The History of the AtomThe History of the Atom

Part 2 – The discovery of Part 2 – The discovery of subatomic particlessubatomic particles

When we last left the atom, it looked like this:

J.J. ThomsonJ.J. Thomson Discovered the Discovered the

electronelectron with with cathode raycathode ray experimentexperiment

Plum puddingPlum pudding model model

Chocolate Chip Cookie

Chocolate chips are distributed throughout a cookie just as electrons are distributed throughout an atom

Today’s Thomson AnalogyToday’s Thomson Analogy

Cathode Ray that Thomson used showing how the electric beam is curved by the stream of electrons

cathode tube

anode

Glass tubestand

Alligator clip

Thomson’s Experiment Video

Robert MillikanRobert Millikan

An An AmericanAmerican Discovered the Discovered the

charge/masscharge/mass of the of the electronelectron

Oil DropOil Drop Experiment Experiment

Oil Drop ExperimentOil Drop Experiment Balanced the Balanced the gravitationalgravitational and and electricelectric forces on forces on

tiny charged droplets of tiny charged droplets of oiloil suspended between suspended between two metal two metal electrodeselectrodes

By knowing the By knowing the electricelectric field, the charge on the field, the charge on the oil dropletoil droplet could be determined. could be determined.

Repeating the experiment for many droplets, he Repeating the experiment for many droplets, he found that the found that the valuesvalues measured were always measured were always multiplesmultiples of the same of the same numbernumber. .

ElectronElectron

SmallestSmallest particle particle Almost no Almost no massmass NegativeNegative charge charge Abbreviated Abbreviated ee--

Ernest RutherfordErnest Rutherford

Discovered that most Discovered that most of the of the atomatom is is emptyempty spacespace using the using the gold gold foilfoil experiment experiment

Discovered the Discovered the nucleusnucleus

Also given credit for Also given credit for coining the name coining the name protonproton

Gold Foil Experiment

Rutherford’sRutherford’s model model

Mostly empty space

Center area is the Nucleus

Little Blue dots are electrons

Rutherford Rutherford AnalogyAnalogy

A peach

A peach has a dense center known as the pit just like an atom has a dense but small center called the nucleus

James ChadwickJames Chadwick

Discovered the Discovered the neutronneutron in 1932 in 1932

ProtonsProtons and and NeutronsNeutrons

NeutronsNeutrons Have Have nono charge charge Are responsible for Are responsible for

isotopesisotopes Roughly same Roughly same massmass

as a as a protonproton Found in the Found in the nucleusnucleus

ProtonsProtons are are PositivePositive charged charged DefineDefine an element an element Has roughly same Has roughly same

massmass as a as a neutronneutron Found in the Found in the nucleusnucleus

NucleusNucleus

Tiny denseTiny dense region region in the in the centercenter of the of the atomatom

Over Over 99%99% of the of the mass of the atom mass of the atom found here.found here.

Made of Made of protonsprotons and and neutronsneutrons

AH Video Clip

HomeworkHomework

Summarize the 9 scientists that we talked Summarize the 9 scientists that we talked about the past 2 days on the Scientist about the past 2 days on the Scientist Timeline PaperTimeline Paper

You will not use all the blanks yet.You will not use all the blanks yet.