Rutherford and the Nuclear Atom

Soon after Thomson developed his model, great insight into atomic structure was provided by one of his former students, Ernest Rutherford (1871-1937), the outstanding experimental physicist of his time. 

It was quite the most incredible event that has ever happened to me in m) life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you. 

Rutherford s mathematical analysis of his results showed that the scattering of positively charged a-particles was caused by repulsion from very dense regions of positive charge in the gold foil. He concluded that the mass of one of these regions is nearly equal to that of a gold atom, but that the diameter is no more than 1/10,000 that of an 

Source of narrow beam of fast-moving a-particles 

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As soon as we start this journey, we encounter an extraordinary feature of our world. When Rutherford proposed the nuclear atom (Section B), he expected to be able to use classical mechanics, the laws of motion proposed by Newton in the seventeenth century, to describe its electronic structure. After all, classical mechanics had been tremendously successful for describing visible objects such as balls and planets. However, it soon became clear that classical mechanics fails when applied to electrons in atoms. New laws, which came to be known as quantum mechanics, were developed in the early part of the twentieth century. 

Rutherford s attitude toward chemistry was stereotyped by his jokes and barbs occasionally directed at his chemical colleagues. The later Manchester physicist P. M. S. Blackett recounted the famous crack, "All science is either physics or stamp collecting, "63 and it was said that Rutherford chafed at receiving the 1908 Nobel Prize in chemistry, rather than in Physics. In a lecture in which he described his theory of the nuclear atom, he joked that the "nucleus is a round, hard objectjust like Professor Perkins head."64 However, Rutherford expressed great respect for his chemist collaborator Frederick Soddy and for other chemists, as well. 

Rutherford published the results of these scattering experiments in mid-1909, and it seemed as if publication of the discovery of the nuclear atom would soon follow. But the plum pudding model remained the working model of the atom. Through the rest of 1909 and most of 1910, Rutherford pondered. 

Ernest Rutherford (1871-1937) was born on a farm in New Zealand. In 1895 he placed second in a scholarship competition to attend Cambridge University but was awarded the scholarship when the winner decided to stay home and get married. As a scientist in England, Rutherford did much of the early work on characterizing radioactivity. He named the a and /3 particles and the g ray and coined the term half-life to describe an important attribute of radioactive elements. His experiments on the behavior of a particles striking thin metal foils led him to postulate the nuclear atom. He also invented the name proton for the nucleus of the hydrogen atom. He received the Nobel Prize in chemistry in 1908. 

From an electrostatic point of view, it is amazing that positively charged protons can be packed so closely together. Yet many nuclei do not spontaneously decompose, so they must be stable. In the early twentieth century when Rutherford postulated the nuclear model of the atom, scientists were puzzled by such a situation. Physicists have since detected many very short-lived subatomic particles (in addition to protons, neutrons, and electrons) as products of nuclear reactions. Well over 100 have been identified. A discussion of these many particles is beyond the scope of a chemistry text. Furthermore their functions are not entirely understood, but it is now thought that they help to overcome the proton-proton repulsions and to bind nuclear particles (nucleons) together. The attractive forces among nucleons appear to be important over only extremely small distances, about 10 cm. 

According to Rutherford, the nuclear atom may be dc scribed as follows. In the centre of the atom there is the nucleus this was tliought of as composed of protons and n- electrons, its cluirge being therefore equal to... 

Atoms and Spectra. We started our discussion by a study of the structure of the atom based on chemical properties and on the indications of the periodic table (Chapter I), and we referred to Rutherford s original theory of the nuclear atom and to the modifications which were applied by Bohr and Sommer-feld in order to keep the chemical theories in line with contemporary physics. Since the study of the physics of atomic-structure probably owes more to spectrum analysis than to any other method of approach we must now deal briefly with the physical interpretation of spectra. 

Rutherford therefore evolved the theory of the nuclear atom. The atom, he decided, contains a very tiny nucleus at its center, which is positively charged and which contains all the protons (and, it was later discovered, the neutrons, too) of the atom. The atomic nucleus has to be very tiny in order to account for the very small fraction of the alpha particles that were deflected, but it must also contain virtually all the mass of the atom. 

What is meant by a nuclear atom Describe the points of Rutherford s model for the nuclear atom and how he tested this model. Based on his experiments, how did Rutherford envision the structure of the atom How did Rutherford s model of the atom s structure differ from Kelvin s "plum pudding" model ... 

All these first rough insights would be the work of other men s years to anchor soundly in theory and experiment. Bohr ran them in to Rutherford. To his surprise, he found the discoverer of the nucleus cautious about his own discovery. Rutherford. .. thought that the meagre evidence [so far obtained] about the nuclear atom was not certain enough to draw such... 

In 1913, two years after Rutherford proposed the nuclear model of the atom (Section 2.2), English physicist Henry Moseley (1887-1915) developed the concept of atomic numbers. Bombarding different elements with high-energy electrons, Moseley found that each element produced X-rays of a unique frequency and that the frequency generally increased as the atomic mass increased. He arranged the X-ray frequencies in order by assigning a unique whole number, called an atomic number, to each element. Moseley correctly identified the atomic number as the number of protons in the nucleus of the atom. (Section 2.3)... 

The basic structure of an atom consists of a nucleus surrounded by a cloud of electrons. This is the nuclear atom , the model of an atom first identified by Ernest Rutherford (1871-1937) in 1911. The nucleus is positively charged, the electrons are negatively charged, and it is the attrartion between these opposite charges that is responsible for the existence and survival of the atom. As is well known, atoms are very small there are over a million carbon atoms in (the printed version of) the full stop at the end of this sentence. A nucleus is even smaller if an atom were enlarged to the size of a football stadium, the nucleus would be the size of a fly at its centre. 

Shortly after Rutherford s conception of the nuclear atom, experiments were performed to determine the masses of individual atoms. These experiments showed that the masses of nearly all atoms were greater than could be accounted for by simply adding up the masses of all the protons and electrons that were known to be present in an atom. This fact led to the concept of the neutron, a particle with no charge but with a mass about the same as that of a proton. Because this particle has no charge, it was very difficult to detect, and the existence of the neutron was not proven experimentally until 1932. All atomic nuclei except that of the simplest hydrogen atom contain neutrons. 

In the last 200 years, vast amounts of data have been accumulated to support atomic theory. When atoms were originally suggested by the early Greeks, no physical evidence existed to support their ideas. Early chemists did a variety of experiments, which culminated in Dalton s model of the atom. Because of the limitations of Dalton s model, modifications were proposed first by Thomson and then by Rutherford, which eventually led to our modern concept of the nuclear atom. These early models of the atom work reasonably well—in fact, we continue to use them to visualize a variety of chemical concepts. There remain questions that these models cannot answer, including an explanation of how atomic structure relates to the periodic table. In this chapter, we will present our modern model of the atom we will see how it varies from and improves upon the earlier atomic models. 

In 1899, Ernest Rutherford began to investigate the nature of the rays emitted from uranium. He found two particles, which he called alpha and beta particles. Soon he realized that uranium, while emitting these particles, was changing into another element. By 1912, over 30 radioactive isotopes were known, and many more are known today. The gamma ray, a third type of emission from radioactive materials similar to an X-ray, was discovered by Paul Villard (1860-1934) in 1900. Rutherford s description of the nuclear atom led scientists to attribute the phenomenon of radioactivity to reactions taking place in the nuclei of atoms. 

Beginning with J.J. Thomson s discovery of the electron in 1897, developments came quickly. In 1911, Ernest Rutherford proposed the nuclear structure of the atom, and by 1920 he had named the proton and the neutron. All of this work was made possible by the discovery of X-rays in 1895, which allowed physicists to probe the atom, and by the discovery of radioactivity in 1896. The phenomenon of radioactivity destroyed the ancient concept of the immutability of the atom once and for all and demonstrated that one element could be transformed into another, thus in a sense achieving the goal that the alchemists had sought in vain. 

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