Quantum Theory, Part 1, Day 2

Is There Something Inside

of the Atom?!?!?

Cathode Rays

Cathode rays are the carriers of electric current from cathode to anode inside a vacuumed tube.

Cathode rays have the following characteristics:– Emit from the cathode when electricity is passed

through an evacuated tube.– Emit in a direction perpendicular to the cathode

surface.– Travel in straight lines.– Cause glass and other materials to fluoresce.– Deflect in a magnetic field similarly to negatively

charged particles.

Crookes Tube

William Crookes

Mask holder

Cathode(-)

Anode(+)

Crookes tube(Cathode ray tube)

Mask holder

Glow

The Effect of an Obstruction on Cathode Rays

Highvoltage

cathode

source ofhigh voltage

yellow-greenfluorescence

shadow

Joseph John Thomson

1897 J. J. Thomson English

– Credited with the discovery of the electron.– His model of the atom featured negatively

charged electrons embedded in a ball of positive charge.• “Plum-Pudding” model

– Awarded the Nobel prize in 1909 for calculating the charge/mass ratio of the electron. • me /e

• The value is determined to be –5.686 X 10-12 kg/C

Thomson’s Model

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

A Cathode Ray Tube

Thomson’s Experiment

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vacuum tube

metal disks

voltage source

Cathode Anode

Thomson’s Experiment

+-voltage sourceOFF

ON

Passing an electric current makes a beam appear

to move from the negative to the positive end

Thomson’s Experiment

+-voltage sourceOFF

ON

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By adding an electric field…

he found that the moving pieces were negative.

Crooke’s Tube

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vacuum tube

metal disks

voltage source

magnet

William Crookes

Cathode Ray Experiment

Deflectionregion

Drift region

Displacement

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-Anodes / collimators

Cathode

Volts

Conclusions• He compared the value with the mass/charge ratio for the

lightest charged particle.

• By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom.

• He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles.

• This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day.

• Since any electrode material produces an identical ray, cathode ray particles are present in all types of matter - a universal negatively charged subatomic particle later named the electron.

J.J. Thomson• He proved that atoms of

any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be balancing the negative charge.

J.J. Thomson

Lord Rutherford’s Gold Foil Experiment (1909)

Worked with Bohr, Geiger, and Marsden in order to prove Thomson’s model.

Used Polonium to produce alpha particles (He+2).

Aimed alpha particles at gold foil by drilling hole in lead block.

Since the mass is evenly distributed in gold atoms, alpha particles should go straight through.

Used gold foil because it could be made only a few atoms thick.

Rutherford’s Apparatus

beam of alpha particles

radioactive substance

fluorescent screencircular - ZnS coated

gold foil

Pb blockPo

Au Foil

Fluorescent Screen, ZnS

What he expected…

Because, he thought the mass was evenly distributed in the atom

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Alone, the subatomic particles were not enough to stop the alpha particles.

What he got…richochetingalpha particles

The Predicted Result:

expected path

expected marks on screen

mark onscreen

likely alphaparticle path

Observed Result:

How he explained it:

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Rutherford’s Model of the Atom

Atom is mostly empty space. Small dense, positive piece

at center (core). Contains most of the atom’s

mass. Alpha particles are deflected

by it if they get close enough. Disproved Thomson’s Model This is called the Nuclear

Model

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It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back to hit you.

"On consideration, I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center carrying a charge."

While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are; they are definitely smaller than 10-18 meters, and they might literally be points, but we do not know.

It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles. (Oh, will this madness ever end?)

Scale of the atom.

Website “The Particle Adventure”

Problem with Rutherford’s Model?!?!

We know that unlike charges attract (i.e. positive and negative), therefore what should happen between the positive nucleus and negative electrons?

Robert Millikan’s Oil Drop

Experiment (1911)

• American• Measured the charge of an electron

Millikan’s Experiment (1911)

Oil Drop Experiment

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oil droplets

oil droplet under observation

Charged plate

Small hole

Charged plate

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Telescope

Robert Millikan(1909)

Balancing electrical and gravitational forces allowed the electron charge to be determined.Mass was calculated using charge to mass ratio (9.1093 x 10-28 g).

Oil

Atomizer

X-rays give oil drops a charge, by transferring electrons to them from the air

The charge of each drop was always a multiple of the same smaller charge.

1.60 x 10-19 C