The understanding that alpha particle scattering is caused by electrostatic forces rather than kinetic collisions was established through the famous experiments conducted by Ernest Rutherford and his colleagues, Hans Geiger and Ernest Marsden, in the early 20th century.
Rutherford's experiments involved bombarding a thin sheet of gold foil with alpha particles, which are positively charged particles consisting of two protons and two neutrons (identical to a helium nucleus). The alpha particles were emitted by a radioactive source and directed toward the gold foil.
According to the prevailing model at the time, known as the Thomson model, the atom was thought to be a positively charged "pudding" with electrons scattered within it. In this model, the positive charge was believed to be distributed evenly throughout the atom, and it was expected that the alpha particles would pass through the gold foil with only minor deflections.
However, the experimental results surprised Rutherford and his team. They observed that while most alpha particles indeed passed through the foil, some of them were deflected at large angles, and a few even bounced back directly.
Rutherford realized that these unexpected observations could not be explained solely by the Thomson model. Instead, he proposed a new model known as the nuclear model, which suggested that atoms have a tiny, dense, positively charged nucleus at their center, surrounded by mostly empty space with electrons orbiting around the nucleus.
To explain the alpha particle scattering, Rutherford concluded that the positively charged alpha particles were being repelled by the positively charged nucleus of the gold atoms. This repulsion could only be explained by the electrostatic forces between the like charges.
Rutherford's interpretation of the experimental results was further supported by the mathematical calculations of the expected scattering angles based on Coulomb's law, which describes the electrostatic interaction between charged particles.
In summary, it was through the analysis of the experimental data and the subsequent development of the nuclear model that Rutherford and his team determined that the alpha particle scattering was primarily caused by the electrostatic forces between the positively charged alpha particles and the positively charged atomic nucleus, rather than by kinetic collisions between the particles.