Periodic Variation in Physical Properties

As we have seen, the electron configurations of the elements show a periodic variation with increasing atomic number. Consequently, 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 influence the chemical behavior of the elements. First, let s look at the concept of effective nuclear charge, which has a direct bearing on many atomic properties. 

The increase in effective nuclear charge from left to right across a period and from top to bottom in a group for representative elements. 

One way to illustrate how electrons in an atom shield one another is to consider the amounts of energy required to remove the two electrons from a helium atom. Experiments show that it takes 3.94 x 10 J to remove the first electron and 8.72 X 10 J to remove the second electron. There is no shielding once the first electron is removed, so the second electron feels the full effect of the +2 nuclear charge. 

Because the core electrons are, on average, closer to the nucleus than valence electrons, core electrons shield valence electrons much more than valence electrons shield one another. Consider the second-period elements from Li to Ne. Moving from left to right, we find the number of core electrons (1 ) remains constant while the nuclear charge increases. However, because the added electron is a valence electron and valence electrons do not shield each other well, the net effect of moving across the period is a greater effective nuclear charge felt by the valence electrons, as shown here. 

The effective nuclear charge also increases as we go down a particular periodic group. However, because the valence electrons are now added to increasingly large shells as n increases, the electrostatic attraction between the nucleus and the valence electrons actually decreases. 

In Chapter 7, we discussed the shielding effect that electrons close to the nucleus have on outer-shell electrons in matty-electron atoms. The presence of shielding electrons reduces the electrostatic attraction between the positively charged protons in the nucleus and the outer electrons. Moreover, the repulsive forces between electrons in a many-electron atom further offset the attractive force exerted by the nucleus. The concept of effective nuclear charge enables us to account for the effects of shielding on periodic properties. 

For atoms with three or more electrons, the electrons in a given shell are shielded by electrons in inner shells (that is, shells closer to the nucleus) but not by electrons in outer shells. Thus, in a lithium atom, whose electron configuration is l5 25 the 25 electron is shielded by the two Is electrons, but the 2s electron does not have a shielding effect on the l5 electrons. In addition, filled inner shells shield outer electrons more effectively than electrons in the same subshell shield each other. 

A number of physical properties, including density, melting point, and boiling point, are related to the sizes of atoms, but atomic size is difficult to define. As we saw in Chapter 7, the electron density in an atom extends far beyond the nucleus, but we normally think of atomic size as the volume containing about 90 percent of the total electron density around the nucleus. When we mnst be even more specific, we define the size of an atom in terms of its atomic radius, which is one-half the distance between the two nuclei in two adjacent metal atoms. 

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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. 

Metamictization of zircon (ZrSi04). Zircons from different localities show a considerable variation in physical properties, and it is generally accepted that these variations are the result of structural damage due to the radiations emitted by U and/or Th impurities over geological periods of time. The extensive literature on zircon has been reviewed by Speer (1982). 

PERIODIC LAW. Originally stated in recognition of an empirical periodic variation of physical and chemical properties of the elements with atomic weight, this law is now understood to he based fundamentally on atomic number and atomic structure. A modern statement is the electronic configurations of the atoms of the elements vary periodically with their atomic number. Consequently, all properties of the elements that depend on their atomic structure (electronic configuration) tend also to change with increasing atomic number in a periodic manner. 

Lothar Meyer and Dmitri Mendeleev independently and simultaneously organized the elements into tables based on their atomic weights. Meyer was more interested in the periodic variation in the physical properties of the elements, whereas Mendeleev was more interested in patterns of chemical reactivity. Therefore, most chemists consider Mendeleev to be responsible for the creation of the modern periodic table. 

Periodic variations in the physical properties of the elements reflect differences in atomic structure. The metaUic character of elements decreases across a period from metals through... 

The objective of theoretical organic chemistry has always been to correlate systematic variation in physical, chemical and (eventually) biological properties of organic molecules, with systematic variation in their molecular strucmre. This, of course, is only a relative correlation that was already possible long before the advent of quanmm mechanics. The formulation of structure-colour correlation rules by Witt [11], Dilthey [12] and Wizinger [13], forms an impressive example. In inorganic chemistry the periodic... 

The compounds in this table were selected on the basis of their laboratory and industrial importance, as well as their value in illustrating trends in the variation of physical properties with position in the periodic table. An effort has been made to include the most frequently encountered inorganic substances a limited number of organometallics are also covered. Many, if not most, of the compounds that are solids at ambient temperature can exist in more than one crystalline modification. In the absence of other information, the data given here can be assumed to apply to the most stable or common crystalline form. In many cases, however, two or more forms are of practical importance, and separate entries will be found in the table. 

Periodic Variation in Properties Overall, physical properties such as atomic and ionic radii of the elements vary in a regular and periodic fashion. Similar variation is also noted in their chemical properties. Chemical properties of special importance are ionization energy, which measures tlie tendency of an atom of an element to lose an electron, and electron affinity, which measures the tendency of an atom to accept an electron. Ionization energy and electron affinity form the basis for understanding chemical bond formation. 

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