March 10, 2019 • 1 min
Toward the end of the 19th century, it became clear that atoms are not indivisible. The existence of characteristic atomic spectra of elements suggested that atoms have an internal structure, and J. J. Thomson’s discovery of the negatively charged electron in 1897 showed that atoms could be broken down into charged particles. Rutherford’s experiments in 1910-1911 revealed that an atom’s positive charge resides in a small, dense nucleus. In 1919, Rutherford made an additional discovery: When alpha particles are fired into nitrogen, one of the products is hydrogen gas. He reasoned that the hydrogen nucleus is a constituent of the nuclei of heavier atoms, such as nitrogen, and that a collision with a fast-moving alpha particle can dislodge one of those hydrogen nuclei. Thus, the hydrogen nucleus is an elementary particle, to which Rutherford gave the name proton. The following decade saw the blossoming of quantum mechanics, including the Schrödinger equation. It is owing to all these physicists that today we are on our way to understanding the principles that underlie atomic structure.