Quantum Theory, Part 1, Day 2

Is There Something Inside of the Atom?!?!?

Cathode Rays n Cathode rays are the carriers of electric current from cathode to anode inside a vacuumed tube. n 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 High voltage cathode source of high voltage yellow-green fluorescence shadow

Joseph John Thomson 1897 J. J. Thomson nEnglish –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. m e /e The value is determined to be –5.686 X kg/C

Thomson’s Model

Source of Electrical Potential Metal Plate Gas-filled glass tube Metal plate Stream of negative particles (electrons) A Cathode Ray Tube

Thomson’s Experiment + - vacuum tube metal disks voltage source CathodeAnode

Thomson’s Experiment + - voltage source OFF ON Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment + - voltage source OFF ON + - By adding an electric field… he found that the moving pieces were negative.

Crooke’s Tube + - vacuum tube metal disks voltage source magnet William Crookes

Cathode Ray Experiment Deflection region Drift region Displacement + - 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) nWorked with Bohr, Geiger, and Marsden in order to prove Thomson’s model. nUsed Polonium to produce alpha particles (He +2 ). nAimed alpha particles at gold foil by drilling hole in lead block. nSince the mass is evenly distributed in gold atoms, alpha particles should go straight through. nUsed gold foil because it could be made only a few atoms thick.

Rutherford’s Apparatus beam of alpha particles radioactive substance fluorescent screen circular - ZnS coated gold foil

Pb block Po Au Foil Fluorescent Screen, ZnS

What he expected…

Because, he thought the mass was evenly distributed in the atom Alone, the subatomic particles were not enough to stop the alpha particles.

What he got… richocheting alpha particles

The Predicted Result: expected path expected marks on screen mark on screen likely alpha particle path Observed Result:

How he explained it: + Rutherford’s Model of the Atom nAtom is mostly empty space. nSmall dense, positive piece at center (core). nContains most of the atom’s mass. nAlpha particles are deflected by it if they get close enough. nDisproved Thomson’s Model nThis 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 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”The Particle Adventure

Problem with Rutherford’s Model?!?! nWe 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 oil droplets oil droplet under observation Charged plate Small hole Charged plate - + Telescope Robert Millikan (1909) Balancing electrical and gravitational forces allowed the electron charge to be determined. Mass was calculated using charge to mass ratio ( x 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 x C