# 6.7.1 Alpha Particle Scattering Experiment

## 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 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

### What did the Alpha Particle Scattering Experiment Show?

• The Rutherford alpha particle scattering experiment showed that:
• 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

• Since atoms were known to be neutral, the negative electrons were thought to be on a positive sphere of charge (plum pudding model) before the nucleus was theorised
• Now it is known that the negative electrons are orbiting the nucleus. Collectively, these make up the atom

#### 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°? • The Rutherford scattering experiment 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 ### Get unlimited access

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