Rutherford Experiment

• In 1911 Ernest Rutherford tried to prove the existence of the Thomson model of the atom, which was that atoms were small solid spheres of positive charge, with free-flowing electrons inside.
• He expected to see the particles pass straight through, like firing a bullet through a bag of sand.
• Instead, although most went straight through, many were scattered at angles up to 1˚.  
• The experiment was broadened to find the maximum degree of scattering, and some particles scattered at angles greater than 90˚.  In other words, they reflected back.  
• The conclusion was that atoms are mostly empty space, with a solid, dense nucleus.

Rutherford’s Conclusions

• Why was there empty space?  The alphas that passed straight through were not interacting with anything, while the ones that showed slight deflection had some small interaction with the positive nuclei they passed.
• Why was the nucleus solid?  Only a solid object could cause the alphas to reflect back at large angles.
• Why was the nucleus positive?  The force with which the alphas reflected could only be explained by a repulsion between two positive charges.

Elementary Particles

• Elementary means that it is not made out of any smaller particles.
• Protons and neutrons were once thought to be elementary, but experiments in the 1950s and 1960s revealed the existence of many more particles.
• Currently there are three known classes of elementary particles:

• Quarks        
• Leptons
• Exchange particles

Quarks

• Two physicists (Murray Gell-Mann and George Zweig) independently proposed the idea of the quark to account for unexplained patterns of behavior in other particles.

Antiparticles

• Antiparticles have the same mass as a regular particle counterpart.  All other properties are opposite
• We denote antiparticles by writing the symbol for the regular particle with a bar over the top.
• Example:

Neutrinos

• First postulated by Wolfgang Pauli.  Certain radioactive decays did not appear to conserve energy or momentum
• The presence of the neutrino with small mass and no charge was suggested as a possible explanation.
• It’s mass is like that of an electron, so can experience small gravitational force
• No charge means it doesn’t experience EM force, but it is affected by the weak force.
• Fall into the category of leptons