The student will learn:

1. Development of modern day Atomic theory

2. Early contributions of Democritus/Dalton

3. Discovery of electron: Thompson/Millikan

4. Discovery of nucleus: Rutherford/Chadwick

History of the Atomic Theory

460-370 BC

Democritus: small invisible particles called atomos.

How?

• 1766-1844 John Dalton Englishman

Dalton’s Atomic theory

• 1. element made up tiny particles called atoms

• 2. atoms of a given element are identical• Ex) all carbon atoms have same physical & chemical properties

atoms of a different element are different in some way

Ex) carbon atoms have different physical & chemical properties than sulfur

Dalton’s Atomic Theory

• 3. Atoms of one element combine with atoms of a different element to form Compounds.

* Law of Definite Proportion» All samples of a compound contain the same ratio / or /

proportion of the elements

* Chemical Formulas

Dalton’s Atomic Theory

4.Chemical reactions involve reorganization of atoms* atoms are not changed just rearranged

* atoms of one element cannot change into atoms of another element.

“ cannot change copper into gold”

Dalton’s Atomic Theory1766-1844

1st discovery of subatomic particles

Discovery of electrons

1897 J.J.Thomson : used a cathode ray tube

Discovery of electrons

Caused a vacuum in tube

Filled it with a gas

A piece of metal at each end + / -

Put electricity through it

Surface opposite the cathode+ glowed

Put magnets next to it = caused a deflection

Placed an object inside it cast a shadow

Placed a wheel inside and it moved

Thomson’s Plum Pudding model of an atom.

Idea of the time….William Thomson aka…. Lord Kelvin

JJ Thomson 1897

Robert A. MillikanAmerican physicist- 1909

Charge of electron 1.592 x 10-19

coulomb

Oil drop experiment: suspended an oil drop between two charged plates.

Discovery of Nucleus

Ernst Rutherford - 1911

Gold foil experiment:

Bombarded a piece of gold foil with alpha particles which were considered positive.

Expected to go all the way through it.

If made of pieces……..

then should be able to break it into pieces……..

Results of foil experiment

if Plum Pudding

model had been correct.

Rutherford’s experiment.

Actual results.

p.72

“as if you had fired a 15 inch artillery shell at a piece of tissue paper and it bounced back”.

Ernst Rutherford

Thought about for 2 years.

Rebounded positive 2He atom means Gold must have a dense middle and be positive.

Named it nucleus and it must be positive.

Ernest Rutherford 1911

James Chadwickdiscovery of neutron

• In 1913 he was in Germany at start of World War I and would be interned in Ruhleben P.O.W. camp just outside of Berlin.

• Hans Geiger and Ernest Rutherford interceded for his release.

• It was his discovery of neutrons that led to “nuclear fission”…….sleeping pills

Sir James ChadwickEnglish - 1913

• Atoms from Dalton 1800

• Electrons from J.J.Thomson 1897

• Nucleus from Rutherford 1911

• Neutron from Chadwick 1932

• Protons???? • Collaborative but assigned to Rutherford….because

needed something positive to make neutral.

What was discovered and who discovered it in the following photo?

Who & what was discovered?