Alpha-particle Scattering Experiment (Cambridge International A-Level Physics)

Alpha-particle scattering 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

Alpha particle scattering experiment

α-particles are fired at a thin gold foil and a detector records the deflection of the particles

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

Deflection patterns of the alpha particles

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

  • This is shown by process A in the image above
  • This suggested the atom is mainly empty space

• Some α-particles deflected through small angles of < 10^o

  • This is shown by process B in the image above
  • 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

  • This is shown by process C in the image above
  • 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

Structure of an atom as determined by the alpha particle experiment

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.

Which of the following graphs best represents the variation of n with θ from 0 to 90°?

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