The Atom

John Dalton’s Atomic Theory => 1808

1. All matter is made up of very small particles called atoms

2. All atoms are indivisible

3. Atoms cannot be created or destroyed

William Crooke => Maltese Cross Experiment

• Cathode rays produced from cathode

• They travel up vacuum tube towards anode => Maltese cross

• Cathode rays fluoresce when they hit the glass

• Cross cast a shadow

Cathode Rays: Cathode rays are steams of negatively charged particles called electrons

William Crooke => Paddle Wheel Experiment

• Cathode rays produced from cathode

• They travel up vacuum tube towards anode

• Move paddle wheel

William Crooke Conclusions

• Cathode rays travel in straight lines

• Cathode rays cause glass to fluoresce

• Cathode rays have enough energy to move a paddle wheel

Plum Pudding Model => JJ Thompson =>1898

• JJ Thompson proposed structure of the atom => incorrect

JJ Thompsom => Electron => 1895

• Cathode rays deflected towards positive plate

• This means that cathode rays have a negative charge

• Conclusion => Cathode rays are made up of negatively charged particles called electrons

Robert Milikan => 1909 => Oil Drop Experiment

• Determined charge of an electron

  • 1.6×10^-19

Ernest Rutheford + Alpha Particles

• Alpha particles: 2 protons + 2 neutron

• Bombarded gold foil with alpha particles

• Discovered proton and nucleus

• Disproved plum pudding model

James Chadwick => Neutron => 1932

• Bombarded beryllium with alpha particles => discovered neutron