Atomic Models

Thomson used these results to formulate his "plum pudding" model of the atom between the years 1903–1907. This model was an adaptation to a similar model first proposed by Lord Kelvin (1824–1907) in 1902. According to this model, the negatively charged electrons of an atom are found within a positively charged material, much like plums embedded in plum pudding. If one of the electrons were displaced, it would move back to its original position. This provided a stable model of a neutral atom. Around the same time period the Japanese physicist Hantaro Nagoaka developed the "Saturnian" atomic model. In 1904, Nagaoka proposed that an atom resembled the planet Saturn. The planet itself was a region of positive charge around which rings of electrons circled. The atom, according to this model, was unstable because electrons moving in rings around a positive charge would gradually lose energy and eventually fall into the center region.

Thomson's "plum pudding" model won favor over Nagaoka's "Saturnian" model, but was accepted for only a few years. In 1911, New Zealand scientist Ernest Rutherford (1871–1937) proposed his own atomic model based on his famous gold foil experiments. Together with his colleagues Hans Geiger (1882–1945) and Ernest Mardsen, Rutherford aimed a stream of alpha particles at a thin sheet of gold foil. Alpha particles have a positive charge (+2) and are about four times as massive as a hydrogen atom. Their hypothesis was that the alpha particles would pass through the gold foil with minimal deflection, since mass and charge were distributed uniformly throughout an atom, as proposed by Thomson. The data did not agree with this assumption.

Some of the alpha particles were deflected at large angles as they passed through the gold foil. Even more surprising, about one in 8,000 particles was deflected straight back toward the source. As Rutherford described it, it was "as if you had fired a 15 in (38 cm) artillery shell at a piece of tissue paper and it came back and hit you." He proposed that the deflected alpha particles must have come in contact with a densely packed positive charge. He called this region of positive charge the nucleus. The nucleus is surrounded by empty space, through which the electrons circled like the planets circle the sun. These experiments demonstrated that the atom has a tiny nucleus. Despite its minimal size, the nucleus contained most of the mass of the atom.

This model was not widely accepted by physicists because it was difficult to explain how such a small portion of the atom could carry most of the mass. It also suggested that the charge of the nucleus determined the properties of the atom, which disagreed with Dmitri Mendeleev's periodic table of the elements. According to Mendeleev, the atomic mass of the element determined its properties, and not the charge on the nucleus. Furthermore, it did not explain what kept the negatively charged electrons from falling into the positively charged nucleus.