Discovery of the Nucleus (DP IB Physics: SL)

• Evidence for the structure of the atom was discovered by Ernest Rutherford in the beginning of the 20th century from the study of α-particle scattering
• The experimental setup consists of alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles

α-particle scattering experiment set up

• α-particles are the nucleus of a helium atom and are positively charged

When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back

• From this experiment, Rutherford results were:
• The majority of α-particles went straight through (A)
  • This suggested the atom is mainly empty space

• Some α-particles deflected through small angles of < 10^o (B)
  • This suggested there is a positive nucleus at the centre (since two positive charges would repel)

• Only a small number of α-particles deflected straight back at angles of > 90^o (C)
  • This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
  • It was therefore concluded that atoms consist of small dense positively charged nuclei, surrounded by negatively charged electrons

An atom: a small positive nucleus, surrounded by negative electrons

• (Note: The atom is around 100,000 times larger than the nucleus!)

     ANSWER:   A

• The Rutherford scattering experience directed parallel beams of α-particles at gold foil
• The observations were:
  • Most of the α-particles went straight through the foil
  • The largest value of n will therefore be at small angles
  • Some of the α-particles were deflected through small angles
  • n drops quickly with increasing angle of deflection θ

• These observations fit with graph A