Ernest Rutherford came to the conclusion that atoms contain a nucleus from his famous gold foil experiment conducted in 1909. In this experiment, he directed a beam of alpha particles at a thin foil of gold and observed how these particles scattered.
Most of the alpha particles passed through the foil with little to no deflection, which suggested that the majority of the atom is empty space. However, he noticed that a small fraction of the alpha particles were deflected at large angles, and some even bounced back toward the source. This unexpected behavior led Rutherford to deduce that there must be a very small, dense, positively charged center within the atom that he called the nucleus.
The nucleus, he proposed, contains most of the atom’s mass and is surrounded by a cloud of negatively charged electrons. This revolutionary idea fundamentally changed the understanding of atomic structure, moving away from the plum pudding model proposed by J.J. Thomson and establishing the groundwork for modern atomic theory.