Krane comments that the evidence points to a mass radius and a charge radius which agree with each other within about 0.1 fermi. These comparisons have made it clear that there is a "tail" where the density of nuclear matter decreases toward zero. The comparison of calculated and experimental radii for nuclei are very sensitive to the exact onset of the overlap between the probe particle and the nuclear matter. The most definitive information about nuclear sizes comes from electron scattering. The assumption of constant density leads to a nuclear density Where r is the radius of the nucleus of mass number A. The data are summarized in the expression called the Fermi model: Various types of scattering experiments suggest that nuclei are roughly spherical and appear to have essentially the same density. Sun-Pluto distance = 3666 x 10 6 miles = 5900 x 10 6 km Sun-Earth distance = 93 x 10 6 miles = 150 x 10 6 km GOLD ATOMĪtomic mass = 196.97 amu (1 mole = 196.97 grams)Īvogadro's number = 6.02 x 10 23 atoms/moleĬalculated atomic radius = 1.3 x 10 -10 mĬalculated nuclear radius = 7.3 x 10 -15 m How does such a compact nucleus stay together?ĭata for Scale Model of Atom A. On this scale, the nearest star would be a little over 10,000 miles away. Relative scale model of an atom and the solar system Do you perceive a gold ring to contain a larger fraction of solid matter than the solar system? It is also common practice to quote the rest mass energy E=m 0c 2 as if it were the mass. However, the nuclear sizes are quite small and need smaller units: Atomic sizes are on the order of 0.1 nm = 1 Angstrom = 10 -10 m Nuclear sizes are on the order of femtometers which in the nuclear context are usually called fermis:Ītomic masses are measured in terms of atomic mass units with the carbon-12 atom defined as having a mass of exactly 12 amu. 1 electron volt = 1eV = 1.6 x 10 -19 joules 1 MeV = 10 6 eV 1 GeV = 10 9 eV 1 TeV = 10 12 eV Nuclear energies are very high compared to atomic processes, and need larger units.
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