Rutherford Discovers Atomic Nucleus

In 1911, the physicist Ernest Rutherford announced that the atom has a tiny, dense, positively charged core, which he later called the nucleus. His conclusion, based on experiments carried out at the University of Manchester, overturned the prevailing picture of the atom and laid the foundation for modern atomic physics. It is one of the great turning points in humanity's understanding of matter.

At the start of the twentieth century, the atom was no longer thought of as indivisible. J. J. Thomson had discovered the electron in 1897, and since atoms are electrically neutral overall, there had to be some source of positive charge as well. Thomson proposed a model, often nicknamed the "plum pudding" model, in which negatively charged electrons were embedded in a diffuse sphere of positive charge. Rutherford, a New Zealand-born researcher who had already won the Nobel Prize in Chemistry in 1908 for his work on radioactivity, had been studying alpha particles, which are fast-moving particles emitted by certain radioactive substances.

The key experiments were carried out by Hans Geiger and a young student, Ernest Marsden, under Rutherford's direction, around 1908 to 1909. They aimed a beam of alpha particles at an extremely thin sheet of gold foil and watched where the particles landed on a fluorescent screen. Most passed straight through, as the Thomson model predicted. But a small fraction were deflected at large angles, and a very few appeared to bounce almost straight back. Rutherford later compared this to firing a large shell at tissue paper and having it come back at you, a remark often retold in slightly varying forms.

It took Rutherford about two years to work out what the results meant. In 1911 he published an analysis showing that the scattering could be explained only if nearly all of an atom's mass and positive charge were concentrated in a minute central region, with the electrons orbiting far outside it. Most of the atom, in other words, is empty space. His estimates showed the nucleus to be thousands of times smaller than the atom as a whole.

The nuclear model had problems of its own, since classical physics suggested orbiting electrons should quickly lose energy and spiral inward. In 1913 Niels Bohr, who had worked with Rutherford in Manchester, addressed this by applying early quantum ideas, producing a model that built on Rutherford's structure. Rutherford went on to achieve the artificial transmutation of nitrogen in 1917 and to identify the proton as a nuclear component. His discovery opened the path toward nuclear physics, and eventually to nuclear energy, medicine, and weapons.