atomic theory. john dalton first to develop a theory on the structure of an atom 1800s based on 5...
TRANSCRIPT
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