AQA A-Level Physics/Evidence for the nucleus

Exam note

You don't need to learn much about this, but it is important to know the significance of Rutherford's scattering experiment as per the specification , since part 3.8.1.1 refers to the Qualitative study of Rutherford scattering and Appreciation of how knowledge and understanding of the structure of the nucleus has changed over time.

Thompson's model of the atom

In 1897, J.J Thompson discovered the electron, which meant that the indivisible model of the atom was no longer valid, since electrons could be separated from the atom itself. As such, he proposed the "plum pudding" model, in which the main body of the atom is a sphere of positive charge in which the electrons are embedded like plums in a pudding.

Rutherford's scattering experiment

Thompson's model was falsified following an experiment carried out by Ernest Rutherford and two of his students in 1909. Rutherford fired a beam of (positively charged) alpha particles at a thin layer of gold foil. The particle deflection angle was recorded with the help of a circular screen which surrounded the gold foil. Rutherford expected them to be deflected very slightly due to the electrons in the atom.

However, the majority of the alpha particles went straight through the atom with no deflection, while a small number were deflected by a large angle. This led Rutherford to come to a number of conclusions.

• The atom is mostly empty space, since the vast majority of the alpha particles passed straight through.
• There has to exist a positively charged nucleus at the centre, since the positive alpha particles were mostly deflected by a large angle. This also means that compared to the empty space that surrounds it, the nucleus must be very small.
• Due to the conservation of momentum, the nucleus must have a large mass to deflect the alpha particles with high velocity.