The discovery of the proton is credited to Ernest Rutherford, although he did not directly discover it himself. Instead, Rutherford conducted an experiment in 1919 known as the "gold foil experiment" or the "Rutherford scattering experiment," which provided evidence for the existence of the proton.
During this experiment, Rutherford and his colleagues fired alpha particles (helium nuclei) at a thin gold foil. They expected the alpha particles to pass through the foil with only minor deflections due to the prevailing understanding of the atom at that time, which suggested that atoms were uniformly distributed spheres.
However, to their surprise, some of the alpha particles were significantly deflected at large angles, and a few even bounced straight back. This unexpected result led Rutherford to propose a new model of the atom, now known as the Rutherford model or the nuclear model.
According to Rutherford's model, most of the mass and positive charge of an atom is concentrated in a small, dense nucleus at the center, while the electrons orbit around it. This model explained the observations from the gold foil experiment and accounted for the large deflections and backscattering of the alpha particles.
Based on the properties of the alpha particles and the behavior observed in the experiment, Rutherford deduced the existence of a positively charged particle in the nucleus, which he named the "proton." He realized that the positive charge of the nucleus must be responsible for the repulsion and deflection of the alpha particles.
Although Rutherford did not directly discover the proton, his interpretation of the gold foil experiment and his subsequent model of the atom laid the foundation for our understanding of atomic structure and the existence of the proton. As a result, he is credited with the discovery of the proton and is considered one of the pioneering figures in the field of nuclear physics.