The Development of Atomic TheoryThe Development of Atomic Theory

Ancient times to Ancient times to the presentthe present

DemocritusDemocritus Atoms are Atoms are

indivisible, indivisible, indestructible, indestructible, fundamental units fundamental units of matter.of matter.

““Atomos”Atomos”

Robert BoyleRobert Boyle The Skeptical The Skeptical

ChemistChemist Quantitative Quantitative

experimentationexperimentation Identification of Identification of

elementselements

Antoine LavoisierAntoine Lavoisier Role of oxygen in Role of oxygen in

combustioncombustion Law of conservation Law of conservation

of massof mass First modern First modern

textbooktextbook

John DaltonJohn Dalton Atomic Theory -Atomic Theory -

Unchallenged for Unchallenged for 100 years100 years

Law of multiple Law of multiple proportionsproportions

Atomic Theory of MatterAtomic Theory of Matter

The theory that The theory that atoms are the atoms are the fundamental fundamental building blocks of building blocks of matter reemerged matter reemerged in the early 19th in the early 19th century, century, championed by championed by John Dalton.John Dalton.

Dalton’s PostulatesDalton’s Postulates

Each element is composed of Each element is composed of extremely small particles called extremely small particles called atoms.atoms.

Dalton’s PostulatesDalton’s Postulates

All atoms of a given element are All atoms of a given element are identical to one another in mass and identical to one another in mass and other properties, but the atoms of other properties, but the atoms of one element are different from the one element are different from the atoms of all other elements.atoms of all other elements.

Dalton’s PostulatesDalton’s Postulates

Atoms of an element are not Atoms of an element are not changed into atoms of a different changed into atoms of a different element by chemical reactions.element by chemical reactions.

Dalton’s PostulatesDalton’s Postulates

Atoms are neither created nor Atoms are neither created nor destroyed in chemical reactions.destroyed in chemical reactions.

Dalton’s PostulatesDalton’s Postulates

Compounds are formed when Compounds are formed when atoms of more than one element atoms of more than one element combine; a given compound combine; a given compound always has the same relative always has the same relative number and kind of atoms.number and kind of atoms.

Early experiments to characterize Early experiments to characterize the atomthe atom

J. J. ThomsonJ. J. Thomson

Discovery of the Discovery of the electronelectron

Cathode ray tubeCathode ray tube http://www.chem.uihttp://www.chem.ui

uc.edu/clcwebsite/uc.edu/clcwebsite/video/Cath.movvideo/Cath.mov

The ElectronThe Electron

Streams of negatively charged particles Streams of negatively charged particles were found to emanate from cathode were found to emanate from cathode tubes.tubes.

The ElectronThe Electron

Thomson measured the charge/mass ratio Thomson measured the charge/mass ratio of the electron to be 1.76 of the electron to be 1.76 10 1088 coulombs/g.coulombs/g.

The Atom, circa 1900:The Atom, circa 1900:

““Plum pudding” Plum pudding” model, put forward by model, put forward by Thomson.Thomson.

Positive sphere of Positive sphere of matter with negative matter with negative electrons imbedded electrons imbedded in it.in it.

Henri BecquerelHenri Becquerel Discovery of Discovery of

radioactivityradioactivity Nobel PrizeNobel Prize

RadioactivityRadioactivity Three types of radiation were Three types of radiation were

discovered by Ernest Rutherford:discovered by Ernest Rutherford: particlesparticles particlesparticles raysrays

Robert MillikanRobert Millikan Oil Drop Oil Drop

ExperimentExperiment Determined the Determined the

charge and mass of charge and mass of the electronthe electron

http://search.eb.cohttp://search.eb.com/nobel/cap/omillikm/nobel/cap/omillik001a4.html001a4.html

Millikan Oil Drop ExperimentMillikan Oil Drop Experiment

Once the Once the charge/mass ratio charge/mass ratio of the electron was of the electron was known, known, determination of determination of either the charge or either the charge or the mass of an the mass of an electron would yield electron would yield the other.the other.

Millikan Oil Drop ExperimentMillikan Oil Drop Experiment

Robert Millikan Robert Millikan (University of (University of Chicago) Chicago) determined the determined the charge on the charge on the electron in 1909.electron in 1909.

Ernest RutherfordErnest Rutherford Existence of the Existence of the

nucleus and its nucleus and its relative sizerelative size

Gold foil Gold foil experimentexperiment

Discovery of the NucleusDiscovery of the Nucleus

Ernest Rutherford Ernest Rutherford shot shot particles at particles at a thin sheet of a thin sheet of gold foil and gold foil and observed the observed the pattern of scatter pattern of scatter of the particles.of the particles.

The Nuclear AtomThe Nuclear Atom

Since some Since some particles were particles were deflected at large deflected at large angles, Thomson’s angles, Thomson’s model could not be model could not be correct.correct.

Niels BohrNiels Bohr Proposed improvements Proposed improvements

to Rutherford’s model.to Rutherford’s model. Definite orbits in which Definite orbits in which

an electron can travel an electron can travel around a nucleus around a nucleus without radiating without radiating energy.energy.

Only works for Only works for hydrogen!hydrogen!

Other Subatomic ParticlesOther Subatomic Particles

Protons were discovered by Protons were discovered by Rutherford in 1919.Rutherford in 1919.

Neutrons were discovered by Neutrons were discovered by James Chadwick in 1932.James Chadwick in 1932.

Erwin SchrödingerErwin Schrödinger

What could be next?What could be next?