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

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

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... 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

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

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

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

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 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

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

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…

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

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!

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.

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

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