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The Bohr-Rutherford Atom. Physics 100 Chapt 23. Nils Bohr. Ernest Rutherford. 1895 J.J. Thomson discovered electron. Vacuum flask. cathode. + +. - - -. “cathode rays”. + +. -. +. anode. Cathode rays have negative charge and very small mass. S. cathode. + +. - - -. + +. -.

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The bohr rutherford atom

The Bohr-Rutherford Atom

Physics 100

Chapt 23

Nils

Bohr

Ernest

Rutherford


1895 j j thomson discovered electron
1895 J.J. Thomson discovered electron

Vacuum flask

cathode

+ +

- - -

“cathode rays”

+ +

-

+

anode


Cathode rays have negative charge and very small mass
Cathode rays have negative chargeand very small mass

S

cathode

+ +

- - -

+ +

-

+

anode

N

m=0.0005MHydrogen


Plum pudding
Plum pudding?

Positively charged

porridge

Negatively charged

raisins (plums)

+

+

+

+

-

-

-

+

+

+

+

+

-

-

-

+

+

+

+

-

-

+

+

-

+

10-10m


Planetary like
Planetary-like?

Positively charged

dense central nucleus

Negatively charged

orbiting electrons

-

-

-

+

-

-

10-10m


Rutherford experiment
Rutherford Experiment

Vacuum

flask

a-rays


What s in the box
What’s in the box?

or is all the mass

concentrated in a dense

“ball-bearing”?

Is all the mass spread

throughout as in a box

of marshmallows”?


Figure it out without opening or shaking
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 shaking1
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 deflectedby a large angle


Rutherford used a ray bullets to distinguish between the plum pudding planetary models
Rutherford used a-ray “bullets” to distinguish between the plum-pudding & planetary models

Plum-pudding:

+

+

+

+

a

-

-

-

+

+

+

+

a

+

-

-

-

+

a

+

+

+

-

a

-

+

+

-

+

no way for a-rays to scatter at wide angles


Distinguishing between the plum pudding planetary models
distinguishing between the plum-pudding & planetary models

-

a

a

-

-

+

a

a

-

-

Occasionally, an a-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
Rutherford saw ~1/10,000a-rays scatter at wide angles

-

a

-

-

a

+

a

a

-

-

from this he inferred a nuclear

size of about 10-14m


Rutherford atom
Rutherford atom

10-10m

Not to scale!!!

If it were to scale,

the nucleus would

be too small to see

10-14m

+

Even though it has more than 99.9% of the atom’s mass


Relative scales
Relative scales

Aloha stadium

Golf ball

x10-4

Nucleus

99.97% of the mass

Atom


Classical theory had trouble with rutherford s atom
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 10-12 secs


Other peculiar discoveries

sola

Other peculiar discoveries:

Solar light spectrum:

Fraunhofer discovered that some wavelengths are

missing from the sun’s black-body spectrum


Other discoveries
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
bright colors from hydrogen match the missing colors in sunlight

Hydrogen spectrum

Solar spectrum


Bohr s idea
Bohr’s idea sunlight

“Allowed”

orbits


Hydrogen energy levels
Hydrogen energy levels sunlight

4

3

1

+


Hydrogen energy levels1
Hydrogen energy levels sunlight

2

1

+


What makes bohr s allowed energy levels allowed
What makes Bohr’s allowed energy levels allowed? sunlight

Recall what happens when we force

waves into confined spaces:


Confined waves
Confined waves sunlight

Only waves with wavelengths that just fit in survive

(all others cancel themselves out)


Electrons in atoms are confined matter waves ala debroglie
Electrons in atoms are confined matter waves ala deBroglie sunlight

However, if the circumference is exactly an

integer number of wavelengths, successive

turns will interfere constructively

This wave, as it goes around, will interfere

with itself destructively and cancel itself out

Bohr’s allowed energy states correspond to those

with orbits that are integer numbers of wavelengths


Bohr orbits
Bohr orbits sunlight


Bohr orbits1
Bohr orbits sunlight


Quantum mechanics
Quantum Mechanics sunlight

Erwin Schrodinger

Schrodinger’s equation


Matter waves are probability waves
Matter waves are “probability” waves sunlight

Probability to detect the

electron at some place

is y2 at that spot

y

Electrons will never be detected here

Electrons are most likely

to be detected here

or here


Quantum mechanical atom
Quantum mechanical atom sunlight

Bohr’s original idea

Probability density

Quantum Mechanics



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