The Bohr-Rutherford Atom Nils Bohr Ernest Rutherford Physics 100 Chapt 23 1895 J.J. Thomson discovered
electron Vacuum flask - + ++ ++ ---
cathode cathode rays anode Cathode rays have negative charge and very small mass S -
+ ++ ++ --- cathode anode N
m=0.0005MHydrogen Plum pudding? Positively charged porridge Negatively charged raisins (plums) ++ +
+ - + + + + + + + + -+ -+ - +
+ 10-10m Planetary-like? Positively charged ense central nucleus - +
- - 10-10m Negatively charged orbiting electrons Rutherford Experiment Vacuum
flask -rays Whats in the box? Is all the mass spread throughout as in a box of marshmallows? or is all the mass concentrated in a dense
ball-bearing? Figure it out without opening (or shaking) Shoot bullets randomly through the box. If it is filled with marshmallows, all the bullets will go straight through without (much) deflection Figure it out without opening (or
shaking) If it contains a ball-bearing most the bullets will go straight through without deflection--but not all Occasionally, a bullet will collide nearly headon to the ball-bearing and be deflected by a large angle Rutherford used -ray bullets to distinguish between the plumpudding & planetary models Plum-pudding:
++ + + - + + + + +
+ + + -+ -+ - + + no way for -rays to scatter at wide angles
distinguishing between the plumpudding & planetary models - + -
- Occasionally, an-rays will be pointed head-on to a nucleus & will scatter at a wide angle Rutherford saw ~1/10,000 a-rays scatter at wide angles -
+ - - from this he inferred a nuclear size of about 10-14m
Rutherford atom 10-10m Not to scale!!! 10-14m + If it were to scale,
the nucleus would be too small to Even though it see has more than 99.9% of the atoms mass
Relative scales Aloha stadium Golf ball x10-4 Atom Nucleus 99.97% of the mass
Classical theory had trouble with Rutherfords atom Orbiting electrons are accelerating Accelerating electrons should radiate light According to Maxwells theory, a Rutherford atom would only survive for only about 10-12 secs
sola Other peculiar discoveries: Solar light spectrum: raunhofer discovered that some wavelengths a missing from the suns black-body spectrum Other discoveries Low pressure gasses, when heated, do not radiate black-body-like spectra; instead they radiate only a few specific colors
bright colors from hydrogen match the missing colors in sunlight Hydrogen spectrum Solar spectrum Allowed orbits Bohrs idea
Hydrogen energy levels 4 3 1 + Hydrogen energy levels 2
1 + What makes Bohrs allowed energy levels allowed? Recall what happens when we force waves into confined spaces: Confined waves nly waves with wavelengths that just fit in surviv
(all others cancel themselves out) Electrons in atoms are confined matter waves ala deBroglie However, if the circumference is exactly an integer number of wavelengths, This wave, as it goes around, will successive
interfere turns will interfereand constructively with itself destructively cancel itself out Bohrs allowed energy states correspond to those with orbits that are integer numbers of wavelengths Bohr orbits
Bohr orbits Quantum Mechanics Erwin Schrodinger Schrodingers equation Matter waves are probability waves Probability to detect the electron at some place
is 2 at that spot Electrons will never be detected here or here Electrons are most likely to be detected here
Quantum mechanical atom Bohrs original idea Probability density Quantum Mechanics Different QM states of the H atom
