The Bohr-Rutherford Atom Physics 100 Chapt 23 Nils Bohr Ernest Rutherford.

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Presentation transcript:

The Bohr-Rutherford Atom Physics 100 Chapt 23 Nils Bohr Ernest Rutherford

1895 J.J. Thomson discovered electron cathode anode “cathode rays” Vacuum flask

Cathode rays have negative charge and very small mass cathode anodeN S m=0.0005M Hydrogen

Plum pudding? m Positively charged porridge Negatively charged raisins (plums)

Planetary-like? m Positively charged dense central nucleus Negatively charged orbiting electrons

Rutherford Experiment Vacuum flask  -rays

What’s 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 head-on to the ball-bearing and be deflected by a large angle

Rutherford used  -ray “bullets” to distinguish between the plum- pudding & planetary models     no way for  -rays to scatter at wide angles Plum-pudding:

    Occasionally, an  -rays will be pointed head-on to a nucleus & will scatter at a wide angle distinguishing between the plum- pudding & planetary models

Rutherford saw ~1/10,000 a-rays scatter at wide angles     from this he inferred a nuclear size of about m

Rutherford atom m m Not to scale!!! If it were to scale, the nucleus would be too small to see Even though it has more than 99.9% of the atom’s mass

Relative scales Aloha stadium Golf ball Atom Nucleus 99.97% of the mass x10 -4

Classical theory had trouble with Rutherford’s atom Orbiting electrons are accelerating Accelerating electrons should radiate light According to Maxwell’s theory, a Rutherford atom would only survive for only about secs

Fraunhofer discovered that some wavelengths are missing from the sun’s black-body spectrum sola Other peculiar discoveries: Solar light 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

Bohr’s idea “Allowed” orbits

Hydrogen energy levels

+ 1 2

What makes Bohr’s allowed energy levels allowed? Recall what happens when we force waves into confined spaces:

Confined waves Only waves with wavelengths that just fit in survive (all others cancel themselves out)

Electrons in atoms are confined matter waves ala deBroglie This wave, as it goes around, will interfere with itself destructively and cancel itself out However, if the circumference is exactly an integer number of wavelengths, successive turns will interfere constructively Bohr’s allowed energy states correspond to those with orbits that are integer numbers of wavelengths

Bohr orbits

Quantum Mechanics Erwin Schrodinger Schrodinger’s equation

Matter waves are “probability” waves  Probability to detect the electron at some place is   2 at that spot Electrons are most likely to be detected here or here Electrons will never be detected here

Quantum mechanical atom Bohr’s original idea Quantum Mechanics Probability density

Different QM states of the H atom