Periodicity and atomic properties

Aufbau principles, 126, 130 ground-state electron configurations and notations, 126-129 history of development, 123-124 Pauli exclusion principle, 123 periodicity of atomic properties, 131-132... 

As we have seen, the electron confignrations of the elements show a periodic variation with increasing atomic nnmber. Conseqnently, there are also periodic variations in physical and chemical behavior. In this section and the next two, we will examine some physical properties of elements that are in the same group or period and additional properties that inflnence the chemical behavior of the elements. First, let s look at the concept of effective nnclear charge, which has a direct bearing on atomic size and on the tendency for ionization. 

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]. 

These treatments of bond energies illustrate that there are fundamental relationships between bond strengths and atomic properties such as electronegativity and polarizability. The crucial point is that bond strength is increased by electronegativity differences in a given row of the periodic table as a result of an increment that is due to electrostatic attraction. 

An established system is limited by the order of known and unknown elements. With the periodic and atomic relations (existent between all atoms and the properties of their elements) were observed the possibility to not only note the absence of some of them, but even to determine (with a high certitude) the properties of the elements still undiscovered it was possible to predict their atomic weight, the density in the free state or as oxides, acids, bases, the oxidation degree, and the ability to be reduced and to form salts and to describe the metalloorganic properties of the compounds for a given element. It was also possible to describe in detail the properties of some compounds of these undiscovered elements. 

The periodic table on the inside front cover is used with permission of Dragoset, R.A., Musgrove, A., Clark, C.W. and Martin, W.C., Periodic Table Atomic Properties of the Elanents, NIST SP 966 handout, September 2010 (National Institute of Standards and Technology, Gaithersburg, MD, www. nist.gov). As of 2012, Element 114 is officially flerovium (FI) and Elanent 116 is livermorium (Lv). 

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. 

One drawback to a molecular dynamics simulation is that the trajectory length calculated in a reasonable time is several orders of magnitude shorter than any chemical process and most physical processes, which occur in nanoseconds or longer. This allows yon to study properties that change w ithin shorter time periods (such as energy finctnations and atomic positions), but not long-term processes like protein folding. 

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. 

General similarities and trends in the chemical properties of the elements had been noticed increasingly since the end of the eighteenth century and predated the observation of periodic variations in physical properties which were not noted until about 1868. However, it is more convenient to invert this order and to look at trends in atomic and physical properties first. 

These, though more difficult to describe quantitatively than the trends in atomic and physical properties described in the preceding subsection, also become apparent when the elements are compared in each group and along each period. Such trends will be discussed in detail in later chapters and it is only necessary here to enumerate briefly the various types of behaviour that frequently recur. 

The chemical and physical properties of dements are a periodic function of atomic number. 

Periodic function A physical or chemical property of elements that varies periodically with atomic number, 152 Periodic Table An arrangement of the elements in rows and columns according to atomic numbers such that elements with similar chemical properties foil in the same column,... 

---