Atomic Theory• Democritus – Continuous Theory• Separate, discrete particles• Cannot be further divided• Called them Atoma• Atom came from it

• Law of definite composition • Every compound is formed of elements combined in specificratios by mass unique for that compound

Atomic Theory• Dalton – 1803• Combinations of different

atoms form compounds• Atoms of different

elements have different masses• Made first atomic table• Revised into periodic table of

elements

• Dalton’s model foundation of modern atomic theory

Atomic Theory• Thomson – Electron

Discovery• Cathode ray beam

unaffected by gravity• Must be wave• Affected by

magnetism• Must be charged• Drawn to positive

charge – must be negatively charged particles

• Stoney called electrons

Atomic Theory• Thomson – Plum

pudding model• Mass of charged

particles almost nothing, but entire atom mass a lot more

• Negatively charged particles embedded in positively charged substance (pudding) that completely surrounds them – neutral charge

Atomic Theory• Rutherford/Geiger

and the Nucleus• Alpha particles –

mass of 7300 electrons• Pass through gold

foil, strike zinc sulfide, flash of light• Some deflected,

some ricocheted! They must be hitting something massive!

• http://www.youtube.com/watch?v=QY8w8xpKKzs

Atomic Theory• Rutherford/Geiger

and the Nucleus• Later discovered

that nucleus positively charged – named positive particles protons• 1836 times more

massive than electrons

• # of protons in nucleus• Atomic number

Atomic Theory• Chadwick and the Neutron• Nuclei more massive than

protons yet positively charged• Must be neutral particles• Particles named neutrons• Electrons orbit much like planets

Atomic Theory• Bohr-Electron energy

levels• Why orbits not decay?• Heated elements produce

spectra – lines of emissions• Electrons exist in specific

energy levels, as energy absorbed, move from one level to another (quantize) for short period, when return, emit light in specific spectra

Atomic Theory• Bohr-Electron energy levels• 6 or 7 levels per atom• Spectra unique to each element• Note the spectra, identify the element

Atomic Theory• Modern Quantum

Theory – DeBroglie’s Hypothesis• Electrons orbit in

waves – not continuous round motion• Always complete

wave, never partial

Atomic Theory• Modern Quantum

Theory - Heisenberg’s uncertainty principle• Electrons orbit in 4

dimensions (time) their position at any point in time cannot be precisely determined!• Orbit in clouds

Atomic Theory• Physicist humor