Scientists discovered that protons were positively charged particles in the early 20th century. The work leading to this discovery was carried out by several physicists, and I'll outline the key milestones in the process.
Discovery of the Electron: In the late 19th century, J.J. Thomson conducted experiments with cathode ray tubes and discovered the existence of negatively charged particles called electrons. This discovery was a crucial step towards understanding the nature of atomic structure.
Rutherford's Gold Foil Experiment: In 1911, Ernest Rutherford conducted an experiment that provided evidence for the existence of a positively charged nucleus in atoms. He directed a beam of alpha particles (positively charged) at a thin sheet of gold foil. According to the prevailing "plum pudding" model, the positively charged particles were expected to pass through the gold foil with only minor deflections. However, Rutherford observed that a small fraction of the alpha particles were significantly deflected, and some even bounced back. This unexpected result led him to propose the presence of a small, dense, positively charged nucleus at the center of the atom.
Discovery of the Proton: Following Rutherford's experiment, scientists sought to identify the particle responsible for the positive charge in the nucleus. In 1919, Rutherford himself conducted experiments where he collided alpha particles with nitrogen gas. Through the interactions, he observed the emission of hydrogen nuclei, which he called "protons." These hydrogen nuclei had a positive charge, and Rutherford concluded that they were constituents of atomic nuclei.
Further Experimental Confirmation: Over the next few years, additional experiments were conducted to solidify the understanding of protons. For example, in 1920, Rutherford and his colleagues successfully transmuted nitrogen into oxygen by bombarding it with alpha particles, thus providing further evidence for the existence of protons.
These experiments collectively demonstrated that protons are positively charged particles residing in the nucleus of an atom. The discovery of protons laid the foundation for our understanding of atomic structure and the development of atomic theory.