Going Further into Going Further into Atomic StructureAtomic Structure

Thomson, Rutherford, and MillikanThomson, Rutherford, and Millikan

Where we left off…Where we left off…

Dalton’s atomic theoryDalton’s atomic theory 5 postulates5 postulates

All matter is made up of atoms.All matter is made up of atoms. An element is made up of atoms that are identical.An element is made up of atoms that are identical. Atoms of different elements are different from each Atoms of different elements are different from each

other.other. Atoms combine in small whole number ratios to form Atoms combine in small whole number ratios to form

compoundscompounds Atoms of the same elements can combine in different Atoms of the same elements can combine in different

whole number ratios to form different compounds. whole number ratios to form different compounds. • ex. CO vs. COex. CO vs. CO22

Geissler or Crookes TubesGeissler or Crookes Tubes

Contain two metal plates Contain two metal plates in sealed glass tubein sealed glass tube

One metal plate (the One metal plate (the “cathode”) emits a “cathode”) emits a glowing “ray” when glowing “ray” when electrical current is electrical current is appliedapplied

Also called “cathode ray Also called “cathode ray tubes”tubes”

Still used in TVs, Still used in TVs, computer monitorscomputer monitors

What is the nature of these What is the nature of these “cathode rays”?“cathode rays”?

Cathode rays cast a shadowCathode rays cast a shadow

Cathode rays can turn a paddlewheelCathode rays can turn a paddlewheel

Cathode Ray BehaviorsCathode Ray Behaviors

Cathode rays could be produced by just Cathode rays could be produced by just about any metal testedabout any metal tested

If directed on a piece of metal, the rays If directed on a piece of metal, the rays could make the metal glowcould make the metal glow

They showed strange behaviors in They showed strange behaviors in presence of magnetspresence of magnets

Cathode rays and magnetsCathode rays and magnets

Cathode rays bend (“are deflected”) in presence of magnetic field

J. J. ThomsonJ. J. Thomson

English physicist, English physicist, 1856-19401856-1940

Nobel Laureate in Nobel Laureate in Physics, 1906Physics, 1906

Earned scholarship to Earned scholarship to Cambridge UniversityCambridge University

Trained 7 Nobel Trained 7 Nobel Laureates Laureates

1897 Experiments1897 Experiments

Thomson studied the behavior of cathode Thomson studied the behavior of cathode rays in the presence of an electric fieldrays in the presence of an electric field http://http://www.aip.org/history/electron/jjappara.htmwww.aip.org/history/electron/jjappara.htm

Deflection data supported negative charge of Deflection data supported negative charge of cathode rayscathode rays

Thomson measured the mass to charge ratio Thomson measured the mass to charge ratio of cathode raysof cathode rays

Thomson’s conclusionsThomson’s conclusions

Cathode rays are made of tiny, negatively Cathode rays are made of tiny, negatively charged particlescharged particles Originally called the particles “corpuscles”Originally called the particles “corpuscles” Later called Later called electronselectrons, a term coined by G. , a term coined by G.

Johnstone Stoney in 1891Johnstone Stoney in 1891 All atoms contain electronsAll atoms contain electrons

Thomson believed that atoms were made up Thomson believed that atoms were made up only of electronsonly of electrons

Thomson’s model of the atomThomson’s model of the atom

Atoms known to be Atoms known to be electrically neutral, so electrically neutral, so there must be positive there must be positive charge in with the charge in with the electronselectrons

Proposed that Proposed that electrons were electrons were embedded in gel-like embedded in gel-like positive chargepositive charge

Called the “plum Called the “plum pudding model”pudding model”

Ernest RutherfordErnest Rutherford New Zealand nativeNew Zealand native Won scholarship to Won scholarship to

Cambridge UniversityCambridge University Studied with J. J. Studied with J. J.

ThomsonThomson Taught at McGill, Taught at McGill,

Manchester and Manchester and CambridgeCambridge

Ongoing interest in Ongoing interest in radioactivityradioactivity

1908 Nobel Laureate1908 Nobel Laureate

The Gold-Foil ExperimentThe Gold-Foil Experiment

Decided to test Decided to test Thomson’s plum pudding Thomson’s plum pudding modelmodel

Worked with Hans Geiger Worked with Hans Geiger and Hans Marsden from and Hans Marsden from 1909-19131909-1913

Beam of “alpha particles” Beam of “alpha particles” was directed at thin gold was directed at thin gold foil and the paths foil and the paths followed by a detection followed by a detection screenscreen

Alpha particles big, positively charged particles Nucleus of a helium atom

Expectations vs. ResultsExpectations vs. Results

If Thomson’s model is correct, what should the If Thomson’s model is correct, what should the alpha particles do when they hit the foil? alpha particles do when they hit the foil? http://www.wwnorton.com/chemistry/overview/ch3.htmhttp://www.wwnorton.com/chemistry/overview/ch3.htm

The alpha particles were expected to go straight The alpha particles were expected to go straight through the foilthrough the foil

A very small percentage of particles showed A very small percentage of particles showed “backscattering” when they hit the foil“backscattering” when they hit the foil

Thomson’s model didn’t hold up to the evidenceThomson’s model didn’t hold up to the evidence Rutherford proposed a modified theoryRutherford proposed a modified theory

Rutherford’s atomic modelRutherford’s atomic model

Most of the mass and positive charge of an atom Most of the mass and positive charge of an atom is concentrated in the centeris concentrated in the center The “nucleus”The “nucleus” Positively charged subatomic particles called Positively charged subatomic particles called

“protons” identified early 1920’s“protons” identified early 1920’s• Atomic number = number of protonsAtomic number = number of protons

Neutrons were identified in 1932 by James ChadwickNeutrons were identified in 1932 by James Chadwick Electrons surround the nucleusElectrons surround the nucleus

Most of the volume of an atom is due to the electronsMost of the volume of an atom is due to the electrons Most of the atom consists of empty spaceMost of the atom consists of empty space # protons = # electrons in neutral atoms# protons = # electrons in neutral atoms

Robert MillikanRobert Millikan

U.S. physicistU.S. physicist University of ChicagoUniversity of Chicago Nobel Prize, 1923Nobel Prize, 1923 First major success: First major success:

finding the charge of finding the charge of an electronan electron

Oil Drop ExperimentOil Drop Experiment Isolate single oil droplet Isolate single oil droplet

between two platesbetween two plates Adjust charge on plates to Adjust charge on plates to

suspend droplet midairsuspend droplet midair Amount of charge needed Amount of charge needed

proportional to number of proportional to number of electrons on dropelectrons on drop

QQcc= -1.6 x 10= -1.6 x 10-19-19 coulombs coulombs With Thomson’s results, With Thomson’s results,

possible to determine the possible to determine the mass of an electronmass of an electron

View simulation at View simulation at http://www.physchem.co.za/Sthttp://www.physchem.co.za/Static%20Electricity/Millikan.htmatic%20Electricity/Millikan.htm