 16
Co+2 60 27 27 33 25
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
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…)
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…
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 physicaland chemical changes
NO EFFECT! Ionizing Radiation—because rays could
ionize gas molecules
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
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!
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.
Gamma Rays (g) High energy electromagnetic radiation
(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
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