3/29/2010. Structure of the Atom. Knowledge of atoms in 1900 CHAPTER 6. Evidence in 1900 indicated that the atom was not a fundamental unit:

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1 3/9/010 CHAPTER 6 Rutherford Scattering 6.1 The Atomic Models of Thomson and Rutherford 6. Definition of Cross Section 6. Rutherford Scattering 6.3 Structure of the Nucleus The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth. An expert is a person who has made all the mistakes that can be made in a very narrow field. Neverexpress yourself more clearly than you are able to think. Predictionis very difficult, especially about the future. Niels Bohr ( ) - Niels Bohr Structure of the Atom Evidence in 1900 indicated that the atom was not a fundamental unit: 1) There seemed to be too many kinds of atoms, each belonging to a distinct chemical element (way more than earth, air, water, and fire!). ) Atoms and electromagnetic phenomena were intimately related (magnetic materials; insulators vs. conductors; different emission spectra). 3) Elements combine with some elements but not with others, a characteristic that hinted at an internal atomic structure (valence). 4) The discoveries of radioactivity, x rays, and the electron (all seemed to involve atoms breaking apart in some way). Knowledge of atoms in 1900 Electrons (discovered in 1897) carried the negative charge. Electrons were very light, even compared to the atom. Protons had not yet been discovered, but clearly positive charge had to be present to achieve charge neutrality. 1

2 3/9/010 Thomson s Atomic Model Thomson s plum-pudding model of the atom had the positive charges spread uniformly throughout a sphere the size of the atom, with electrons embedded in the uniform background. In Thomson s view, when the atom was heated, the electrons could vibrate about their equilibrium positions, thus producing electromagnetic radiation. Unfortunately, Thomson couldn t explain spectra with this model. Alpha () Particle Scattering can be produce by any type of particle, however the particle must have small wavelength for good diffraction and resolution. Alpha particle is such a particle. It is produce in a radioactive decay of proton 18 Rn Po + He ++ He ++ is a ionized helium nucleus (q+e) and is called the alpha () particle Scattering with Alpha () Particle Exercise 6-1 Show that when particles scatter from an atom, the scattering angle is inversely proportional to the distance for closest approach.

3 3/9/010 Experiments of Geiger and Marsden Geiger, and Marsden conceived a new technique for investigating the structure of matter by scattering particles from atoms. Experiments of Geiger and Marsden Geiger showed that many particles were scattered from thin gold-leaf targets at backward angles greater than 90. Large scattering angles mean the target is more massive than a projectile Electrons can t back-scatter particles. Exercise 6- Calculate the maximum scattering angle corresponding to the maximum momentum change. Before After It can be shown that the maximum momentum transfer to the particle is: Determine θ max by letting p max be perpendicular to the direction of motion: p m max ev θ max p p mev M v too small! 3

4 3/9/010 Try multiple scattering from electrons If an particle is scattered by N electrons: N the number of atoms across the thin gold layer, t m: n The distance between atoms, d n -1/3, is: N t / d still too small! Rutherford s Atomic Model even if the particle is scattered from all 79 electrons in each atom of gold. Ernest Rutherford ( ) Experimental results were not consistent with Thomson s atomic model. Rutherford proposed that an atom has a positively charged core (nucleus) surrounded by the negative electrons. Geiger and Marsden confirmed the idea in Scattering Cross Section (σ) The cross section σ π b is related to the probability (P a particle being scattered by a nucleus. Scattering Rate σ Incident Flux σ) for Scattering cross section is measured in barns 1b 10-8 m Any particle inside the circle of area π b 0 will be similarly (or more) scattered. σ depends on energy and therefore wavelength. 4

5 3/9/010 Rutherford Scattering Scattering experiments help us study matter too small to be observed directly. There s a relationship between the impact parameter b and the scattering angle θ. When b is small, r is small. the Coulomb force is large. θ can be large and the particle can be repelled backward. kq1q cot θ b Mv 0 cot(θ/) π θ Rutherford Scattering Equation In actual experiments, a detector is positioned from θ to θ + dθ that corresponds to incident particles between b and b + db. Scattering rate as a function of angle dσ π hc 1 z Z d cosθ Ek 1 cos ( θ ) Rutherford scattering experiment See figures 6.4,6.5,6.6 and 6.7 in text for experimental results Exercise 6.3: Derive the Rutherford Scattering formulae b kq q cot θ Mv 1 dσ π hc 1 z Z d cosθ Ek 1 cos ( θ ) 5

6 3/9/010 Rutherford Scattering: See Figure a), No penetration of nucleus, Nucleus behaves like point charge, Coulomb force law Does not imply that nucleus is a point charge Force law is still correct even if the nucleus was ball of radius R as long as the alpha particle does not penetrate the nucleus Alpha particle penetration: Rutherford scattering does not hold Modification is required to account for charge behind the alpha particle as it penetrates the nucleus Size of Nucleus Exercise 6.4: For a head on collision of an alpha particle with a nucleus show that the distance of closest approach is kze rm E k 6

7 3/9/010 Rutherford scattering formula can be used to find the size of the nucleus Increase the energy of the incoming particle, the distance of closest approach will be smaller. At some r m (penetration) the results from scattering experiment will not agree with Rutherford s prediction and that r m with give the nuclear size. Example: For a alpha particle of 7.7 MeV, the radius of the gold nucleus is (79)(1.44 ev. nm) ev 5 14 rm 3 10 nm 3 10 m 6 Nuclear size is measured in Fermi or Femtometers Lightest atom ~ 1fm Heaviest atom ~ 10 atom Electron scattering experiments give R (1. fm) A 1 3 The nucleus is made up of closely packed spheres of protons and neutrons Experiments with 1 GeV electrons hitting the nucleus reveal that there is appreciable deviation from Rutherford scattering cross section, showing that neutron and proton s are not point like but finite size. Measurement on size of proton and neutron ~ 1fm 7