IB DP Physics: HL复习笔记7.3.1 Discovery of the Nucleus

The Rutherford-Geiger-Marsden Experiment

  • 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 < 10o (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 > 90o (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!)

Worked Example

In an α-particle scattering experiment, a student set up the apparatus below to determine the number n of α-particle incident per unit time on a detector held at various angles θ.WE-Rutherford-scattering-question-image-1Which of the following graphs best represents the variation of n with θ from 0 to 90°?WE-Rutherford-scattering-question-image-2

     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