Nucleus/nuclear model

Now our nuclear model suffices. We can build up the atoms for all elements. Each atom has a nucleus consisting of protons and neutrons. The protons are responsible for all of the nuclear charge and part of the mass. The neutrons are responsible for the rest of the mass of the nucleus. The neutron plays a role in binding the nucleus together, apparently adding attractive forces which predominate over the electrical repulsions among the protons. ... 

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. 

The observation that atoms of a single element can have different masses helped scientists refine the nuclear model still further. They realized that an atomic nucleus must contain subatomic particles other than protons and proposed that it also contains electrically neutral particles called neutrons (denoted n). Because neutrons have no electric charge, their presence does not affect the nuclear charge or the number of electrons in the atom. However, they do add substantially to the mass of the nucleus, so different numbers of neutrons in a nucleus give rise to atoms of different masses, even though the atoms belong to the same element. As we can see from Table B.l, neutrons and protons are very similar apart from their charge they are jointly known as nucleons. 

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. 

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. 

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... 

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. 

A mono-nuclear model, which allows only one nucleation, the next nucleation taking place only after the entire surface is covered by growth layers originating from the earlier nucleus. 

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. 

Elements are made of tiny particles called atoms, which can combine in simple numerical ratios according to the law of multiple proportions. Atoms are composed of three fundamental particles Protons are positively charged, electrons are negatively charged, and neutrons are neutral. According to the nuclear model of an atom proposed by Ernest Rutherford, protons and neutrons are clustered into a dense core called the nucleus, while electrons move around the nucleus at a relatively large distance. 

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. 

As we will see later, the chemistry of an atom mainly results from its electrons. For this reason chemists can be satisfied with a relatively crude nuclear model. The nucleus is assumed to contain protons, which have a positive... 

E. Rutherford Established the nuclear model of the atom (very small, heavy nucleus surrounded by mostly empty space)... 

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. 

As you learned in Chapter 4, Rutherford proposed that all of an atom s positive charge and virtually all of its mass are concentrated in a nucleus that is surrounded by fast-moving electrons. Although his nuclear model was a major scientific development, it lacked detail about how electrons occupy the space surrounding the nucleus. In this chapter, you will learn how electrons are arranged in an atom and how that arrangement plays a role in chemical behavior. 

Many scientists in the early twentieth century found Rutherford s nuclear atomic model to be fundamentally incomplete. To physicists, the model did not explain how the atom s electrons are arranged in the space around the nucleus. Nor did it address the question of why the negatively charged electrons are not pulled into the atom s positively charged nucleus. Chemists found Rutherford s nuclear model lacking because it did not begin to account for the differences in chemical behavior among the various elements. 

The nuclear current consists of a classical, external part (jmc) == jcxt describing the nucleus in its ground state and a second quantized part j uc describing internal nuclear degrees of freedom. Specification of this fluctuating current employs nuclear models. The action principle yields equations of motion for the coupled Dirac-Maxwell helds and A>M ... 

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