Nuclear model of the atom

In 1911, Rutherford proposed the nuclear model of the atom, consisting of a very small positively charged nucleus surrounded by outer electrons at relatively large distances from the nucleus. He showed that this nuclear model can be used to explain the very large deflections sometimes suffered by a particles in passing through matter. What holds the electrons in place is an attractive electrical 

University of Chicago Press, Chicago (1987). 

Partington, A Short History of Chemistry, p. 360, 3rd ed., Dover Publications, New York (1989) M.I. Sobel, Light, pp. 82 85, University of Chicago Press, Chicago (1987). 

In 1913, Moseley developed a method for determining the nuclear charge from the frequency of x rays emitted by an element on collision with high-velocity electrons.Each element has its own characteristic x rays, which are almost like a fingerprint of the element. 

The nuclei of atoms are now known to be made up of protons and neutrons, a 

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We develop the modern model of an atom in Chapter 1. At this stage, all we need to know is that according to the current nuclear model of the atom, an atom consists of a small positively charged nucleus, which is responsible for almost all its mass, surrounded by negatively charged electrons (denoted e ). Compared with the size of the nucleus (about 10 14 m in diameter), the space occupied by the electrons is enormous... 

In the nuclear model of the atom, the positive charge and almost all of the mass is concentrated in the tiny nucleus, and the surrounding negatively charged electrons take up most of the space. The atomic number is the number of protons in the nucleus. 

What Do We Need to Know Already We need to be familiar with the nuclear model of the atom and the general layout of the periodic table (Fundamentals Section B). We also need the concepts of kinetic and potential energy (Section A). 

This chapter builds an understanding of atomic structure in four steps. First, we review the experiments that led to our current nuclear model of the atom and see how spectroscopy reveals information about the arrangement of electrons around the nucleus. Then we describe the experiments that led to the replacement of classical mechanics by quantum mechanics, introduce some of its central features, and illustrate them by considering a very simple system. Next, we apply those ideas to the simplest atom of all, the hydrogen atom. Finally, we extend these concepts to the atoms of all the elements of the periodic table and see the origin of the periodicity of the elements. 

We are now ready to build a quantum mechanical model of a hydrogen atom. Our task is to combine our knowledge that an electron has wavelike properties and is described by a wavefunction with the nuclear model of the atom, and explain the ladder of energy levels suggested by spectroscopy. 

Describe the experiments that led to the formulation of the nuclear model of the atom (Section 1.1). 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Chemistry is the science that deals with the interaction between different forms of matter to bring about a change in the nature of the interacting matter. According to the nuclear model of the atom it is evident that chemical interaction is due to interacting electron clouds. In order to understand chemical interactions it is therefore necessary to understand, in the first place the behaviour of electrons. 

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... 

A little earlier, in 1903 (Lenard 1903), Philipp Eduard Anton von Lenard (1862-1947) had carried out some scattering experiments in which he bombarded various metallic foils with high-energy cathode rays. He observed that the majority of electrons passed through the foils undeflected - from this he concluded that the majority of the volume occupied by the metallic atoms must be empty space. This idea was more fully developed by Rutherford (1911), who proposed the nuclear model of the atom which, despite much further elaboration, we still use today for the most basic explanations. 

In the early part of the twentieth century, then, a simple model of atomic structure became accepted, now known as the Rutherford nuclear model of the atom, or, subsequently, the Bohr-Rutherford model. This supposed that most of the mass of the atom is concentrated in the nucleus, which consists of protons (positively charged particles) and neutrons (electrically neutral particles, of approximately the same mass). The number of protons in the nucleus is called the atomic number, which essentially defines the nature of... 

Rutherford performed several calculations that led him to an inescapable conclusion the atom is made up mainly of empty space, with a small, massive region of concentrated charge at the centre. Soon afterward, the charge on this central region was determined to be positive, and was named the atomic nucleus. Because Rutherford s atomic model, shown in Figure 3.5 on the next page, pictures electrons in motion around an atomic nucleus, chemists often call this the nuclear model of the atom. You may also see it referred to as a planetary model because the electrons resemble the planets in motion around a central body. 

Simplicity was never an attribute attached to the concept of nucleus in the way that it was attached for many years to the concept of atom. By the time the nuclear model of the atom was formulated, the atom was already known to be composite and at least some atoms impermanent. If the nucleus contained most of the mass of an atom, and if a radioactive atom spontaneously ejected pieces like a particles, then the nucleus must be where those ejected particles came from. Of course, we teach our students that nuclei are made of protons and neutrons. How did these particles come to be known Rutherford had a hand in their stories as well. 

Our next task is to combine the wavelike properties of electrons with the nuclear model of the atom and hope to explain the strange ladder of energy levels in a hydrogen atom observed experimentally. In doing so, we shall greatly enrich our model of the internal structure of an atom. 

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. 

Ernest Rutherford a, (3, and -y emissions gold-foil experiment nuclear model of the atom Determined the nature of radioactive particles. His gold-foil experiment established the presence of a positively charged nucleus and that the atom is mostly empty space. 

The nuclear model of the atom, as envisioned by Rutherford and Bohr, had much in common with the solar system. In each there is a massive core that exerts a controlling influence over less massive satellites orbiting around the central core. In both the solar system and the atom, the force between the central core and the orbiting satellites decreases as the square of their separation. In the case of the solar system, it was Johannes Kepler, early in the seventeenth century, who first allowed hard data—data he knew to be accurate—to sit in judgment on his speculations about the orbits of the Sun s planets. 

Consider Ernest Rutherford s alpha-particle bombardment experiment illustrated in Figure 2.11. How did the results of this experiment lead Rutherford away from the plum pudding model of the atom to propose the nuclear model of the atom ... 

Rutherford s nuclear model of the atom explains the results of the gold foil experiment. Most alpha particles pass straight through, being only slightly deflected by electrons, if at all. The strong force of repulsion between the positive nucleus and the positive alpha particles causes the large deflections. 

List the strengths and weaknesses of Rutherford s nuclear model of the atom. (4.2)... 

He returned to England in 1908, and it was there, at Manchester University, that he and his coworkers Hans Geiger and Ernst Marsden performed the famous gold foil experiments that led to the nuclear model of the atom. Not only did he perform much important research in physics and chemistry, but he also guided the work of ten future recipients of the Nobel Prize. 

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. 

Why are the results of Rutherford s gold foil experiment more consistent with a nuclear model of the atom than with the chocolate-chip cookie dough model ... 

Describe the nuclear model of the atom, including the general location of the protons, neutrons, and electrons, the relative size of the nucleus compared to the size of the atom, and the modern description of the electron. 

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