`alpha`- particles scattering experiment helped to detect the existence of the nucleus. The main observation of this experiment is

Explanation

As the alpha particles and nucleus are positively charged, the alpha particles always seater away after reaching at Heme distance from the nucleus. This distance between the nucleus and the alpha particle is the distance of closest approach. To make alpha particles come in contact with the nucleus, lot of energy will be required in the form of the kinetic energy of the alpha particles to overcome the electrostatic repulsion between the both.