Atomic Theory

John Dalton

• First to develop a theory on the structure of an atom

• 1800’s• Based on 5 major

principles

Dalton’s 5 Principles

• 1) All matter is made of indivisible atoms• 2) All atoms of a given element are identical in their chemical and physical properties• 3) Atoms of different elements have different chemical and physical properties

Dalton’s notation of elements: 21 = water, 22 = ammonia,

Dalton’s 5 Principles

• 4) Atoms of different elements combine in simple, whole number ratios to form compounds

Ex: 2 H + 1 O H2O 5) Atoms cannot be subdivided, created,

or destroyed when they combine, separate or rearrange in reactions

*Law of Conservation of Mass

Problems with Dalton’s ideas• JJ Thomson was able to prove that

atoms were made of smaller particles that contained charge

• Sent electricity through a cathode ray tube – gas inside glowed

J.J. Thomson

• Used a cathode ray tube to prove the small particles must have a NEGATIVE charge = Electron

• Electric beam bent with a magnet

• Cathode Ray Tube

J.J. Thomson• Atoms were electrically

neutral, so there must be a + particle to cancel out the – charge from the electron

• 1897 – developed the “Plum Pudding” model of an atom• Positive “ball”

containing electrons

Robert Millikan - 1909• Oil-drop Experiment• Determined the charge

on an electron to be 1.602 × 10−19 coulomb

• Determined mass of electron to be 1/2000 of a proton

Ernest Rutherford

• Student of Thomson

• Conducted the Gold Foil Experiment in 1909 to disprove the Plum Pudding model

The Gold Foil Experiment

• Sent a beam of + charges (alpha particles) through a very thin piece of gold foil

• Angles of deflection were measured

Gold Foil - Conclusion• Since most alpha (+)

particles passed straight through, most of the foil must actually be “empty regions” – not a solid + sphere like Thomson believed

• Positive charge and most of the mass of an atom must be found in the core/center = NUCLEUS

Rutherford’s Model of an Atom

Niels Bohr - 1913• There are only certain discrete

orbits allowed in an atom and no others

• Electrons must be in one of these orbits

Erwin Schroedinger• 1926• Modified atomic structure to

include electron’s location in a “cloud” or “shell” instead of a predictable orbit

James Chadwick

• 1932 – discovered the neutron

• A neutron contained no charge and a mass equivalent to that of a proton