He proposed that each atom consisted of a positively charged substance (i.e., a tiny chunk of matter) in which was embedded the appropriate number of negatively charged electrons to keep the atom electrically neutral. He further suggested the atom was like a cup of pudding (the positively charged matter) in which plums or raisins (the electrons) were suspended. Thomson’s “plum pudding” model of the atom lasted only briefly and was displaced in 1911, when Baron Ernest Rutherford (1871–1937) proposed the nuclear model of the atom. Rutherford based the new atomic model on his brilliant interpretation of a laboratory experiment that yielded unanticipated backscattering of several alpha particles as a stream of such particles bombarded a thin gold foil target. Taking into account the kinetic energy of the alpha particles and the much larger mass of the target gold atoms, he concluded that alpha particles could scatter backward only if the gold atom had most of its mass concentrated in a small, positively charged center. He named this dense central region of positive charge the nucleus.
Rutherford’s nuclear model totally revolutionized the way scientists viewed the atom and started the field of nuclear science. His model implied structure within the atom—namely, that most of an atom’s mass is concentrated in a very small central region. Electrons orbit this central region at great relative distances, much as the planets of our solar system orbit the Sun. To truly appreciate the dimensions and distances within the nuclear atom, however, you must recognize that the tiny atomic nucleus has a radius on the order of 1–10 × 10−15 meters, while the atom itself has a radius (as defined by its cloud of orbiting electrons) of about 10−10 meter. In other words, the radius of the electron’s orbit is about one hundred thousand times larger than the radius of the nucleus
One of the startling implications of Rutherford’s new atomic model was the fact that an atom is really mostly empty space. Another important conclusion was that electrically neutral atoms must have as many positive charges in the nucleus as there are negatively charged electrons orbiting the nucleus. It was several decades before scientists could satisfactorily explain how so many positively charged protons could remain close together in such a small volume despite coulomb repulsion forces (like charges repel). The answer lay in the neutron and its role in the nucleus in supporting something physicists call the strong nuclear force—an extremely powerful attractive force between nucleons (both protons and neutrons) that operates over a range of about 10−15 m or less.
Experimenting between 1919 and 1920, Rutherford showed that a unit positive charge in the atomic nucleus was identical with the nucleus of the hydrogen atom (minus its electron). He named this charged nuclear particle the proton and then speculated that a proton-sized neutral particle might also be present in the nucleus. Rutherford’s speculation was not validated until 1932, when his scientific associate, Sir James Chadwick (1891–1974), performed a defining experiment that revealed the existence of the neutron. Chadwick’s work completed development of the basic nuclear atom model—a model capable of supporting the emergence of the modern nuclear age during the 1930s and 1940s.