Nuclear theory of the atom

From the beginning of his studies Langmuir was especially interested in the structure of the atom. The nature of the atom s structure was still very much in doubt. Many had crossed swords with nature to wrest this secret from her. Kelvin had pictured the atom as consisting of mobile electrons embedded in a sphere of positive electrification. J. J. Thomson had developed this same idea but his model, too, had failed because it could not account for many contradictory phenomena. Rutherford s nuclear theory of the atom as a solar system was also objected to as incomplete. The greatest difficulty to the acceptance of these models was that they all lacked a consistent explanation of the peculiar spectra of gaseous elements when heated to incandescence. 

Geiger, Hans Wilhelm (1882-1945) German physicist, who carried out research with Rutherford at Manchester University before returning to Germany in 1912. In 1908 he and Rutherford produced the Geiger counter, improved in 1928 as the Geiger-Mtiller counter. In 1909 his scattering experiments with alpha particles led to Rutherford s nuclear theory of the atom. 

Rutherford created a new model to explain his results ( Figure 4.5). He concluded that matter must not be as uniform as it appears. It must contain large regions of empty space dotted with small regions of very dense matter. In order to explain the deflections he observed, the mass and positive charge of an atom must all be concentrated in a space much smaller than the size of the atom itself. Based on this idea, he developed the nuclear theory of the atom, which has three basic parts ... 

Throughout this book, we represent atoms as spheres. For example, a carbon atom is represented by a black sphere as shown here. In light of the nuclear theory of the atom, would C-12 and C-13 look different in this representation of atoms Why or why not ... 

What are the main ideas in the nuclear theory of the atom ... 

In this chapter we consider some of the experiments that led to the conclusion that the atom is electrical in nature. Dalton s concept of a structureless atom provided no mechanism to explain these observa tions. These experiments, started over 150 years ago, also culminated in the discovery of X rays and radioactivity (Section 25.1). The spontaneous disintegration of naturally radioactive atoms into smaller particles contradicts the Daltonian hypothesis that atoms are unalterable. In turn, these discoveries inaugurated a more complete theory of the structure of atoms—the nuclear theory of the atom—and reaffirmed the atom as the unit of chemical changes. 

Artificial transmutations. Everyone is more or less fomiliar with the dreams, shattered by Dalton, of the alchemists who attempted to transform a cheap metal into gold. With the postulation of the nuclear theory of the atom, artificial transmutation came to appear feasible. The first induced transmutation was demonstrated in 1919 by Rutherford, who exposed nitrogen to a particles from radium and detected the production of protons ... 

The nuclear theory of the atom assumed that the negatively charged electron was in orbit about a more massive nucleus. However, Maxwell s theory of electromagnetism requires that when charged matter changes its direction, it must emit radiation as it accelerates. But electrons in atoms don t emit radiation as they orbit the nucleus, as far as scientists could tell. 

In 1909, Ernest Rutherford probed the inner structure of the atom by working with a form of radioactivity called alpha radiation and developed the nuclear theory of the atom. 

Another consequence of the quantum theory of the atomic and nuclear systems is that no two protons, or two neutrons, can have exactly the same wave function. The practical appHcation of this rule is that only a specific number of particles can occupy any particular atomic or nuclear level. This prevents all of the electrons of the atom, or protons and neutrons in the nucleus, from deexciting to the single lowest state. 

This experiment established the nuclear model of the atom. A key point derived from this is that the electrons circling the nucleus are in fixed stable orbits, just like the planets around the sun. Furthermore, each orbital or shell contains a fixed number of electrons additional electrons are added to the next stable orbital above that which is full. This stable orbital model is a departure from classical electromagnetic theory (which predicts unstable orbitals, in which the electrons spiral into the nucleus and are destroyed), and can only be explained by quantum theory. The fixed numbers for each orbital were determined to be two in the first level, eight in the second level, eight in the third level (but extendible to 18) and so on. Using this simple model, chemists derived the systematic structure of the Periodic Table (see Appendix 5), and began to... 

The theory of the atomic energy levels developed in the previous chapter is incomplete, since we systematically ignored the nuclear spin which leads to an additional splitting of the energy levels. This effect will be the subject of our discussion below. 

A little-known paper of fundamental importance to modern atomic theory was published by Hantaro Nagaoka in 1904 [10]. Apart from oblique citation, it was soon buried and forgotten. With hindsight it deserved better than that. It contained the seminal ideas underlying the nuclear model of the atom, the standing-wave nature of orbital electrons and radiationless stationary states. It was so far ahead of contemporary thinking that later imitators either failed to appreciate its significance, or pretended to be unaware of it. 

The most recent views on the Periodic Classification suggest an entirely different method of removing the anomaly, namely, that the atomic weight is not the correct property to use in arranging the elements rather should one be chosen which increases by a constant amount in passing from one element to the next. Physical theory indicates the positive nuclear charge of the atom to be one such property, whilst experimentally it is found that the X-ray spectra afford such a property, and eliminate the nickel-cobalt anomaly.1... 

At that time, the molecular theory of matter was not universally accepted. This had to wait for Einstein s 1905 explanation [32] of a study of the irregular movement of microscopic particles suspended in a liquid by the botanist Robert Brown [12] in 1828 - Brownian motion. Rutherford s 1911 paper [101] establishing the nuclear model of the atom was followed Bohr s quantum theory of the atom [8] and the flowering of Quantum Mechanics during the 1920s. Quantum ChemisUy, now mature field of scientific investigation and one of the centtal pillars of Computational Chemistty, was established in two seminal papers published in 1927 by Heitler and London [40] and Born and Qppenheimer [9]. 

Although Thomson s atomic model would soon be discarded by Rutherford when he introduced his nuclear model of the atom, it did succeed in establishing two important concepts. One was that the electron held the key to chemical periodicity, and the other was the notion that the atoms of successive elements in the periodic table differ by the addition of a single electron. Both of these ideas were to become important aspects of Niels Bohr s atomic theory of periodicity, which would soon be published. 

The atomic theory and nuclear model of the atom are both theories because they attempt to provide a broader understanding and model behavior of chemical systems. 

Physical theory at the time Rutherford proposed his nuclear model of the atom was not able to explain how this model could give a stable atom. Explain the nature of this difficulty. 

CHAPTER 5 Atomic Theory The Nuclear Model of the Atom... 

Dalton s atomic theory, nuclear model of the atom, planetary model of the atom, Rutherford s scattering experiment... 

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