THE PERIODICITY OF ATOMIC PROPERTIES

A similar type of research took place also in nuclear physics during the thirties with a systematic characterization of different properties for a number of atomic nuclei [24]. As an example can be mentioned the studies of the neutron cross sections as a function of the number of neutrons or protons in the nuclei, which showed systematic variations with very small values at certain numbers corresponding to nuclei with 20, 50, 82 and 126 neutrons. This discovered periodicity was rather different compared with the periodicity of atomic properties as the first ionization potential and electron affinity for alkali and noble gas atoms. Speaking at a meeting of the Chemical Society on April 19, 1934, the centenary of the birth of Mendeleev, Rutherford concluded, /< may be that a Mendeleev of the future may address the Fellows of this Society on the Natural Order of Atomic Nuclei and history may repeat itself [25]. Measurements of for example nuclear spins for a number of isotopes also showed a similar type of periodicity as found in neutron cross sections. This kind of periodicity could not at that time be understood from the commonly used liquid drop model [26] but based on the single particle model formulated by Mayer, Haxel, Jensen and Suess in 1949 [27]. 

As an introduction to the main topics of this book - atomic structure and the periodicity of atomic properties - the foundations of the subject, which lie in quantum mechanics, and the nature of atomic particles and electromagnetic radiation are described. 

The periodicity of chemical properties, which Is summarized In the periodic table. Is one of the most useful organizing principles in chemistry. Periodic patterns also provide information about electron arrangements in atoms. 

For example, E. G. Mazurs (note 2, p. 105) expresses the discord as follows The periodicity of atomic structure must be accepted as a Natural Law. Therefore, scientists have to change their minds, get away from the conservatism that accepts only Mendeleev s chemical table as right, and adjust the other phenomena to this phenomenon that is, derive the chemical and physical properties of the elements from the electronic structure of the atoms. ... 

There s nothing unique about the periodicity of atomic radii shown in Figure 5.1. Any of several dozen other physical or chemical properties can be plotted in a similar way with similar results. We ll look at several examples of such periodicity in this chapter and the next. 

The modern periodic table contains a tremendous amount of useful information. In this section we will discuss the origin of this valuable tool later, we will see how the quantum mechanical model for the atom explains the periodicity of chemical properties. Certainly one of the greatest successes of the quantum mechanical model is its ability to account for the arrangement of the elements on the periodic table. 

From the dependence of atomic polarizability by the cubic atomic radii rises immediately the correspondence with the atomic volume also. Therefore, atomic polarizability states as a fully atomic related property and have to closely follow the periodicity of atomic volumes, the Lothar Meyer s periodic curve. 

More down to earth, Plotnikov examined the effect that different atoms had on the spectra and noticed that metal iodides absorbed at longer wavelength than bromides and even more of chlorides, again an indication of the periodicity of photochemical properties, and likewise X increased with the atomic weight of the cation. 

From a modern-day perspective, the use of the atomic number instead of the atomic weight to define an element seems evident, as we know that this atomic number Z, which is the number of protons in the nucleus, is approximately proportional to the atomic weight. Indeed, it is the variation in the number of neutrons in the nucleus of atoms with the same atomic number that gives rise to isotopes of the same element. As the number of electrons in a neutral atom is the same as the number of protons in the nucleus (the atomic number), and the chemical properties depend largely on the number of electrons, it is quite natural that the periodicity of chemical properties should reflect the atomic number, which is in turn, as we have just pointed out, closely paralleled by the atomic weight. Nevertheless, this solution was far from obvious at the beginning of the twentieth century, because in order to save the periodic table and the periodic system, it looked as though chemists would have to abandon Mendeleev s definition of an element. 

The trends in chemical and physical properties of the elements described beautifully in the periodic table and the ability of early spectroscopists to fit atomic line spectra by simple mathematical formulas and to interpret atomic electronic states in terms of empirical quantum numbers provide compelling evidence that some relatively simple framework must exist for understanding the electronic structures of all atoms. The great predictive power of the concept of atomic valence further suggests that molecular electronic structure should be understandable in terms of those of the constituent atoms. 

The development of the structural theory of the atom was the result of advances made by physics. In the 1920s, the physical chemist Langmuir (Nobel Prize in chemistry 1932) wrote, The problem of the structure of atoms has been attacked mainly by physicists who have given little consideration to the chemical properties which must be explained by a theory of atomic structure. The vast store of knowledge of chemical properties and relationship, such as summarized by the Periodic Table, should serve as a better foundation for a theory of atomic structure than the relativity meager experimental data along purely physical lines. ... 

Another property that is closely related to electronegativity and position in the periodic table is polarizability. Polarizability is related to the size of atoms and ions and the... 

In this paper, the electronic structure of disordered Cu-Zn alloys are studied by calculations on models with Cu and Zn atoms distributed randomly on the sites of fee and bcc lattices. Concentrations of 10%, 25%, 50%, 75%, and 90% are used. The lattice spacings are the same for all the bcc models, 5.5 Bohr radii, and for all the fee models, 6.9 Bohr radii. With these lattice constants, the atomic volumes of the atoms are essentially the same in the two different crystal structures. Most of the bcc models contain 432 atoms and the fee models contain 500 atoms. These clusters are periodically reproduced to fill all space. Some of these calculations have been described previously. The test that is used to demonstrate that these clusters are large enough to be self-averaging is to repeat selected calculations with models that have the same concentration but a completely different arrangement of Cu and Zn atoms. We found differences that are quite small, and will be specified below in the discussions of specific properties. 

Why Do We Need to Know This Material Atoms are the fundamental building blocks of matter. They are the currency of chemistry in the sense that almost all the explanations of chemical phenomena are expressed in terms of atoms. This chapter explores the periodic variation of atomic properties and shows how quantum mechanics is used to account for the structures and therefore the properties of atoms. 

What Are the Key Ideas The structures of atoms determine their properties consequently, the behavior of elements is related to their locations in the periodic table. 

What Do We Need to Know Already The information in this chapter is organized around the principles of atomic structure and specifically the periodic table (Chapter 1). However, the chapter draws on all the preceding chapters, because it uses those principles to account for the properties of the elements. 

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